Patent Publication Number: US-4322792-A

Title: Common front-end control for a peripheral controller connected to a computer

Description:
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LIST OF TABLES                                                            
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Table O-SS Status States                                                  
Table I    CFE Glossary                                                   
Table II   Backplane Signals on Distribution Card                         
           Interfaces to CFE (Common Front End)                           
Table III  Backplane Signals between CFE and                              
           Maintenance Card                                               
Table IV   CFE Foreplane Connectors                                       
Table V    PDB - Peripheral Connector                                     
Table VIa  49-bit Micro-code Word (22-48)                                 
Table VIb  49-bit Micro-code Word (00-21)                                 
Table VII  Address Codes: Maintenance Card to CFE                         
           DLP Maintenance Mode Addressing                                
Table VIII Maintenance Card Lines for Driving PROM                        
           Address Lines (A0-A9)                                          
Table IX   CFE Signals displayed with given Display                       
           Select Codes                                                   
Table Xa   CFE-PROM Address Selection                                     
Table Xb   Enable Lines for Expanded AO Selection                         
Table XI   RAM Address Selection (5-5)                                    
Table XII  Encoded Function (5-6)                                         
Table XIII Shift Register Control (5-7)                                   
Table XIV  INRAM Bus Selection (5-8)                                      
Table XV   Least Bit Selection (AO) (5-9)                                 
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     CROSS REFERENCES TO RELATED APPLICATIONS AND PATENTS 
     The following cases which involve the use of an input-output subsystem connecting a main host computer and various peripheral units are included herein by reference. 
     U.S. Pat. No. 4,162,520, inventors Darwen Cook and Donald Millers, II. The case described the peripheral-controller known as a Line Control Processor which controlled and handled data transfers between a given peripheral terminal unit and the main host system. 
     U.S. Pat. No. 4,074,352 entitled &#34;Modular Block Unit for I/O Subsystem&#34;, inventors Darwen Cook and Donald Millers, II. This case described a base module unit which housed and supported a group of eight peripheral-controllers and interfaced them to a main host computer system. 
     U.S. Pat. No. 4,106,092, entitled &#34;Interface System Providing Interfaces to Central Processing Unit and Modular Processor-Controllers for I/O Subsystem&#34;, inventor Donald Millers, II. This patent described a unit in the main host system, designated as an I/O translator or &#34;IOT&#34;, which controls and regulates data transfer between the main host system and a plurality of base modules and their peripheral-controllers. 
     U.S. Pat. No. 4,189,769, entitled &#34;Input/Output Subsystem for Digital Data Processor System&#34;, and allowed on July 31, 1979, inventors Darwen Cook and Donald Millers, II. This case describes a subsystem wherein a plurality of peripheral-controllers are organized in base modules for data communications with a main host system. The peripheral-controllers and the base module form an input-output subsystem for controlling data transfers to/from a large number of peripheral units to the main host computer system. 
     A patent application U.S. Ser. No. 103,743 entitled &#34;Data Link Processor For Magnetic Tape Data Transfer System&#34;, filed Dec. 14, 1979, inventors Kenneth Baun and Jimmy Saunders. 
     A patent application U.S. Ser. No. 103,452 entitled &#34;Peripheral Dependent Circuit For Peripheral Controller&#34;, filed Dec. 14, 1979, inventor Kenneth Baun. 
     A patent application U.S. Ser. No. 103,739 entitled &#34;I/O Subystem Using Data Link Processors&#34;, filed Dec. 14, 1979, inventors Kenneth Baun and Donald Millers II. 
     The above issued patents form a foundation and background for the present application and are included by reference into this specification. 
     SUMMARY OF INVENTION 
     The basic functions of a data link processor (or intelligent I/O interface control unit as it was previously called in the cited patents) is to provide the specific processing and control functions for data transfers between a specific peripheral and a main host computer system. 
     There has newly been developed an improved peripheral controller (data link processor) which, instead of being individually designed and oriented for one particular type of peripheral, now has isolated certain common function elements for all types of peripheral-controllers to form a data link processor of improved capability. The common-to-all unit is designated as the Common Front End which forms a basic part of the data link processor. This Common Front End, when used in conjunction with a Peripheral Dependent Logic Board (which adapts the data link processor to a particular peripheral unit) is known as a Common I/O data link processor. 
     The line control processor (peripheral-controller) in U.S. Pat. No. 4,162,520 was described as an input-output interface data transfer control unit. While many of the general overall operating functions remain the same in regard to following the commands of I/O descriptors from the main system and the returning of Result Descriptors to the main system, the improved peripheral-controller now designated as the &#34;data link processor&#34; (DLP) provides a PROM with micro-code words for use of the required control functions rather than the complexities of processor logic and plurality of multiplexors used in the peripheral-controller line control processor described in U.S. Pat. No. 4,162,520. 
     The improved peripheral-controller, now designated as the &#34;data link processor&#34;, is made of two functional units which comprise (a) a Common Front End (CFE) card which provides the control logic and micro-code words for the handling of data transfers and a (b) Peripheral Dependent Board (PDB) which interfaces the peripheral device and provides the line discipline and protocols suitable to the requirements of the particular peripheral device. The Common Front End card is a standardized unit for all different types of peripheral devices used except for the actual micro-code which is loaded into its PROM. The Peripheral Dependent Board is individually tailored to the line disciplines, protocols and other requirements of any individual type of peripheral unit. 
     Thus a peripheral-controller for an I/O subsystem can be efficiently formed of a universally-common unit (Common Front End) which is a standardized unit plus a unit (PDB) specifically tailored to interface the peripheral device. 
     The preferred embodiment described herein is directed to the card component of the data link processor called the Common Front End or comon control circuit card. This CFE card provides the commonly used functional circuitry for all types of data link processors (peripheral-controllers) and uses a PROM control store for micro-code word operators which can be addressed and sequenced to operate the logic for execution of data transfers thus unburdening the main host computer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an overall system diagram showing the relationship of the data link processor to the main host system and the peripheral terminal units. 
     FIG. 2 is a block diagram of a plurality of typical base modules which house and support a plurality of data link processors. 
     FIG. 3 is a schematic drawing of the printed circuit cards which make up the data link processor. 
     FIG. 4A is a circuit diagram of the common front end card of the data link processor; FIG. 4B is a circuit of the common front end clear circuitry; FIG. 4C shows the clock control circuitry for the CFE; FIG. 4D shows the connection logic circuitry of the common front end; FIG. 4E is a timing diagram showing how data transfers are effectuated during certain clock periods. 
     FIG. 5A is a general block diagram of a typical Peripheral Dependent Board; FIG. 5B is a circuit block diagram for the entire data link processor; FIG. 5C is a schematic drawing of the queue organization in the RAM buffer for a peripheral terminal designated as the magnetic tape data link processor. 
    
    
     The overall environment or system configuration in which a data link processor, such as a magnetic tape data link processor (MT-DLP) operates, is shown in FIG. 1. The main host computer system 10 having a host dependent port 10 p , connects message level interface buses 15 i  to specialized peripheral-controllers such as data link processor DLP 20 10  and 20 11  through a distribution control card 20 1d . Likewise, the message level interface 15 i  connects to a distribution control card (DC) 20 Od , which supports a plurality of other data link processors, such as the magnetic tape DLP, 20 O3 . 
     In the particularly preferred system embodiment for the magnetic tape data link processor, the DLP 20 O3  is connected by bus to a Master Electronic Control (MEC) 50 M . The MEC 50 M  may connect to and control up to 16 magnetic tape transport units (MTTU) 53. The Master Electronic Control has been described in a publication entitled B 9495 Magnetic Tape Subsystem Technical Manual copyright 1976, Burroughs Corporation, Detroit, Michigan 48232 at pages 31 through 46. 
     FIG. 2 shows a more generalized situation in which a host computer 10 is connected by a message level interface 15 i  to two base modules 20 O  and 20 1 . Each of these base modules has its own respective distribution cards (DC) respectively designated 20 Od  and 20 1d . 
     Each base module supports a plurality of data link processors (peripheral-controllers). For example, base module 20 O  may have a console data link processor 20 OO  which controls a console unit 50 c . Likewise, data link processors 20 O1 , 20 O2  may respectively handle control and data communication transfers between items such as a card printer, card punch or other peripheral units. Each particular data link processor is specifically oriented for serving the requirement of each type of peripheral unit. 
     Likewise, base module 20 1  has its own distribution card (DC) 20 1d  which can connect to data link processors such as 20 10 , 20 11 , each of which data link processors is specifically tailored to handle the data transfer and control operation for a particular type of peripheral terminal unit. Each base module will be seen to have its own maintenance card unit (20 Om  for base module 20 O  and maintenance card 20 1m  for base module 20 1 ). The maintenance card provides diagnostic and maintenance coperations for the data link processors of a given base module. 
     As previously described in U.S. Pat. Nos. 4,106,092 and 4,162,520, each base module has a series of backplane connectors and grooved slides whereby a large number of printed circuit-component cards may be inserted and slid-in for connection to the backplane of the base module. Thus, the distribution card 20 Od , the maintenance card 20 Om  and other printed circuit cards which constitute the peripheral-controllers or other function cards, may be inserted and connected into the common backplane of the base module. 
     FIG. 3 is a schematic illustration of how a specific data link processor (peripheral-controller) may be physically structured with slide-in printed circuit cards. Thus, a Common Front End card 10 c  is seen having a series of backplane connections 10 b , for connection to the motherboard of the base module, and having frontplane or foreplane connectors 80 a1 , 80 b1 , 80 c1 , and DLP address jumper pins 83 which are used to set the specific address of the particular data link processor (DLP) involved. 
     Connected to the CFE card 10 c  by means of the foreplane connectors is the Peripheral Dependent Board (PDB) card 80. This card has foreplane connectors 80 a , 80 b , 80 c , which connect to its associated Common Front End (CFE) card 10 c . The Peripheral Dependent Board 80 also has a foreplane connector 80 d  for connection to the MEC, Master Electronic Control unit 50 M . The PDB card 80 has jumper pins 82 for identification (ID) whereby an encoded identification signal for the board can be effectuated. 
     Thus, the specialized data link processor (peripheral controller) as 20 O3  can be seen to comprise two slide-in cards, one of which, the Common Front End card 10 c , is common to all types of peripheral controllers; while the Peripheral Dependent Board, PDB card 80, is a specialized card providing the circuitry functions required for the control, handling and specialized protocols particularly required for a specific type of peripheral unit. 
     OVERALL SYSTEM DESCRIPTION 
     As previously discussed in U.S. Pat. Nos. 4,162,520 and 4,106,092, use was made of an input/output subsystem operating between a main hose computer and one or more peripheral devices. The input/output subsystem used a plurality of base module cabinets, each of which supported a group of base modules. The base modules consisted of up to eight &#34;Line Control Processors&#34; (peripheral-controllers) which were designated as intelligent input/output processing units which controlled data transfers between any specific peripheral device and the main host system. Since these Line Control Processors (LCP) provided an identification code or &#34;data link&#34; for each task assigned by the main host system, these Line Control Processors came, in subsequent design, to be known as Data Link Processors (DLP) and henceforth will be referred to by the term of Data Link Processors. 
     The data link processor I/O subsystem interfaces a variety of peripheral (I/O) devices with one, or more than one, host system. This setup simplifies the I/O device maintainability by allowing maintenance of the I/O subsystem to be performed without being specifically tailored to the vagaries of main host systems. The data link processor subsystem is distinguished by its adaptability to any host computer system having the MLI (message level interface) capabilities described in U.S. Pat. Nos. 4,162,520 and 4,106,092. 
     The data link processor I/O subsystem can be utilized by host systems having totally different internal characteristics, for example, the same I/O subsystem may be shared by any combination of different Burroughs B 900 systems, as the B 2900, B 3900, B 4900 and the B 6900 system. 
     The message level interface is a connected line discipline by which the host system or host systems communicate with the I/O subsystem. In the preferred embodiment of the latest configuration, each of the host systems utilizes a unit called a Host Dependent Port (HDP) to achieve the message level communication. 
     The peripheral devices which are attached to and operate with a data link processor I/O subsystem operate in a semiautonomous environment, that is, a host system must initiate a peripheral device to perform an operation, by sending the data link processor an I/O descriptor. After the DLP is initiated, it (DLP) performs the operation (OP) independently of the host system. At the conclusion of the operation, the DLP reconnects back to the host system which initiated the operation and returns information called a &#34;result descriptor&#34; (R/D). During this operation, the DLP can &#34;reconnect&#34; to the host system to transfer data via the data link processor. 
     The ability of a DLP to disconnect from a host system and proceed with an I/O transfer operation frees the host system to perform other operations during the time that an I/O transfer operation is in progress within the data link processor subsystem. 
     The ability to reconnect to a host system requires the data link processor to contain a &#34;linking mechanism&#34; to re-establish the communication path between the peripheral device and the host system that initiated that peripheral device. The logic circuit that provides for this reconnection is contained in the data link processor. Alternatively the logic circuit that provides for this connection and reconnection may be contained in a system controller (SC) or a unit such as the IOT (input/output translator unit, described in the aforementioned U.S. Patents). Every peripheral device that is connected to the DLP I/O subsystem uses one of this type of &#34;connection-reconnection&#34; control devices. 
     The message level interface that connects the host system to a DLP I/O subsystem is capable of conducting all communication between the host system and the peripheral subsystem including connection, reconnection and disconnection. 
     Two types of communications are made use of between the host system and the I/O subsystem. These are (a) &#34;control&#34; communications done by control levels and (b) &#34;data transferring&#34; communications. In both types of communication information (either control levels or data) pass between the host system and the peripheral subsystem. 
     When information passes from the main host system to the DLP I/O subsystem, the operation is described as a &#34;Write&#34; operation. When information is passed from the I/O subsystem to the main host system, the operation is described as a &#34;Read&#34; operation. For example, when a peripheral device requests to be reconnected to the main host system or to another available main host system, the communication starts in the I/O subsystem with information passed on to the host system and is, therefore, a &#34;Read&#34; type of operation. When a host system initiates a peripheral device and the communication starts from the host system with information being passed from the host system to the peripheral, this is called a &#34;Write&#34; type of operation. 
     Those peripheral devices of the &#34;input&#34; type will produce only &#34;Read&#34; data for the host system. Those peripheral devices of the &#34;output&#34; type will only receive data from the host system, i.e., a &#34;Write data&#34; operation. Thus, input or output peripheral units require either a &#34;Read data&#34; or a &#34;Write data&#34; operation. Read data and Write data are not the same thing as &#34;read or write&#34; operations because &#34;read or write&#34; only describes the direction of the flow of information on the message level interface. A read function performed by an input peripheral device requires a &#34;Write&#34; operation to initiate the peripheral device first, followed by a &#34;Read&#34; operation to pass the data to the host system. 
     As described in U.S. Pat. No. 4,074,352, the LCP base module is the basic building block for the input-output subsystem. In the case where a universally &#34;Common Front End&#34; card is used in each data link processor (DLP), the subsystem can be designated as the &#34;Common I/O Subsystem&#34;. The DLP base module consists of a backplane which connects slide-in cards, any two of which form a DLP. A base module consists of one to six Distribution Cards (DC) for communication to/from the system and houses from one to eight data link processors (DLP&#39;s), each of which handles data communications to/from a specific peripheral terminal unit. Each base module also includes one Maintenance Card (MC), two termination boards and space for additional cards for specialized purposes, but which are not essential to a normal operating situation. 
     Each data link processor (DLP) consists of a Common Front End card (CFE) and peripheral dependent logic which is located on separate circuit board known as the Peripheral Dependent Board (PDB). Read only memories designated as PROMs located on the Common Front End card contain micro-code words which are the only elements dependent on the type of peripheral device used, the remaining of the elements being standard independent components. The Common Front End card and the Peripheral Dependent Board communicate to each other via three 50-pin frontplane connectors seen in FIG. 3. 
     The data link processor (20 03 , FIG. 1) contains the hardware and power required to interface the host dependent port 10 p  of the main host system 10 with a plurality of peripheral devices 53. The hardware of the subsystem consists of a base module as 20 0 , FIG. 2, which include the following: 
     1. a control console (50 c , FIG. 2) 
     2. the message level interface cabling (MLI), 15 i . 
     3. the base backplane 20 0B  for the data link processors, FIG. 3. 
     4. the base modules which fit into the cabinet and connect to the base backplane. 
     5. the data link processors (DLP) which are typically organized such that 8 data link processors are supported by each of the base modules (FIG. 2). 
     As described in the heretofore mentioned patents, the base modules were fitted with one or more Distribution Cards (DC) depending on the number of host systems which could be connected to that base module, plus a termination card and a Maintenance Card (MC). 
     As described in the afore-cited patents, the &#34;poll test&#34; is defined as the procedure used by a host system and its host dependent port to establish communication with a DLP subsystem. The host system initiates a connection to a data link processor by performing a &#34;poll test&#34; sequence. The result of this sequence is as follows: 
     1. a connection is made from the host system to the data link processor that also includes the status condition of the addressed data link processor. 
     2. a signal indication that a particular data link processor is &#34;not active&#34; in that particular DLP address, that is, the DLP is not physically present, or it has been taken off-line by the Maintenance Card, or it is unavailable because of a PROM parity error. 
     3. a signal indication that another Distribution Card (DC) is actively engaged in the particular base module and thus a path to the requested DLP is not available at this time. 
     4. a signal indication that the address requested did not carry the proper parity. 
     To begin a &#34;poll test&#34;, the host system 10 sends a Channel Select to the particular base module that it has selected to be connected. The Channel Select signal provides an indication to the Distribution Card (DC) that it has been selected for a poll test. The host system sends the address of the DLP on data lines D-8 (MSB, most significant bit) through D-1 (LSB, least significant bit). 
     A LEM (line expansion module) card can be connected to a base module to enable 16 DLPs to be connected to a single MLI 15 i . A BCC (base control card) can be used to provide identity signals for a base module. 
     If a line expansion module (LEM) is in use, the host system sends the Distribution Card the address of the requested base module on data lines C-8 (MSB) through C-1 (LSB). If a base control card (BCC) is to be addressed, then bit A-8 is used. The host dependent port (HDP) then sends Address Select to all of its base modules. This causes the Distribution Cards that receive the Channel Select to begin a poll test, and signifies to all other Distribution Cards that the host system interface is busy. This busy indication is needed by non-connected Distribution Cards to resolve Distribution Card priority in handling DLP requests for connection in situations where there is a multiple Distribution Card base environment. 
     The Distribution Card (DC), which is selected by the Channel Select and the Address Select signal, acknowledges by returning a DLP strobe signal back to the host system 10. If another Distribution Card is actively engaged on the base module backplane, or a Distribution Card of higher priority is also taking a poll test connection, then a &#34;port busy&#34; indication accompanies the acknowledge signal. If the vertical parity carried on the address is incorrect, a parity error indication accompanies the acknowledged signal. If the base module is available for connection and the address parity is correct, then the host system receives only the acknowledge signal. The Distribution Card addresses and connects to the addressed DLP. 
     The Distribution Card connects with the selected DLP by decoding the DLP address and driving one of eight address lines. The DLP specified by the address line has the address receiver jumpered to the correct address line. The DLP receiving this address sends a DLP connected signal (LCPCON) back to the host system to indicate its presence. 
     Upon receiving an &#34;acknowledge&#34; signal, and no indication of a port busy or parity error signal, the host system drops its Channel Select signal. This makes all lines available for connected communication. However, if an exception condition is reported, the host system must disconnect from the DLP. 
     When the Distribution Card detects the absence of a Channel Select, it sends another acknowledge (strobe signal). If the DLP addressed, has responded with LCPCON, the Distribution Card &#34;enables&#34; the DLP&#39;s status signal to the host system with this second &#34;acknowledge&#34;. With the second acknowledge, the DLP and host system are connected via the Distribution Card. If the selected DLP did not respond (because it was off-line or not installed) the host system receives (together with the acknowledge signal) a DLP status signal of &#34;zero&#34; indicating that the DLP is not available. 
     If the DLP is unable to handle a poll test at this time (because it is busy) the DLP status indicates this condition. It is the function of the host system to detect and to respond to this condition by &#34;disconnecting&#34; or converting to a poll request. The connection established with this &#34;poll test&#34; remains intact as long as &#34;address select&#34; remains true. 
     The DLP participation in the poll test-host system connection routine is minimal. The DLP only receives its unique address line and returns LCPCON in acknowledgment and enables the status line. If there are multiple Distribution Cards installed in a base module, then access to the backplane of the base module is achieved by a request granted mechanism in the path selection module (PSM). The PSM also provides for Distribution Card priority resolution. 
     POLL TEST TO POLL REQUEST CONVERSION 
     If the host system 10 is performing a poll test, and the initial DLP status indicates that the DLP desires access to the host system for information exchange because of a previous I/O descriptor, the host system has three options: 
     (a) disconnect 
     (b) selectively clear a DLP 
     (c) handle and service the DLP 
     If the option (c) is selected, the host system converts to a &#34;poll request&#34;. A &#34;poll request&#34; is defined as the procedure used by a data link processor to establish communication with a host system. A &#34;poll request&#34; is the opposite of a &#34;poll test&#34; because the origination of the communication is reversed, that is, with the DLP being the transmitter and the host system being the receiver and responder. 
     In conjunction with any initial status in the &#34;read&#34; direction (information flow toward the host system) the Distribution Card sends the DLP data toward the host system. If the initial status is &#34;Send descriptor link&#34;, this data is the first word of the descriptor link and contains the host return field. The host system must check this field to be certain that it can handle the DLP information transfer. If the host system cannot handle it, the DLP must &#34;disconnect&#34; before sending any host system strobes. 
     DLP INITIATED CONNECTION (POLL REQUEST) 
     When access is required to the host system and a disconnection has occurred, the DLP re-establishes connection by initiating a &#34;poll request&#34; sequence. Because all DLP&#39;s may request connection at the same time, a decision is made on a priority basis. Priority is partitioned into two types--(a) global priority, and (b) base module priority. The global priority of a DLP is based on the type of peripheral device which it services. There are 6 standard levels of global priority for DLPs. The global priority of a particular DLP is assigned with regard to the host system access requirements of the peripheral device, such as speed, stream mode, etc. 
     An additional and higher level of global priority is provided to designate an emergency request. This is designated as global priority equal to 7. An emergency request is defined as a condition requiring immediate access to the host system in order to avoid difficulty in error recovery or operator action. Global priority is used by the main host system to determine the priority of each requesting Distribution Card. Base module priority is the priority of each DLP within the base. Base module priority is used by the Distribution Card to determine the priority of each requesting DLP. 
     Base module priority is determined by the DLP number of each DLP in the base module. This number corresponds to the DLP address jumpered on each DLP, for example, the DLP address 7 is equal to DLP number 7 which is equal to the base module priority equal to 7. The established priority levels are arranged such that the highest base module priority is 7 while the lowest is 0. Only one DLP within each base module is assigned to each priority number. Thus, a base module has been organized to contain a maximum of up to 8 DLPs. 
     To begin a poll request, the DLP raises the &#34;request&#34; level jumpered to one of eight DLP request lines in the DLP, depending upon and equal to the address of the DLP. Upon detecting a DLP request, if the host system is idle, the Distribution Card raises an interrupt request to the host system. When the host system detects &#34;Interrupt Request&#34; it sends &#34;Access Granted&#34; to all the base modules and begins a poll request sequence. Access Granted allows all Distribution Cards that have requests on-line to begin poll requesting. Access Granted also signifies that the message level interface (MLI) is busy to all Distribution Cards that are not requesting. 
     CONNECTED STATE 
     In the connected state, the Distribution Card (DC) provides the communication path between the host system and the selected data link processor. All communication between the host system and the base module is asynchronous. This requires that send/acknowledge on the message level interface be pulses rather than levels. The host system send/acknowledge (AG+SIO) and the DLP send/acknowledge (LCPST/) are pulses on the message level interface (MLI). 
     The base module of the I/O subsystem can support up to eight data link processors which are mounted on slide-in cards which connect to the backplane of the base module. In each base module facilities are provided for slide-in cards which can be installed and which work with the data link processors and the main host system. These cards may be summarized as follows: 
     DISTRIBUTION CARD (DC) 
     The Distribution Card provides an electrical and functional interface between a given DLP base module and a host system. The circuitry of this card provides host system connection for 1-8 data link processors (DLPs). The routines for establishing a connection between a given DLP and the host system are implemented by logic circuitry contained on the Distribution Card. A connection can be initiated by either the host system or by a data link processor. When the host system initiates the connections, this is designated as &#34;poll test&#34;; when the data link processor initiates connections, these are designated as &#34;poll requests&#34;. 
     All communications between a host system and a base module are asynchronous. The Distribution Card synchronizes this communication. The Distribution Card provides for the capabilities of up to 6 transfer paths into a base module from one or more host systems. Each separate path into a base module requires a separate Distribution Card, if a multiple configuration is used, then a card known as the PSM or path selection module is required. Once a Distribution Card has provided a connection between the host system and a selected data link processor, the Distribution Card assumes a &#34;connected&#34; state and then becomes practically transparent to communication transfers between the host system and the selected data link processor. 
     PATH SELECTION MODULE (PSM) 
     The path selection module is a card inserted into the base module and is required when the base module contains two or more Distribution Cards. The path selection module governs access to the base backplane, and it selects and routes the DLP request, in addition to also handling Master Clear of all base modules and in addition to Selective Clear of selected base modules. 
     BCML/TTL CONVERTER (BURROUGHS CURRENT MODE LOGIC/TRANSISTOR TYPE LOGIC) CONVERTER 
     This optional converter card is used to interface a main system host dependent port (HDP) that is using Burroughs current mode logic architecture with the standard transistor type logic format of the data link processor subsystem. 
     TERMINATION CARDS 
     Each DLP base standardly requires two termination cards (TC). These cards are designated as numbers 1 and 2 and contain the required transistors and capacitors in order to properly balance and load the base module backplane lines. 
     MAINTENANCE CARD 
     Since a single backplane is provided in common for all of the data link processors of a given base module, this permits a large proportion of the DLP maintenance to be centralized. This centralized maintenance circuitry is located on the Maintenance Card that is installed at one end of the DLP base module (opposite from the position of the Distribution Card). The Maintenance Card contains the clock generation circuitry for the entire base module, in addition to other maintenance circuitry, such as circuitry for diagnostic procedures. These diagnostic procedures include the ability to simulate the peripheral interface, to control the clock to the data link processor, and to access the storage elements of the data link processor for comparison with standardly expected results. 
     As stated, the clock generation logic circuitry of the entire base module is located on the Maintenance Card. Since every card in a given base module receives this signal, the actual signal distribution has been divided into two lines in order to halve the driver requirements. The actual clock is an 8-megahertz, 50% duty cycle positive pulse on the DLP backplane. 
     DATA LINK PROCESSOR (DLP) 
     A data link processor provides the control logic and the host system interface funtion for the associated peripheral units which it services. The DLP logic that controls the peripheral device is device-dependent; therefore, some DLPs contain different hardware and/or different micro-codes. Communication with the main host system is via the message level interface by way of the Distribution Card. Requests for host system access originate from a data link processor. The DLP disconnects from the host system while transferring information to or from the peripheral device it services. A DLP consists of two or more cards (generally 10 by 13 inches each). Each card can contain a maximum of 96 TTL chips arranged in a 6×16 array. The cards plug into adjoining slots in the base module backplane. A data link processor is made up of a Common Front End card (CFE) and one or more Peripheral Dependent Boards (PDB). The CFE and first PDB are connected together by three foreplane connectors, containing 50-pin each (FIG. 3). If more than one PDB is required for a CFE, then the PDB cards are connected to each other by a fourth 50-pin foreplane connector. The CFE card contains the necessary logic to store and execute the micro-code that is dictated by the particular Peripheral Dependent Board being used. Random access memory (RAM) chips and programmable read-only memory (PROM) chips are located on the Common Front End card. The unique logic required by a particular peripheral device is located on the PDB 80 and can vary according to the particular type of peripheral device being serviced. On the other hand, the CFE 10 c  is a standardized card which provides functions common to all types of Peripheral Dependent Boards and peripheral devices serviced. 
     The function of the DLP is to interface a particular peripheral device to the main host system. The required communication between the host system and the DLP is standardized in method and content. Every DLP communicates to the main host system using this standard discipline. The DLP masks the peripheral idiosyncrasies from the host system. Thus, the DLP is independent of the host system because of this. The data link processor performs its peripheral-controlling functions in response to information received over its interface to the host system. 
     Data link processor communication with the host system is accomplished by use of standard DLP &#34;status&#34; states. These status states indicate to the host system the requirements of the DLP (at any given unit of time). Once a connection is established between a data link processor and a host system, all information transfers are performed as indicated by the DLP &#34;status&#34; signal. Thus, the entire communication process is oriented about the DLP and is &#34;DLP status driven&#34;. The &#34;status state signals&#34; are common for every type of data link processor. This has been described in detail in U.S. Pat. No. 4,162,520. 
     Every data link processor contains a message buffer of a size sufficiently applicable to its associated type of peripheral device. Data transmissions between the DLP and the host system operate to and from this message buffer. This eliminates access errors for peripherals having fixed record lengths such as card readers, printers or card punches. Stream mode devices such as tapes and disks contain and utilize two or more buffers to accomplish variable length message transfers. 
     DLP STATUS FLOW SIGNALS 
     The DLP status flow is designed to allow orderly message transfers. When it is said that the host system is &#34;DLP status driven&#34;, this means that the DLP indicates its requirements to the host system by sending its status state signal. These status states are transmitted continuously by the DLP once it is connected to the host system. The host system examines these status lines at DLP &#34;send/acknowledge&#34; time. Every DLP implements this common signal flow in a manner designed to provide as much commonality as possible so that each DLP will have a standardized routine. Status states are generated for host system usage only; they are not used logically by the DLP. They are used to make the host system aware of the internal operation routine of the DLP. Thus, this allows the host system to anticipate somewhat the requirements of the DLP. 
     STATUS STATE SIGNALS FOR THE DATA LINK PROCESSOR 
     As was previously described in U.S. Pat. No. 4,162,520 on column 70 and in connection with FIG. 6A and Table VII, each condition or situation of the operating routine of the DLP will have a Mnemonic and a &#34;status state count number&#34;. In the present system which uses the data link processor and a host dependent port, HDP, rather than the input/output translator (IOT) of the above mentioned patent, the following table O-SS outlines the description of each of the status states of the data link processor. It will be seen that each of the states follow the same routine pattern of that shown for the earlier line control processors. 
     
                                           TABLE O-SS                              
__________________________________________________________________________
Status      Status State                                                  
Mnemonic    Count Usage or Meaning                                        
__________________________________________________________________________
Cleared     STC=0 Entered by the DLP when it is cleared. This             
                  status is also shown if the DLP is not present,         
                  it has a PROM parity error, or it has been              
                  taken off-line by the Maintenance Card.                 
Disconnect  STC=1 Used by the DLP to indicate that no more                
                  transfers are possible during the connection, or        
                  to indicate that the DLP is unable to accept a          
                  new I/O descriptor.                                     
Reserved    STC=2 Reserved for expansion. Detection of this status        
                  results in an error.                                    
Idle        STC=3 Indicates that the DLP can accept a new I/O             
                  descriptor, or that a DLP receiving this new            
                  descriptor requires additional descriptor words.        
Read        STC=4 Indicates that data is being transferred to the         
                  host system by the DLP.                                 
Send Descriptor Link                                                      
            STC=5 Indicates that the descriptor link is being sent,       
                  to the host system.                                     
Receive Descriptor Link                                                   
            STC=6 Indicates that the DLP needs to receive, or is          
                  receiving the descriptor link.                          
Result Descriptor                                                         
            STC=7 Indicates that the result descriptor is being sent      
                  to the host system.                                     
Write       STC=8 Indicates that the DLP needs data from the              
                  host system.                                            
Encoded Status                                                            
            STC=9 Indicates that the DLP is sending special status        
                  information on the data lines.                          
Port Busy   STC=10                                                        
                  The host is waiting on port busy, but the LEM           
                  has a request from another DLP.                         
I/O Descriptor LPW                                                        
            STC=11                                                        
                  Indicates that the DLP requires the I/O                 
                  descriptor LPW.                                         
Break       STC=12                                                        
                  Indicates the end of a data message, and the            
                  DLP now wants an LPW.                                   
Break Enable                                                              
            STC=13                                                        
                  Indicates the desire by the DLP to transmit             
                  another message to the host system. The host            
                  system may accept or refuse this request.               
Character Transfer                                                        
            STC=14                                                        
                  Used by certain DLPs to resolve the contents            
                  of the last data word that has been received            
                  from the host system.                                   
Result Descriptor LPW                                                     
            STC=15                                                        
                  Indicates that the final word of the result             
                  descriptor is being sent to the host system and         
                  is followed by the appropriate LPW.                     
__________________________________________________________________________
 
    
     INFORMATION TRANSFERS 
     All information transfers between the DLP I/O subsystem and the host system are asynchronous and accomplished by means of control strobes. On the other hand, transfers between Distribution Card and the DLP are synchronous. The various types of information transfers are briefly described hereinunder. 
     SYSTEM TRANSMIT 
     When information is ready to be sent to the I/O subsystem, the host system emits a pulse (SIO) to the DLP I/O subsystem. When the subsystem has received data it then emits a pulse (LCPST) over to the host system. At this point the next transfer takes place beginning with a SIO signal. Resynchronizing of the asynchronous signals occurs in the Distribution Card (DC). Upon receipt of the host system strobe, the Distribution Card synchronously sets the STIOL level to the DLP. STIOL is synchronously reset when LCPSTL is &#34;true&#34; from the DLP. The answer to the host system occurs as soon as LCPSTL is &#34;true&#34; from the DLP. If this is the case, STIOL is &#34;true&#34; for only one clock period and the host system strobes are immediately answered. Data from this system is latched in the Distribution Card on the trailing edge of the host system strobe. 
     SYSTEM RECEIVE 
     When the host system can accept another word of data from the DLP I/O subsystem, it emits a pulse (SIO) to the subsystem. In turn, when the subsystem is capable of sending a new word it emits a pulse (LCPSTL) to the host system. Upon receipt of the host system strobe, the Distribution Card synchronously sets the STIOL &#34;level&#34; to indicate that a new cycle can begin. The new cycle is completed and the host system is strobed when LCPSTL is &#34;true&#34; and STIOL is synchronously reset. LCPSTL can be &#34;true&#34; before the SIO is received. If this is the case, STIOL is &#34;true&#34; for only one clock period and the host system&#39;s strobe pulses are immediately followed by subsystem strobe pulses with new data. Data to the host system is latched in the Distribution Card on the leading edge on the subsystem strobe pulse to the host system. 
     LINE TURN 
     In the course of a message transfer, it is sometimes necessary to change the information direction. The host system and the DLP coordinate in this turnaround of bi-directional lines. The DLP controls the data direction in the base module with a backplane line called I/O send (IOSND/). I/O send, when low, directs the data lines towards flow into the host system. The DLP initiates the &#34;line turn&#34; with a status transition that requires a change in information direction. Two situations occur here: 
     1. Host System Transmit to Host System Receive: If the host system detects a status change when it receives an acknowledge (on the information transfer that requires it to receive information) the host system sends another strobe to acknowledge the &#34;status change&#34;. The DLP detecting the host systems &#34;acknowledge&#34; raises the I/O send and begins transmission to the host system. 
     2. Host System Receive to Host System Transmit: If the host system acknowledges an information transfer in conjunction with a status change that requires a line turnaround, the DLP inactivates the I/O send and sends another DLP strobe to the host system. When the host system receives the &#34;acknowledge&#34; (that the base lines have been turned), then the host system begins to transmit to the DLP. 
     DLP BASE ADDRESS 
     As the DLP base backplane is made up of common lines running the length of the base, the printed circuit cards, which form the DLP, can function in almost any set of slide-in card locations. The base address chosen for a DLP is &#34;jumpered&#34; on the DLP card (FIG. 3). The base address serves only to make the DLP unique within the base. The DLP&#39;s global priority is not affected by its base address. This priority is selected on the Distribution Card (DC). 
     There are two backplane line functions that are linear: the DLP request and the DLP address. They are allocated eight lines each (0-7). The request and address jumpers must correspond. 
     DLP ID (IDENTIFICATION) 
     Upon receiving a TEST/ID OP code, the DLP returns a two-word result descriptor (R/D). The second word contains DLP ID information. Digits A and B of the ID word are a predetermined bit pattern specifying the type of DLP. Digits C and D of the ID word are a bit pattern specified by field-installed jumpers, and are used to uniquely identify the DLP. The ID word for the DLP is formatted as follows: ##STR1## 
     I/O DESCRIPTORS 
     The I/O descriptor is transmitted in 17 parallel bits including odd parity. The DLP OP codes are limited to the following four types: 
     1. Read 
     2. Write 
     3. Test 
     4. Echo 
     Operations that do not transfer data are considered &#34;test&#34;. Thus, a test is defined as an operation that results in the host system receiving a result descriptor only. Echo is a maintenance operation that causes the DLP to accept a buffer load of data from the host system, and then return it back to the host system. This allows a quick confidence check of the I/O data path as far as the DLP buffer is concerned. Also, the various translation logics can be checked by an echo operation. 
     DLPs that require further information associated with the basic operations obtain that information in the form of variants. The first I/O descriptor transfer contains the four OP code bits and up to 12 variant code bits. Further variants are transferred in 16 parallel bit increments and are not limited to any size. 
     RESULT DESCRIPTORS 
     A result descriptor is transmitted to the host system along 17 parallel bits including odd parity. The first four bits (one digit) of the first word of the DLP result descriptor are common for every DLP. These first four bits are shown as follows: 
     
         ______________________________________                                    
Bit          Significance                                                 
______________________________________                                    
A8           Not ready peripheral                                         
A4           I/O Descriptor error                                         
A2           MLI Vertical parity error                                    
A1           MLI longitudinal parity error                                
______________________________________                                    
 
    
     The B, C and D digits of the &#34;DLP result descriptor&#34; will vary according to the type of data link processor involved. 
     FIGS. 4A-1, 4A-2 show a block diagram of the Common Front-End, 10 c , (which is sometimes designated as a &#34;Common I/O&#34; unit). The maintenance control bus 30, coming from the Maintenance Card 20 om  provides input to a receiver 15 which has one output to the address multiplexer 12 and another output on bus 37 to the peripheral dependent board (PDB) 80. 
     The Distribution Card data bus 32 provides input to receiver 16 while line 31 provides another input (RCV/) to receiver 16 from the peripheral dependent board 80. One output of receiver 16 is fed to the address multiplexor 12; while the other output forms bus 38 as a data bus to the PDB 80. 
     Receiver 17 has one input from the Maintenance Card data bus 34 and another control, SIMRCV/ (simulate receive) on input line 33 from the peripheral dependent board PDB 80. Receiver 17 provides an output to the address multiplexor 12 and an output to the data bus 38. 
     Bus 35 from PDB 80 provides another input to the address multiplexor 12, while the low order address bit (AO) from the PDB 80 on line 36 provides an input to the PROM 13. PROM 13 provides a bus to the PROM register 14, which register also has an input from AND gate 24 which has two inputs; one of which constitutes the PROMCLK/ line and the other input is from the parity check circuit 18 to signal whether or not a parity error has been detected. 
     PROM register 14 has an output on bus 40 for maintenance display signals and connects to the common backplane of the base module. Another output of the PROM register 14 connects to the Request Status Latches circuit 19 which feeds its output to drivers 20 to provide signals designated IOSF (I/O Send flip-flop) Status, REQ (request) and EMREQ (emergency request) which are interrupts to the host. This bus also connects to the common backplane. 
     The PROM register 14 outputs include: the control lines of bus 43, the PDB usage bus 44 and the multiplexor enable bus 45, all of which connect to the peripheral dependent board, PDB 80. 
     A Random Access Memory or RAM buffer storage 22 will be seen having four sets of inputs from the peripheral dependent board 80. These inputs are: the chip select line 50, the write enable line 51, the RAM data in-bus 52 and the RAM address bus 53. The output bus 22a of RAM 22 is designated as the RAM data-out bus which connects to the peripheral dependent board 80. 
     COMMON FRONT END-CARD (CFE) 
     Referring to FIG. 4A which shows a block diagram of the Common Front-End (CFE) 10 c , the central operative element of the Common Front-End is the PROM controller and storage unit 13. The PROM storage 13 consists of 13 separate PROM chips which may have a total storage capacity of 1024 52-bit words. This includes odd parity. 
     As seen in FIG. 4A the common front-end also contains the receivers 15, 16, and 17, for the data link processor (DLP) interfaces with the distribution card on data bus 32 and the maintenance card on control bus 30. The &#34;enable&#34; signals for these buses are driven by the peripheral dependent board 80. 
     The data link processor RAM storage buffer 22 has a capacity of 1024 17-bit words which includes odd parity. The RAM storage unit 22 is controlled entirely by the peripheral dependent board 80 (FIG. 5B). The following Table I lists a glossary of various signals and terms used in the Common Front-end. 
     
                                           TABLE IA                                
__________________________________________________________________________
COMMON FRONT-END CARD: GLOSSARY OF TERMS                                  
__________________________________________________________________________
A0                       BROP                                             
PROM address bit 0.      When high, a 16-way PROM address branch is se-   
A1                       lected.                                          
PROM address bit 1.      BRST                                             
A2                       When high, use stack register for PROM address.  
PROM address bit 2.      BUFFEND/                                         
A3                       Active low, from PDB, used to stop burst mode.   
PROM address bit 3.      CLK8                                             
A4                       8-megahertz clock.                               
PROM address bit 4.      CLK8/                                            
A5                       8-megahertz clock not.                           
PROM address bit 5.      CLKEN                                            
A6                       High active, clock conditioning level.           
PROM address bit 6       CLKEN/                                           
A7                       Low active, used to enable clock on PDB and      
PROM address bit 7.      CFE.                                             
A8                       CLKST                                            
PROM address bit 8.      When high, PROM clock is disabled.               
A9                       CLOCK..0                                         
PROM ADDRESS bit 9.      8-megahertz backplane clock from MC.             
ADLOC/                   CLOCK/                                           
When high, DLP is MC addressed or DLP address                             
                         CFE control logic clock.                         
is not valid.            CLR/                                             
ADRVLD/0                 Active low, logic clear term.                    
When low, LOCnn/.0 is valid.                                              
                         CLRD                                             
AF                       Active high, logic control term.                 
When high, strobe I/O has been received.                                  
                         CLRLAT                                           
AF/                      Active high, logic term used to control SCLR.    
When low, strobe I/O is sent to the PDB.                                  
                         CONECT/                                          
BASLCL/0                 Active low, DC is connected to DLP.              
When low, the base is in local.                                           
                         CS/                                              
BR6                      Active low, RAM-chip-select level.               
A branch line from PDB for PROM address selec-                            
                         DATAA8/0 - DATAPR/0                              
tion.                    17-bit data bus from DC.                         
DBUSA8 - PARITY/0        LCLCLR/0                                         
17-bit data bus.         Active low, clear level from MC.                 
DBUSn                    LCPAD                                            
9-bit bus used as input to PROM address MPXs.                             
                         Active high, DLP is addressed by DC or MC.       
DIOSND/0                 LCPAD/                                           
Active low, I/O SEND level to MC.                                         
                         Active low, DLP is addressed by DC or MC.        
DLCPST/0                 LCPADF                                           
Active low, DLP strobe to MC.                                             
                         Active high, DLP is addressed by DC or MC.       
DPLY01/0 - DPLY10/0      LCPCON/0                                         
Ten display lines to MC. Active low, DLP is connected to DC.              
DSEL1/ - DSEL4/          LCPRQn/0                                         
Multiplexor address lines for data selection to dis-                      
                         Active low, DLP request levels to DC.            
play lines.              LCPSTL                                           
DSEL8/                   Active high, DLP strobe level.                   
Active low, multiplexor chip select line for input                        
                         LCPSTL/0                                         
to display lines.        Active low, DLP strobe level to DC.              
DSIMA8/0 - PARSIM/0      LCSTU1/0 - LCSTU8/0                              
17-bit data bus from MC. Four DLP status lines to DC.                     
DSIMn                    LOCAL/                                           
9 bits of DSIM lines, used to become DBUSn.                               
                         Active high, DLP is not MC addressed, or         
                         address                                          
DSTAT1/0 - DSTAT8/0      is not valid.                                    
Four DLP status lines to MC.                                              
                         LOCAL/.1                                         
EMREQ                    Active low, DLP is MC addressed.                 
Active high, DLP emergency request.                                       
                         LOCnn/.0                                         
EMRREQ/0                 Local address lines from MC.                     
Active low, DLP emergency request to DC.                                  
                         MLCPAD/0                                         
GPRIF/                   Active low, DLP is addressed by MC.              
Peripheral bus control term to PDB.                                       
                         MSTCLR/0                                         
GPRIF/.0                 Active low, base power-up clear from MC.         
Active low, from MC, disconnects PDB peripheral                           
                         MSTIOL/0                                         
cable.                   Active low, maintenance strobe I/O from MC.      
GRP0/                    MTERM/.0                                         
Active low, controls 16-line display to MC.                               
                         Active low, maintenance terminate from MC.       
INRAMA8 - INRAMPR        OFFLN                                            
17 RAM input data lines. Active high, MC has localized the CFE.           
IOSF                     OFFLN/                                           
I/O SEND flip-flop to PDB.                                                
                         Active low, the DLP is in local.                 
IOSND/.0                                                                  
I/O SEND to DC.                                                           
#BRANCH1 - #BRANCH5      #LDSTK/                                          
PROM branching control lines.                                             
                         Active low, allows stack register load of        
                         current                                          
#CONST0 - #CONST7        PROM address. This level is held high during     
                         all                                              
Multipurpose PROM outputs, PDB-dependent.                                 
                         microcode subroutines.                           
#G3 - #L4                #NEXT0 - #NEXT8                                  
PDB dependent PROM outputs (see PDB GLOS-                                 
                         PROM address bits.                               
SARY OF TERMS).                                                           
#LCPSTL/                 #PARITY                                          
Active low, DLP strobe level, from PROM con-                              
                         PROM parity bit (odd).                           
troller, to DC.                                                           
#LDINT/                                                                   
Active low, (load interface) PROM MLI bus con-                            
                         +5V                                              
trol level               VCC from power supply.                           
__________________________________________________________________________
 
    
     
                                           TABLE IB                                
__________________________________________________________________________
GLOSSARY OF TERMS - PERIPHERAL DEPENDENT BOARD                            
__________________________________________________________________________
ABANDON                  CLK8                                             
If set, this signal allows an operation to the MEC                        
                         The enabled 8-megahertz DLP clock.               
to complete without further microcode control.                            
                         CLR+PER                                          
The result of the operation is discarded.                                 
                         Clear or PROM parity error. Clears the MEC in-   
ADDCLR/                  terface.                                         
Address clear. When low, clears the SAD and                               
                         CS/                                              
PAD counters to zero.    Chip select. When low, this signal enables the   
ADDEQ                    RAM for reading and writing functions.           
Address equal. When high, indicates equality of                           
                         #CONSTn                                          
the SAD and PAD counts.  Constant lines 0-7. Originate in the microcode   
                         and                                              
#ADDSLA                  are used to determine the values to be driven    
Address select A. Used in combination with                                
                         when loading the MEC interface register; they    
                         also                                             
#ADDSLB to select one of three addresses: SAD,                            
                         choose the function to be performed by the       
                         shift                                            
PAD, or QAD.             register and are used for certain QAD bits.      
#ADDSLB                  CONNECT                                          
Address select B. See #ADDSLA.                                            
                         When high, this signal indicates a connected     
                         con-                                             
AF                       dition with the host system.                     
Asynchronous flip-flop. Follows the STIOL by                              
                         DATAV                                            
one cycle.               Data available. When high, indicates a clock     
                         from                                             
#BRANCHn                 the MEC has been detected.                       
Used to select the various signals for PROM ad-                           
                         DATAVK                                           
dress bit A0.            Data available reset. Used to clear the DATAV    
BUFDONE/                 flag.                                            
Buffer limit. When low, indicates end-of-buffer                           
                         FLAGA                                            
count in PAD.            When high, indicates a non-BCD digit is in the   
                         A                                                
BUFLIM/                  or B positions of the data bus (also see         
                         OPDEC).                                          
Buffer limit. When low, indicates end-of-buffer                           
                         FLAGB                                            
count in SAD.            Not used by the PEMT-DLP.                        
BUFSEL                   FLAGC                                            
Buffer select. The most significant digit for both                        
                         Not used by the PEMT-DLP.                        
PAD and SAD, determines which half of the RAM                             
                         #FLGCLR/                                         
is used for data storage.                                                 
                         Flag clear. Originates in the microcode and      
                         when                                             
BUSY                     low, clears the MLIPAR, LPWERR, and PERV-        
Comes from the MEC and indicates the MEC is                               
                         PAR flags.                                       
ready for data transfer. BUSY goes low after the                          
                         #FSLn                                            
result has been sent by the MEC.                                          
                         Function select bits. Originate in the           
                         microcode                                        
CLEAR                    and are decoded into seven active functions and  
                         a                                                
Comes from the CFE and includes local clear, etc.                         
                         null state.                                      
Clears the MEC interface, stopping any operation                          
in progress.                                                              
GPRIF                    MUXSLA                                           
Gated peripheral. Used in peripheral simulation.                          
                         Multiplexor select A. This signal is used in     
                         combi-                                           
When driven high, causes the MEC-related signals                          
                         nation with MUXSLB to select the RAM, the shift  
to be diverted to the Maintenance Card control.                           
                         register, the receive register, or the LPW       
                         register                                         
IDnn                     to drive the INRAM bus.                          
Identification jumpers select. Selects the ID jump-                       
                         MUXSLB                                           
ers for reading. Also used to stop burst mode un-                         
                         Multiplexor select B. See MUXSLA.                
der microcode control.   NVP                                              
IGNORE                   New vertical parity. This signal is generated to 
                         in-                                              
If set, allows the DLP to ignore data coming from                         
                         sure odd parity on the INRAM bus.                
the host system while handling data from the                              
                         OPDEC                                            
MEC.                     Operation decode. Reflects the decoded output    
                         of                                               
INFOnn                   the present operation as presented by the        
                         output                                           
MEC information lines. Consist of 17 bidirectional                        
                         of the 16-way operation decode microcode         
                         branch.                                          
lines which are used to transfer information to and                       
                         ODDAB                                            
from the MEC.            Indicates that an odd number of ones exist in    
                         the                                              
INRAM                    A and B digits of the INRAM bus.                 
A major data bus that can be driven by the RAM,                           
                         ODDCD                                            
the shift register, the receive register, or the LPW                      
                         Indicates that an odd number of ones exist in    
                         the                                              
register                 C and D digits of the INRAM bus.                 
IOSF                     OFFLN                                            
I/O SEND flip-flop. When IOSF is set (high) the                           
                         Off-line. When high, prevents the DLP from com-  
PEMT-DLP is driving the MLI. When reset (low)                             
                         municating with the host system.                 
the DLP can receive MLI information.                                      
                         OPEND                                            
LPnn                     End of operation. Causes SELECT to be dropped    
Individual LPW outputs from the LPW register.                             
                         to allow the result to be transferred and        
                         discarded.                                       
LPWCLK                   PADCLR                                           
LPW clock. Causes the outputs of the LPW regis-                           
                         Peripheral address clear. Drives the PAD to all  
ter and the system register to be exclusive ORed                          
                         zeroes.                                          
into the LPW register.   PADCTU                                           
LPWPRE                   Peripheral address count up. Increments the      
                         PAD.                                             
LPW preset. Causes all ones to be fed to the LPW                          
                         PADCTD                                           
register in the presetting function. It also clocks                       
                         Peripheral address count down. Decrements the    
the LPW error flip-flop. PAD.                                             
LPWERR                   PADEND                                           
LPW error flip-flop. Accumulates the detected                             
                         Peripheral address end. When high, indicates     
                         that                                             
LPW errors on the data received from the host                             
                         the last count is currently in the PAD. The      
                         next                                             
system.                  increment will cause the PAD to go to zero.      
LOCAL                    PADENDF                                          
When high, this signal indicates that the Mainte-                         
                         Peripheral address end flip-flop. Set by         
                         PADEND.                                          
nance Card has access to the PEMT-DLP.                                    
PADLD                    QADn                                             
Peripheral address load. Causes the lower 8 bits of                       
                         Individual queue address bits (n). Represents    
                         the                                              
PAD to be loaded from the C and D digits of the                           
                         tape drive unit number for assigning a queue to  
                         a                                                
INRAM bus.               magnetic tape unit.                              
PADn                     QUECLR                                           
Individual (n) peripheral address bits.                                   
                         Queue clear. Clears QAD to unit number zero.     
PARERR                   Q1                                               
Parity error. When high, indicates a mismatch be-                         
                         MEC clock resynchronization.                     
tween INRAMPR and the NVP calculated for the                              
                         Q2                                               
word on the INRAM bus.   MEC clock resynchronization.                     
PERF                     Q3                                               
PROM Parity error flip-flop. When high, indicates                         
                         MEC clock trailing edge detect.                  
an error has been detected on the PROM output                             
                         RAMWE                                            
register, stopping the DLP clock.                                         
                         RAM write enable. Causes the INRAM bus to be     
PERVPAR                  written into the RAM.                            
Peripheral vertical parity error flag. When high, in-                     
                         RCV                                              
dicates a vertical parity error was detected on in-                       
                         Receive enable. Allows the data bus to be input  
                         to                                               
formation received from the MEC.                                          
                         the receive register.                            
PINTCLR                  RDYST                                            
Peripheral interface clear. Deactivates all signals                       
                         Ready status. Received from the MEC to indicate  
from the DLP to the MEC. that the MEC is ready to receive an operation.   
PINTLD                   READY                                            
Peripheral interface load. Loads the control signal                       
                         Received from the MEC after it has taken the     
to the MEC.              initiate word.                                   
PRCV                     RECnn                                            
Peripheral receive. Enables the tristate receivers                        
                         Individual receive register bits.                
to feed the INFO bus into the receive register.                           
                         SADBRW                                           
PSHDN                    System address register borrow. A flag to        
                         indicate                                         
Push down. Causes a one to be pushed out of the                           
                         that the four least significant bits of the SAD  
                         are                                              
poll register.           zero. Also used when decrementing a decimal      
PSHUP                    count.                                           
Push up. Causes a one to be pushed into the poll                          
                         SADCLR                                           
register.                SAD clear. Forces the SAD count to zero.         
P1                       SADCTD                                           
Poll flag one. Used for buffer management to indi-                        
                         SAD count-down. Decrements the SAD.              
cate that at least one buffer is ready for host sys-                      
                         SADCTU                                           
tem access.              SAD count-up. Increments the SAD.                
P3                       #SADCTU                                          
Poll flag three. Indicates that at least three buffers                    
                         SAD count-up (from microcode). Increments the    
are ready, and the hardware must consider raising                         
                         SAD on its low-going edge, and when held high,   
emergency request.       allows the SAD to be automatically incremented   
P4                       by AF during burst mode.                         
Poll flag four. Four buffers are ready, and the                           
PEMT-DLP must consider a possible access error.                           
SADEND                   SHFTCLR                                          
When high, indicates that the SAD is at its highest                       
                         Shift clear. Sets the shift register bits to all 
                         zeroes.                                          
count. The next up-count will result in a zero.                           
                         SIMRCV                                           
SADENDF                  Simulate receive. Enables the DSIM lines to      
                         feed                                             
SAD end flip-flop. Stores the fact that the SA-                           
                         the receive register.                            
DEND signal went high.                                                    
                         SIMSND                                           
SADINCR                  Simulate send. Enables the system register to    
SAD increment. Causes SAD to count-up at each                             
                         drive the DSIM lines.                            
AF during read burst.                                                     
                         SLA                                              
SADLD                    When high, selects AF to clock the receive       
                         regis-                                           
SAD load. Causes the A and B digits of the                                
                         ter, otherwise Q2 is used.                       
INRAM bus to be loaded into the SAD.                                      
                         SLAF                                             
SADn                     Stores the fact that SLA was high.               
Individual system address register bits.                                  
                         SND                                              
SDnn                     Send enable. Allows the system register to       
                         drive                                            
Individual system data register output bits.                              
                         the data lines.                                  
SELECT                   SPARE                                            
A signal from the DLP to the MEC to start an op-                          
                         Not used by the MT-DLP.                          
eration. Dropping SELECT solicits the result                              
word from the MEC.       SPR-LCL                                          
                         Not used by the MT-DLP.                          
SEMREQ                                                                    
Simultaneous emergency request. Indicates that an                         
                         SREGLD                                           
emergency request already exists in the base.                             
                         System register load. The rising edge of this    
                         signal                                           
SEND                     clocks the INRAM bus into the system data        
                         regis-                                           
When high, indicates that a DLP is driving the                            
                         ter. The trailing edge of this signal causes     
                         the                                              
INFO lines to the MEC.   LPW register to clock.                           
SHDBIT                   STOPB                                            
Shift-down bit. This bit is loaded into bit C8 of the                     
                         Stop burst. When high, indicates that the DLP    
                         is                                               
shift register on shift-down operations.                                  
                         not in burst mode.                               
#SHFTOP                  STOPBF                                           
Shift operation. Generates the clock to the shift                         
                         Stop burst flip-flop. Records the fact that      
                         STOPB                                            
register.                went high.                                       
SHFTCLK                  SW1                                              
Shift clock. Causes the selected shift function to                        
                         Switch one. Used during maintenance to load the  
be performed.            address of the next microcode word.              
SYSVPAR                  XMIT                                             
System vertical parity. Indicates that a vertical                         
                         Transmit. Enables the transmit register to load  
parity error was detected on information received                         
                         and drive the INFO lines.                        
from the host system.                                                     
                         ZERO                                             
TERMF                    Indicates that the word on the INRAM bus is all  
Terminates flip-flop. Indicates that the host system                      
                         zeroes (the parity bit is not included).         
has raised the terminate line.                                            
                         ZEROAB                                           
TERMBRST                 Indicates that the A and B digits of the INRAM   
Terminated burst. Indicates that the burst mode                           
                         bus are zero.                                    
was stopped by a terminate from the host system.                          
                         ZEROCD                                           
WD                       Indicates that the C and D digits of the INRAM   
Shift word bits. From the shift register outputs.                         
                         bus are zero.                                    
__________________________________________________________________________
 
    
     DISTRIBUTION CARD INTERFACE 
     As previously indicted in FIG. 2, data link processors are housed in a base module unit. Each data link processor, consisting of two cards, slides into the base module housing having a common backplane to which the printed circuit boards of a data link processor are connected. 
     All communications between the Common Front End 10 c  and the Distribution Cards such as 20 od , 20 1d  are performed through the data link processor base module backplane. The backplane is common to all cards that are installed in a base module. 
     Table II shows a list of all the backplane signals that occur on the Distribution Card interface to the Common Front End. The 17-bit wide data portion of the bus 32, FIG. 4A, is received from the Distribution Card (DC) on the Common Front End (CFE). This same 17-bit bus is driven in the opposite direction (by drivers on the PDB) when the data link processor is sending data back to the Distribution Card. The enabling levels that control the direction of this bus are generated on the peripheral dependent board. 
     
                       TABLE II                                                    
______________________________________                                    
Distribution Card/CFE Interface                                           
Level    Pin #   Direction                                                
                          Definition                                      
______________________________________                                    
LCSTU8/0 123     to DC    DLP status bit 8                                
LCSTU4/0 023     to DC    DLP status bit 4                                
LCSTU2/0 126     to DC    DLP status bit 2                                
LCSTU1/0 026     to DC    DLP status bit 1                                
LCPCON/0 074     to DC    DLP is connected                                
IOSND/.0 124     to DC    DLP is sending                                  
EMRREQ/0 191     to DC    DLP has emergency request                       
LCPRQ7/0 163     to DC    DLP #7 has request                              
LCPRQ6/0 063     to DC    DLP #6 has request                              
LCPRQ5/0 164     to DC    DLP #5 has request                              
LCPRQ4/0 064     to DC    DLP #4 has request                              
LCPRQ3/0 165     to DC    DLP #3 has request                              
LCPRQ2/0 065     to DC    DLP #2 has request                              
LCPRQ1/0 166     to DC    DLP #1 has request                              
LCPRQ0/0 066     to DC    DLP #0 has request                              
LCPSTL/0 024     to DC    DLP strobe                                      
STIOL/.0 127     to CFE   Strobe I/O                                      
TERM/..0 072     to CFE   Terminate                                       
SELCLR/0 167     to CFE   Selective clear                                 
LCPAD7/0 176     to CFE   DLP #7 is addressed                             
LCPAD6/0 076     to CFE   DLP #6 is addressed                             
LCPAD5/0 177     to CFE   DLP #5 is addressed                             
LCPAD4/0 077     to CFE   DLP #4 is addressed                             
LCPAD3/0 178     to CFE   DLP #3 is addressed                             
LCPAD2/0 078     to CFE   DLP #2 is addressed                             
LCPAD1/0 179     to CFE   DLP #1 is addressed                             
LCPAD0/0 079     to CFE   DLP #0 is addressed                             
DATAA8/0 106     Bi-Dir   Data bit                                        
DATAA4/0 006     Bi-Dir   Data bit                                        
DATAA2/0 108     Bi-Dir   Data bit                                        
DATAA1/0 008     Bi-Dir   Data bit                                        
DATAB8/0 110     Bi-Dir   Data bit                                        
DATAB4/0 010     Bi-Dir   Data bit                                        
DATAB2/0 117     Bi-Dir   Data bit                                        
DATAB1/0 017     Bi-Dir   Data bit                                        
DATAC8/0 119     Bi-Dir   Data bit                                        
DATAC4/0 019     Bi-Dir   Data bit                                        
DATAC2/0 121     Bi-Dir   Data bit                                        
DATAC1/0 021     Bi-Dir   Data bit                                        
DATAD8/0 132     Bi-Dir   Data bit                                        
DATAD4/0 032     Bi-Dir   Data bit                                        
DATAD2/0 134     Bi-Dir   Data bit                                        
DATAD1/0 034     Bi-Dir   Data bit                                        
PARITY/0 136     Bi-Dir   Data parity bit (odd)                           
______________________________________                                    
 
    
     MAINTENANCE CARD INTERFACE 
     All communications between the Common Front End and the Maintenance Card (as 20 om ) take place on the data link processor&#39;s base module backplane. Table III shows a list of all the backplane signals that occur between the Common Front End and the Maintenance Card. 
     
                       TABLE III                                                   
______________________________________                                    
Maintenance Card/CFE Interface                                            
Level    Pin #   Direction                                                
                          Definition                                      
______________________________________                                    
DSTAT8/0 044     to MC    DLP status bit 8                                
DSTAT4/0 145     to MC    DLP status bit 4                                
DSTAT2/0 045     to MC    DLP status bit 2                                
DSTAT1/0 146     to MC    DLP status bit 1                                
DLCPST/0 140     to MC    DLP strobe                                      
DIOSND/0 144     to MC    DLP is sending                                  
MSTIOL/0 040     to CFE   Strobe I/O                                      
MTERM/.0 184     to CFE   Terminate                                       
MLCPAD/0 154     to CFE   DLP is addressed                                
BASLCL/0 161     to CFE   Base is in local                                
MSTCLR/0 172     to CFE   Base clear                                      
LCLCLR/0 174     to CFE   Local clear                                     
SWH.1/.0 058     to CFE   Maintenance switch #1                           
SP/....0 147     to CFE   Single-pulse mode                               
START/.0 149     to CFE   Allow clock(s)                                  
CLOCK ..0                                                                 
         048     to CFE   8 Mhz clock                                     
ADRVLD/0 043     to CFE   Local address is valid                          
LOC16/.0 187     to CFE   Local address bit 16                            
LOC08/.0 087     to CFE   Local address bit 8                             
LOC04/.0 188     to CFE   Local address bit 4                             
LOC02/.0 088     to CFE   Local address bit 2                             
LOC01/.0 189     to CFE   Local address bit 1                             
OFFLNE/0 073     to CFE   DLP is off line                                 
GPRIF/.0 061     to CFE   Gate off peripheral interface                   
DSEL8/.0 060     to CFE   Display select bit                              
DSEL4/.0 085     to CFE   Display select bit 4                            
DSEL2/.0 186     to CFE   Display select bit 2                            
DSEL1/.0 086     to CFE   Display select bit 1                            
DPLY01/0 054     to MC    Display line                                    
DPLY02/0 155     to MC    Display line                                    
DPLY03/0 055     to MC    Display line                                    
DPLY04/0 156     to MC    Display line                                    
DPLY05/0 056     to MC    Display line                                    
DPLY06/0 157     to MC    Display line                                    
DPLY07/0 057     to MC    Display line                                    
DPLY08/0 067     to MC    Display line                                    
DPLY09/0 168     to MC    Display line                                    
DPLY10/0 068     to MC    Display line                                    
DSIMA8/0 112     Bi-Dir   Data simulate line A8                           
DSIMA4/0 012     Bi-Dir   Data simulate line A4                           
DSIMA2/0 113     Bi-Dir   Data simulate line A2                           
DSIMA1/0 013     Bi-Dir   Data simulate line A1                           
DSIMB8/0 114     Bi-Dir   Data simulate line B8                           
DSIMB4/0 014     Bi-Dir   Data simulate line B4                           
DSIMB2/0 115     Bi-Dir   Data simulate line B2                           
DSIMB1/0 015     Bi-Dir   Data simulate line B1                           
DSIMC8/0 027     Bi-Dir   Data simulate line C8                           
DSIMC4/0 128     Bi-Dir   Data simulate line C4                           
DSIMC2/0 028     Bi-Dir   Data simulate line C2                           
DSIMC1/0 129     Bi-Dir   Data simulate line C1                           
DSIMD8/0 029     Bi-Dir   Data simulate line D8                           
DSIMD4/0 130     Bi-Dir   Data simulate line D4                           
DSIMD2/0 030     Bi-Dir   Data simulate line D2                           
DSIMD1/0 131     Bi-Dir   Data simulate line D1                           
PARSIM/0 137     Bi-Dir   Data simulate parity line (odd)                 
______________________________________                                    
 
    
     MAINTENANCE FACILITIES 
     In FIG. 2 certain maintenance facilities are provided. These consist of a console 50 c  (which may include a cathode ray tube CRT, a mini-disk, etc.) and the Maintenance Card, as 20 om , plus other software packages and hardware which are included in the DLP. Under programmatic control, the console 50 c  can be used to manipulate the DLP in such a manner as to determine its internal state for given operations and to compare it to a known correct state by use of the Maintenance Card 20 om . Thus diagnosis of a failing DLP can be made. 
     The console is the interface between the host system and the host system operator, as well as being the maintenance interface to the I/O subsystem. Maintenance of the magnetic tape data link processor originates at the console, when in the off-line mode; and from the host system, when in the on-line mode of operation. A data base can be supplied on flexible diskettes or magnetic tape to perform diagnosis of the data link processor (DLP). The selection of the test and type of module can be done at the host system by a field engineer or a host system operator. 
     DIAGNOSTIC TESTING MODES 
     There are two modes of diagnostic testing, (a) off-line and (b) on-line. In either mode the units undergoing tests are not available to the host system as a resource and must be placed off-line prior to running diagnostics. Diagnostic programs use the console-to-maintenance card interface in the diagnosis of the subsystem modules. These programs can perform a card test on the addressed unit by means of a maintenance data base that is stored on flexible diskettes or is resident in the host system. 
     OFF-LINE MODE 
     This mode implies the following: 
     1. The host systems resources are not available 
     2. The test data base is console-diskette-resident 
     3. The operator must supply control information. 
     ON-LINE MODE 
     This mode implies the following: 
     1. The host systems resources are available 
     2. The test data base is host-system-resident 
     3. A host resident program performs the diagnosis. 
     Confidence test programs can be developed which use the message level interface (MLI) to ascertain a confidence level of the I/O subsystem unit or units under test. These tests may be used to isolate a failing unit so that maintenance can be invoked to determine if the cause of the failure is in an I/O subsystem module or in the peripheral device. 
     PERIPHERAL-DEPENDENT BOARD INTERFACE 
     As seen in FIG. 3 the peripheral dependent board (PDB) and the Common Front End card are provided with foreplane connectors 80 a , 80 b , 80 c , 80 d , and 80 a1 , 80 b1 , 80 c1 . The interface between the Common Front End and the peripheral dependent board is made of three 50-pin foreplane connectors 80 a , 80 b  and 80 c . Table IV lists the connectors and shows the pin numbers together with the logic names of the signals involved. 
     
                                           TABLE IV                                
__________________________________________________________________________
CFE Foreplane Connectors                                                  
          Pin  $-Side                                                     
                     #-Side          Pin  $-Side #-Side                   
Connector Number                                                          
               Signal                                                     
                     Signal    Connector                                  
                                     Number                               
                                          Signal Signal                   
__________________________________________________________________________
(80.sub.a1)                                                               
    1     00   RAMPAR                                                     
                     RAMA4     2     38   CLR/   GND                      
    1     01   RAMA8 RAMA1     2     39   SEMREQ/                         
                                                 GND                      
    1     02   RAMA2 RAMB4     2     40   INRAMD8                         
                                                 INRAMD2                  
    1     03   RAMB8 RAMB1     2     41   INRAMD4                         
                                                 INRAMD1                  
    1     04   RAMB2 GND       2     42   INRAMPR                         
                                                 TEST14                   
    1     05   RAMC8 RAMC4     2     43   TEST13 TEST12                   
    1     06   RAMC2 RAMC1     2     44   TEST11 TEST10                   
    1     07   RAMD8 RAMD4     2     45   TEST9  GND                      
    1     08   RAMD2 RAMD1     2     46   TEST6  TEST8                    
    1     09   INRAMA8                                                    
                     INRAMB8   2     47   OFFLN/ TEST5                    
    1     10   INRAMA4                                                    
                     INRAMB4   2     48   BUFFEND/                        
                                                 AF/                      
    1     11   INRAMA2                                                    
                     INRAMB2   2     49   CLKEN/ GND                      
    1     12   INRAMA1                                                    
                     INRAMB1                                              
                           (80.sub.c1)                                    
                               3     50   OPDEC8 OPDEC4                   
    1     13   INRAMC8                                                    
                     INRAMC2   3     51   OPDEC2 OPDEC1                   
    1     14   INRAMC4                                                    
                     INRAMC1   3     52   TERMF/ SEL6/                    
    1     15   RCV/  SIMRCV/   3     53   SEL5/  SEL4/                    
    1     16   DBUSA8                                                     
                     DBUSA4    3     54   BR6    GND                      
    1     17   DBUSA2                                                     
                     DBUSA1    3     55   #L4    #L3                      
    1     18   DBUSB8                                                     
                     DBUSB4    3     56   #L2    #L1                      
    1     19   DBUSB2                                                     
                     DBUSB1    3     57   #K4    #K3                      
    1     20   DBUSC8                                                     
                     GND       3     58   #K2    #K1                      
    1     21   DBUSC2                                                     
                     DBUSC4    3     59   #J4    #J3                      
    1     22   DBUSD8                                                     
                     DBUSC1    3     60   #J2    #J1                      
    1     23   DBUSD2                                                     
                     DBUSD4    3     61   #I4    #I3                      
    1     24   DBUSPR                                                     
                     DBUSD1    3     62   #I2    #I1                      
(80.sub.b1)                                                               
    2     25   GND   WE/       3     63   #H4    #H3                      
    2     26   CS/   RAMAD9    3     64   #H2    #H1                      
    2     27   RAMAD8                                                     
                     RAMAD7    3     65   #G4    STOPB/                   
    2     28   RAMAD6                                                     
                     RAMAD5    3     66   #G3    #CONST6                  
    2     29   RAMAD4                                                     
                     GND       3     67   #CONST7                         
                                                 #CONST4                  
    2     30   RAMAD2                                                     
                     RAMAD3    3     68   #CONST5                         
                                                 #CONST2                  
    2     31   RAMAD0                                                     
                     RAMAD1    3     69   #CONST3                         
                                                 #CONST0                  
    2     32   PERF/ GPRIF/    3     70   #CONST1                         
                                                 GND                      
    2     33   DSEL8/                                                     
                     IOSF      3     71   #BRANCH2                        
                                                 #BRANCH3                 
    2     34   DSEL2/                                                     
                     DSEL4/    3     72   SEL3/  #BRANCH1                 
    2     35   SW1/  DSEL1/    3     73   GND    SEL2/                    
    2     36   CONECT/                                                    
                     GND       3     74   A0     GND                      
    2     37   LOCAL/                                                     
                     GND                                                  
__________________________________________________________________________
 
    
     PERIPHERAL CONNECTION TO PDB 
     Connector 80 d  is called the PDB Peripheral Interface Connector and connects the peripheral dependent board to the MEC 50 M  which interfaces the magnetic tape transport units 53. Table V shows the signals and connections for the PDB peripheral connector. 
     
                       TABLE V                                                     
______________________________________                                    
PDB-Peripheral Connector Pin Mapping                                      
Peripheral                                                                
Connector  25 Conductor     Signal                                        
On PDB     Coax Cable       Name                                          
______________________________________                                    
S75        SD               INFO15.1                                      
S76        #D               INFO16.1                                      
S77        SE               INFO13.1                                      
S78        #E               INFO14.1                                      
S79        SG               INFO11.1                                      
S80        #G               INFO12.1                                      
S81        SH               SEND...1                                      
S82        #H               SELECT.1                                      
S83        SJ               INFO09.1                                      
S84        #J               INFO10.1                                      
S85        SK               PARITY.1                                      
S86        #K               MECLCL.1                                      
S87        SM               INFO07.1                                      
S88        #M               INFO08.1                                      
S89        SN               INFO02.1                                      
S90        #N               RDYST..1                                      
S91        SQ               INFO05.1                                      
S92        #Q               INFO06.1                                      
S93        SR               BUSY...1                                      
S94        #R               READY..1                                      
S95        ST               INFO03.1                                      
S96        #T               INFO04.1                                      
S97        SU               CLOCK..1                                      
S98        #U               SPARE..1                                      
S99        SW               INFO01.1                                      
______________________________________                                    
 NOTE                                                                     
 #75 thru #99 are grounded.                                               
 
    
     THE PROM CONTROLLER 
     The PROM controller 13 shown in FIG. 4A is made up of 13 PROM chips located on the CFE card 10 c . These chips are combined to make up a 52-bit micro-code instruction word. The address lines of the 13 PROM chips are bussed together so that all of the individual address lines are common to every single chip. The chip select on each PROM chip is always enabled (grounded). 
     The data output of the 13-chip PROM matrix forms the 52-bit word. This word is read out of the address that is present on the common input address line A0-A9. The PROMs herein are basically unclocked devices so that means is required to synchronize their outputs with an eight megahertz clock which feeds through gate 24. This is done by making use of register chips 14. The register chips contain eight flip-flops each. Thus, seven chips are used to synchronize and latch the 52-bit micro-code word. This latched micro-code instruction word is used to control the operation of the entire data link processor. Every eight-megahertz clock pulse latches the next subsequent word into the register chip 14. 
     Different types of data link processors require their own unique micro-code. Thus, all Common Front End cards will contain identical hardware except for the 13 PROM chips. Although the PROM word physically contains 52-bits, only 49 bits are used by the micro-code program. The remaining three bits are not parity checked. Tables VIa and VIb indicate the 49-bit micro-code word, by bit position and name. All PROM output signal names are preceded by a &#34;pound sign&#34; (#) so that they are easily recognizable. Bit 32 of the micro-code word is the odd parity bit. The CFE card is made to continually check for odd parity and it halts the micro-code program if a parity error (even parity) occurs on any 49-bit PROM micro-code word. 
     
                       TABLE VIa                                                   
______________________________________                                    
MT-DLP                                                                    
Common PROM                                                               
Output Signals                                                            
PROM      PROM                                                            
CFE       Word            PDB                                             
Name      Bit No.         Name                                            
______________________________________                                    
#Z4       48              #NEXT9                                          
#A1       47              #NEXT7                                          
#A2       46              #NEXT6                                          
#A3       45              #NEXT5                                          
#A4       44              #NEXT4                                          
#B1       43              #NEXT3                                          
#B2       42              #NEXT2                                          
#B3       41              #NEXT1                                          
#B4       40              #NEXT0                                          
#C1       39              #NEXT8                                          
#C2       38              #BRANCH1                                        
#C3       37              #BRANCH2                                        
#C4       36              #BRANCH3                                        
#D1       35              #BRANCH4                                        
#D2       34              #BRANCH5                                        
#D3       33              #LCPSTL/                                        
#D4       32              #PARITY                                         
#E1       31              #CONST7                                         
#E2       30              #CONST6                                         
#E3       29              #CONST5                                         
#E4       28              #CONST4                                         
#F1       27              #CONST3                                         
#F2       26              #CONST2                                         
#F3       25              #CONST1                                         
#F4       24              #CONST0                                         
#G1       23              #LDINT/                                         
#G2       22              #LDSTK/                                         
______________________________________                                    
 
    
     
                       TABLE VIb                                                   
______________________________________                                    
MT-DLP                                                                    
PDB PROM                                                                  
Output Signals                                                            
PROM      PROM                                                            
CEF       Word            PDB                                             
Name      Bit No.         Name                                            
______________________________________                                    
#G3       21              #PADLD/                                         
#G4       20              #PADCTU                                         
#H1       19              #RAMWE                                          
#H2       18              #PSHDN                                          
#H3       17              #SADCTU/                                        
#H4       16              #SREGLD/                                        
#I1       15              #SHFTOP                                         
#I2       14              #FSL4                                           
#I3       13              #FSL2                                           
#I4       12              #FSL1                                           
#J1       11              #OPDEC                                          
#J2       10              #DATAVK                                         
#J3       09              #PINTLD/                                        
#J4       08              #IDSEL/                                         
#K1       07              #ADDSLA                                         
#K2       06              #ADDSLB                                         
#K3       05              #MUXSLA/                                        
#K4       04              #MUXSLB/                                        
#L1       03              #FLGCLR/                                        
#L2       02              #TREGLD                                         
#L3       01              #LPWPRE                                         
#L4       00              #BRANCH6                                        
______________________________________                                    
 
    
     MAINTENANCE CONTROL 
     As seen in FIG. 4A, the Common Front End contains receivers 15, 16, 17 which include decoding logic. Receiver 17 is used for operation of the maintenance control lines 33, 34. Table VII shows the addressing signals for maintenance mode of the data link processor. Thus, this Table lists all possible Maintenance Card to Common Front End addressing codes with the response of any given data link processor. The Maintenance Card (MC) has the ability to address any one of eight DLPs in the base module. 
     
                                           TABLE VII                               
__________________________________________________________________________
ADDRESS CODES: DLP Maintenance Mode Addressing                            
(Maintenance Card to CFE)                                                 
Base    Address Address                                                   
Local   Valid   Equal   Action                                            
(BASLCL/0)                                                                
        (ADRVLD/0)                                                        
                (LOCnn/.0)                                                
                        Produced                                          
__________________________________________________________________________
1       1       X       Normal on-line mode                               
1       0       1                                                         
1       0       0       Standard local mode, all                          
                        maintenance available                             
0       0       0       Local mode with base in                           
                        local                                             
0       0       1       Base in local and the DLP                         
                        clocks are disabled                               
0       1       X       Base single-pulse                                 
__________________________________________________________________________
 1 = high.                                                                
 0 = low.                                                                 
 X = don&#39;t care.                                                          
 
    
     The high order address line from the Maintenance Card (LOC 16/.0) must be &#34;high&#34; in order to address a DLP. It will be seen that all backplane signals are low active. The other four address lines provide encoding for DLP selection. The CFE does not decode the maintenance address until the Maintenance Card indicates the address is valid and stabilized by driving ADRVLD/0 low. 
     The Maintenance Card drives four lines which are used to enable unique maintenance features in any given DLP or connection module. 
     When in local mode, the CFE uses one of these lines (SWH.1/.0) for allowing the Maintenance Card to drive the address lines of the CFE PROM 13. When this line (SWH.1/.0) is low, the peripheral dependent board drives RCV/high and drives SIMRCV/low, as well as not driving the 17-line data bus 32. 
     Table VIII shows a list of the Maintenance Card lines which drive the PROM address lines in this local mode. This feature of the maintenance operation is used to verify PROM controller 13 integrity. 
     
                       TABLE VIII                                                  
______________________________________                                    
Maintenance Lines Driving PROM Address                                    
Data Simulation                                                           
PROM                                                                      
Address             Maintenance                                           
Line                Line                                                  
______________________________________                                    
A9                  DSIMC8                                                
A8                  DSIMC4                                                
A7                  DSIMA8                                                
A6                  DSIMA4                                                
A5                  DSIMA2                                                
A4                  DSIMA1                                                
A3                  DSIMB8                                                
A2                  DSIMB4                                                
A1                  DSIMB2                                                
A0                  DSIMB1                                                
______________________________________                                    
 
    
     RAM BUFFER 
     The Common Front End 10 c  contains a random access memory buffer (RAM) 22. This RAM buffer (data RAM) consists of 1,024 17-bit words. All inputs and outputs to this RAM are received by or driven by the peripheral dependent board PDB 80. The designation for the open collector line (60 nanosecond read access RAM) is RWON. This storage area is used to store the data, the OP code, the descriptor links, the descriptor links longitudinal parity word (LPW) and the various flags that are required to properly control the operation of the data link processor. 
     DLP ADDRESS AND REQUEST JUMPERS 
     There are eight backplane lines which are used by the Distribution Card(s) to address any one given data link processor (DLP). Likewise, eight backplane lines are used by the data link processors (DLPs) to indicate a service request to the Distribution Cards. The 16 lines are unique in that only one data link processor (DLP) can use a given request line. Further, the request lines are graded by priority. Once the priority of a data link processor is determined, that priority request line is &#34;jumpered&#34; for use on the Common Front End Card (FIG. 3). The request and the address lines are identically numbered and operate as pairs. Therefore, once the request priority level is determined and jumpered, the corresponding address line is jumpered on the Common Front End Card. 
     DLP LOCAL ADDRESS JUMPERS 
     The Common Front End Card requires a minimum of two and a maximum of three jumpers (FIG. 3) to implement its local maintenance address. This address is used by a Maintenance Card, as 20 om , to address a data link processor, as 20 oo . The data link processor&#39;s local address must always correspond to its on-line DLP address. 
     STACK REGISTER 
     The stack register 11 consists of three binary counter chips. This register contains the value of the current PROM address, or the value of the address to be used when returning from a subroutine by way of a stack-branch operation. 
     The Magnetic Tape Data Link Processor will be seen to be made of two slide-in printed circuit cards. These are the Commond Front End (CFE) card and the Peripheral Dependent Board (PDB). Each of these two boards are totally different in function and structure; however, when they are used together they make up the totality of the Magnetic Tape DLP. 
     The primary function of the CFE 10 is to provide a device in which to store and execute the applicable micro-code. The micro-code is used to sequence the operation of the data link processor (DLP). The Random Access Memory (RAM 22 ) is housed on the Common Front End Card and provides internal data link processor storage for various uses such as data storage, translation storage, etc. 
     The peripheral dependent board (PDB), on the other hand, contains the necessary logic to interface the peripheral device (magnetic tape) to the host system through the message level interface (MLI). (The message level interface 15 i  will also be seen in U.S. Pat. No. 4,162,520 in FIGS. 1A, 2 and 3). The logical elements of the PDB are controlled by using the micro-code coming from the Common Front End (CFE). However, also, many signals are passed from the peripheral dependent board to the Common Front End to allow logical decision-making by the micro-code. 
     As seen in FIG. 4A-2 there are interfaces to the front plane (double arrow) and to the backplane (single arrow) which involve the Maintenance Card. These MC lines will be seen as lines 30, 34 and 40. 
     MAINTENANCE CARD CONNECTION 
     There is a Maintenance Card-Common Front End interface (MC/CFE) which is used when the Maintenance Card (MC) requires connection to a data link processor in order to perform normal maintenance functions. The Maintenance Card tries to connect to a DLP by driving the appropriate local maintenance address line (LOCnn/.0) and the signal ADRVL/0 (address valid) low. This action causes the addressed CFE 10 c  to recognize the MC connection and to drive the logic term LOCAL/.1 low. The LOCAL/.1 term is used to enable a backplane receiver chip which allows the MC to take the DLP off-line (OFFLNE/0 signal coming from the MC goes low). 
     If the MC takes a DLP off-line, it is unavailable to the host system. If the DLP is off-line it does not see any DC (Distribution Control Card) attempts at connection, and all DLP requests to the DC are inhibited. 
     The Maintenance Card always takes the data link processor off-line when maintenance routines are called for. This is done so that maintenance functions (such as single-pulse, and PROM verification) do not interfere with normal distribution control card operations to the on-line data link processors. The logic term LOCAL/.1, when low, is also used to enable the following functions: 
     1. It allows GPRIF/.0 (from the Maintenance Card) to turn off the data link processors peripheral interface. 
     2. It becomes LOCAL/ which is used by the peripheral dependent board PDB for bus interface directional control logic. 
     3. It is used to allow the Maintenance Card (MC) local clear and PROM verification (SW1/) function. 
     4. It is used by the Common Front End Card (CFE) for the development of the clock-enable term CLKEN. 
     DISTRIBUTION CARD CONNECTION 
     The following discussion involves the connection mechanism as used by the Distribution Control Card/Common Front End Card interface (DC/CFE) when the Distribution Control Card requires connection to a data link processor. This connection will be initiated by a Distribution Control Card (DC) poll test or by a data link processor (DLP) initiated poll request. The connection logic is shown in FIG. 4D. 
     The CONST lines 4-7 are fed as input to a 4-bit binary counter J3-C. The output of this counter is fed to a tri-state inverter C4-C to provide, as output, the &#34;status&#34; lines LCPSTU--. The data link processor address LCPADn/0 and the off-line signal OFFLN provide inputs to buffer chip M5-C whose output is fed through inverter P4-C to form the LCPAD signal to NAND gate M3-C. NAND gate M3-C also has signal inputs for parity error (PERF/) and off-line (OFFLN/). The output of NAND M3-C is the signal CONECT/ which connects to the PDB bus directional control PROM and also provides inputs to inverters C4-C, C4-C1, E4-C and also to the buffer F4-C. The CONECT/ also is formed as input to NAND B3-C having an output which is fed to NOR gate A4-C. The NOR gate A4-C is fed to inverter B4-C to form the logic control signal CLRD. The outputs of the inverter C4-C1 form the signal IOSND/.0 (input-output send) and the signal LCPCON/0 (data link processor connected). The buffer F4-C provides a distribution card strobe output and a distribution card terminate signal. The inverter E4-C provides a strobe level signal LCPSTL/0 (data link processor strobe level). 
     The Distribution Control Card (DC) will attempt to connect to a data link processor (DLP) by driving the appropriate DLP address line (LCPADn/0) low. This backplane signal is applied to a buffer chip (M5-C) which is enabled when the data link processor (DLP) is on-line. The output of the buffer chip is then fed through an inverter (P4-C) and applied to a 3-legged NAND gate (M3-C) to become the term-CONECT/. This CONECT/ term is used to enable the following set of signals on the DC/CFE interface: 
     1. LCPCON/0. This line goes low when the DLP is connected. 
     2. LCPSTUn/0. These are the four data link processors status lines. 
     3. IOSND/.0. This is the state of the I/O-send flip-flop. 
     4. LCPSTL/0. This is the data link processor strobe signal. 
     5. STIOL/0. This is the host system strobe signal. 
     6. TERM/..0. This is the host system terminate signal. 
     7. SELCLR/0. This is the Distribution Card (DC) to the data link processor (DLP) selective clear signal. 
     8. DATAxn/0. This is the 17-bit data bus. 
     All these backplane signals are low active and are enabled only when (a) the CFE is properly addressed by the Distribution Control Card (DC) and (b) a connection is possible, that is, when the data link processor is on-line, and there is no PROM parity error. The actual state of the enabled lines is dependent upon the logical condition of the data link processor and the Distribution Control Card (DC) at the time of the connection. 
     DATA LINK PROCESSOR REQUESTS 
     A DLP request is the method by which a data link processor can notify the Distribution Control Card that it needs host system attention. A request can be considered to be a DLP interrupt (REQ) to the Distribution Card (DC). 
     A DLP request is made to the Distribution Control Card when the DLP drives its jumpered request line (LCPRQn/0) low, or when the DLP drives its jumpered request line and EMRREQ/0 (emergency request) low. Both types of these requests can be generated on the Common Front End Card (CFE) by the PROM stored micro-code program. When the micro-code program drives the PROM output line #LDINT/ (load interface) low, the signals #CONST1 (for emergency request), and #CONST2 (for request) are loaded into a register called the request (FIG. 4A) latch 19 (FIG. 4A) at CLK8/time. Combinational type logic on the Common Front End allows &#34;emergency request&#34; to be enabled on the backplane anytime it occurs, if the data link processor is &#34;on-line&#34;. If a DLP issues an emergency request, it also issues a non-emergency request. This is done so that the Distribution Control Card can determine which DLP is doing the emergency-requesting. 
     The backplane line EMRREQ/0 is common to all data link processors in the base module, and the LCPRQn/0 lines are each unique. If a data link processor issues a non-emergency request, it is &#34;enabled&#34; on the backplane only if the data link processor is on-line and no other data link processors in the base module are doing any emergency-requesting. 
     DATA LINK PROCESSOR DATA TRANSFER RATE 
     Two possible modes of operation govern the data transfer rate between the data link processor DLP and the main host system 10. These modes are called (a) demand mode and (b) burst mode. 
     Demand mode causes data transfers to occur at a rate of less than four-megahertz. Burst mode permits data word (16-bit) transfers to occur at a four-megahertz rate, that is, 64 megabits per second. 
     The data link processor (DLP) and the Distribution Control Card (DC) send &#34;strobe acknowledge&#34; signals back and forth to each other when they present or accept data. The DLP sends the term LCPSTL/0 (DLP strobe level) to the Distribution Control Card, and the Disribution Control Card sends STIOL/.0 to the data link processor. These strobing signals are exchanged during both the demand mode and the burst mode operations. The data transfer rate is determined by the speed at which these strobe signals are exchanged. FIG. 4E show a data transfer timing diagram from the Distribution Control Card (DC) to the data link processor. The Common Front End Card (CFE) receives a STIOL/.0 signal from the Distribution Control Card, and synchronizes it with the 8-megahertz clock by the use of a Schottky J-K flip-flop whose outputs are named AF (asynchronous flop) and AF/. The synchronized strobe level AF is used on the Common Front End Card for PROM address bit AO micro-code testing. The AF/ signal is available for use on the peripheral dependent board (PDB) by way of a foreplane connector pin. 
     DEMAND MODE 
     During buffer loading operations, the Distribution Card (DC) informs the data link processor that data is available by driving STIOL/.0 low. The data link processor micro-code program informs the Distribution Card that it has accepted the data that is on the Distribution Card interface line by driving the PROM logic term # LCPSTL/ low. 
     #LCPSTL/ becomes LCPSTL/0 to the Distribution Card and causes the Distribution Card (DC) to drive the STIOL/.0 high until the next new word of data is available. 
     During buffer read operations, the Distribution Card informs the data link processor that it is ready to accept new data by driving STIOL/.0 low. The data link processor micro-code instructions inform the Distribution Card that new data is available on the Distribution Card interface line by driving the PROM logic term #LCPSTL/ low. #LCPSTL/ becomes LCPSTL/0 to the Distribution Control Card. The Distribution Card informs the data link processor that it has accepted the data by driving the STIOL/.0 high. 
     The direction of the data flow is controlled by the PROM output term #CONST3. The term #CONST3 is clocked into the request latch 19 at PROM signal #LDINT/ (Load system interface) time and becomes IOSF (I/O send flop). The state of IOSF is sent to the Distribution Control Card as IOSND/.0, and is used to inform the Distribution Control Card of the interface bus line direction. The state of IOSF is also sent to the peripheral dependent board (PDB) 80. The PDB 80 uses IOSF to assist in developing the interface bus line directional control logic. This logic determines which data link processor bus is active and what drivers or receivers must be used. 
     BURST MODE 
     A data link processor (DLP) selects the burst mode of operation by driving both of the PROM logic terms LDINT/ (load system interface) and CONST 0 low. This action causes a STOPB flip-flop to set. The setting of the STOPB flip-flop causes the logic term STOPB to go low. When STOPB goes low, and the data link processor is connected, the Distribution Control (DC) interface signal LCPSTL/0 is held low until the STOPB flip-flop is reset. (STOPB=stop burst; when &#34;high&#34;, indicates DLP not in burst mode). 
     During a buffer load operation, the action of LCPSTL/0 remaining low causes the Distribution Card to see an immediate response to its own strobe (STIOL/.0), which it sent when it presented new data on the interface data lines. A data link processor running in the burst mode can accept data at this rate because of its micro-code programming and its eight-megahertz clock rate. 
     During buffer &#34;read&#34; operations, the action of LCPSTL/0 remaining low causes the Distribution Card to see that new data is being presented on the interface data lines immediately. The Distribution Card responds by driving STIOL/.0 high after it has taken in the data. 
     The burst mode of operation is ended if the peripheral dependent board 80 drives the logic term BUFFEND/ &#34;low&#34; in conjunction with a Distribution Card strobe. This action causes the STOPB flip-flop to reset. The resetting of the STOPB flip-flop causes LCPSTL/0 to go high. The peripheral dependent board drives BUFFEND/ &#34;low&#34; when the designated end location of the RAM buffer 22 is reached. The peripheral dependent board generates BUFFEND/ for buffer load and buffer read operations. The STOPB flip-flop can also be reset if the data link processor receives a &#34;terminate&#34; or &#34;clear&#34; from the Distribution Control Card (DC) or the Maintenance Card. 
     CLEAR FUNCTIONS 
     The CFE card 10 c  makes use of combinational type logic to implement its clearing functions. A description of clearing functions applicable to all data link processors using the Common Front End Card follows: 
     Power-Up Clear 
     A power-up clear signal is provided to the base module from either the power supply of the cabinet or from some external source as determined by the host system 10. This signal is attached via coaxial cable to a pin of the maintenance card backplane connector. The Maintenance Card uses this signal to create the master clear, MSTCLR/0. The signal MSTCLR/0 clears the data link processor if it is on-line. 
     Base Clear 
     The base clear function is provided by a pushbutton switch located on the Maintenance Card foreplane connector. The switch is ORed with the power-up clear and performs the same function as a power-up clear. 
     Maintenance Local Clear 
     If the Maintenance Card is connected to a data link processor, the Maintenance Card can clear the data link processor by driving LCLCLR/0 low (clear level from Maintenance Card). 
     Host System Master Clear 
     The host system 10 can issue a master clear signal across its message level interface 15 i  (MLI). The Distribution Control Card (DC) contains a host system jumper option which, if installed, causes the master clear (MSTCLR/0) to go &#34;low&#34;. This signal clears all the data link processors that are not addressed by the Maintenance Card. If the Distribution Control Card host system option jumper is not installed, then the host master clear signal is not passed through the Distribution Control Card to the backplane. 
     Host System Selective Clear 
     The host system 10 can clear a single data link processor during a standard poll test operation by driving the message level interface (MLI) line TRM+MC/1 low. This action causes the Distribution Card to drive SELCLR/0 low. The SELCLR/0 being &#34;low&#34; causes the connected data link processor to clear. 
     FIG. 3 shows in schematic form the physical structure of the Common Front End (CFE) card 10 c  while FIG. 4A shows the basic circuit block diagram of the Common Front End Card. Included in the CFE is the clear circuitry shown in FIG. 4B. It will be seen that NOR gate 113 has inputs from NAND gates 110 and 111 in addition to an input from inverter 114 which is fed by buffer 112. The output of gate 113 is fed to buffer 115 and also to inverter 116. Inverter 116 feeds its signal to a Schottky data register chip 117 which provides an output to the PROM address multiplexor 12 of FIG. 4A. The output of data register 117 is also used to provide one of the inputs to NAND gate 118. 
     A &#34;low&#34; signal which results on the output of the NOR gate 113 occurs if any one of the following conditions is met: 
     1. MSTCLR/0 and OFFLN are both &#34;low&#34;. 
     2. SELCLR/0 and CONECT/ are both low. 
     3. LCLCLR/0 and LOCAL/ are both low. 
     The &#34;low&#34; output of NOR gate 113 is used to perform the following functions: 
     1. The output is fed through a buffer chip 115 to become signal CLR/. CLR/ clears the PROM parity error flip-flop (and flip-flops SOTB, AF, and TERMF) on the CFE 10 c . 
     2. CLR/ is sent to the peripheral dependent board (PDB) 80 to clear the specific peripheral-dependent logic. 
     3. The low output signal of NOR gate 113 is sent through an inverter 116 and is applied to one input of the data register chip 117. It is sent as the logic term CLRD. The signal CLRD is double synchronized with CLOCK/ and becomes the signal SCLR (synchronized clear). 
     4. The signal SCLR from register 117 is used to force the PROM clock-disabling term CLKST (clock stop) &#34;low&#34;. This is done in the event that a PROM parity error may have caused CLKST to go high. 
     5. The SCLR signal is used to disable the PROM address multiplexor chips 12. This forces the PROM address lines to all zeros. Address zero is the starting address of all data link processor micro-code programs. 
     CFE CLOCK CONTROL 
     The clock control logic in the CFE 10 c  uses combinational type logic (NAND gates, NOR gates, inverters, buffers, and a Schottky data register chip), in order to enable or disable the always-present 8-megahertz backplane clock (CLOCK . . . 0). The circuitry for the CFE clock control is shown in FIG. 4C. The CFE clock control logic constantly monitors the state of the maintenance bus 30 to determine how it should supply clock signals to the data link processor. The Table VII there was shown the available clock control options and the maintenance bus line states that are necessary to cause the various options to be active. Table VIII showed the maintenance lines for driving the PROM address lines A0-A9. 
     In the CFE clock control circuitry shown in FIG. 4C, it will be seen that the NAND gate A3 has three inputs and the NAND gate 13-1 has four inputs. The first input to NAND gate A3 is from the data register C3 along the SCLR line. The second input to gate A3 is the line PERF/. The PERF signal is the PROM parity flip-flop signal. When &#34;high&#34;, it indicates that an error has been detected on the PROM output register 14, thus stopping the DLP clock. The PERF/ signal is the complement of PERF. The third input to gate A3 is from the PROMCLK/ output of NOR H4-1. 
     The first input to gate 13-1 is the SCLR line. The second input to gate 13-1 is the PERF/ line. The third input to gate 13-1 is from the CLKEN line and the fourth input to gate 13-1 is the output of the buffer N5-1. 
     The NAND gate M3 has one input from inverter P4 (ADLOC) and another input coming from the inverter B4-1 (BASLCL/0). 
     The input for the data register C3 comes from NOR gate A3-2 which has one input from inverter B4, one input coming from NOR gate A3-1, and one input coming from the line START/.0. The NOR gate A3-1 has two inputs, one of which is the line BASLCL/0 and the other input is the line LOCAL/.1. 
     The output of gate A3 is fed to the input of NOR gate H4-1. The output of NAND gate 13-1 is fed to the input of NAND gate H4-13. The output of inverter H5 (CLOCK . . . 0) is fed to both the input of gate H4-2 and gate H4-13. 
     The output of gate A3 is fed to the input of NOR gate H4-1 to form the signal line PROMCLK/. The output of NAND gate H4-13 is fed to buffer 14-1 and to buffer G4. NORMAL ON-LINE MODE: When certain of the following conditions are met, then all data link processor clocks are active at the 8-megahertz rate. These conditions are: 
     1. BASLCL/0 is high (the base module is not in local). 
     2. LOCAL/.1 is high (the DLP is not Maintenance Card (MC) addressed). 
     3. PERF/ is high (no PROM parity error). The signals BASLCL/0 and LOCAL/.1, both being &#34;high&#34;, causes a &#34;high&#34; on to one input of the Schottky data register C3. This register chip is clocked by the always-present signal CLOCK/. This signal is derived from the backplane clock signal CLOCK . . . 0 after a triple inversion. One output of the register chip C3 becomes the logic term clock enable (CLKEN). The signal clock enable is NANDed on two gates (A3-4 and 13-1) using the signals PERF/, SW1/, and the output of NAND gate M3. Gate M3 has inputs which are: 
     a. The signal ADLOC/ (after inversion). The ADLOC/ is low when the DLP is not addressed by the MC, or when ADRVLD/0 is high. 
     b. The backplane signal BASLCL/0 (after inversion). 
     The output of NAND gate A3-4 becomes the signal logic term B (P-CLKEN) and this is NANDed with the signal CLOCK . . . 0 (after inversion by inverter H5 which is fed as one input to gate H4-2). The output of gate H4-2 is fed through a NOR gate H4-1 to form the signal PROMCLK/. This signal PROMCLK/ is used to clock the PROM data registers 14 and the stack register 11 of FIG. 4A. 
     The output of NAND gate 13-1 becomes the signal CLKEN/. This signal CLKEN/, when &#34;low&#34;, enables the 8-megahertz backplane clocks on the peripheral dependent board, PDB 80. 
     The CLKEN/ signal is NANDed with the signal CLOCK . . . 0 (after inversion) at the gate H4-13. The output of gate H4-13 is fed through one inverter buffer 14-1 and also through one non-inverting buffer G4 to become the signals respectively CLK8/ and CLK8 (8-megahertz clock. These clock signals are used in the Common Front End Card 10 c . 
     STANDARD LOCAL MODE 
     The standard local mode of &#34;maintenance&#34; is operative under the following conditions: (Also see Table VII). 
     1. BASLCL/0 is high (base is not in local). 
     2. ADRVLD/0 (address valid) is low; and the LOCnn/.0 lines on the maintenance bus are valid. 
     3. The LOCnn/.0 lines equal the local address jumpers on the CFE 10 c . This equal comparison together with the ADRVLD/0 being low, forces LOCAL/.1 into low. LOCAL/.1 low means that the DLP is &#34;Maintenance Card addressed&#34;. 
     In this mode all the maintenance functions are available; for example, the Maintenance Card can now: 
     1. select the single-pulse mode. 
     2. perform PROM micro-code verification. 
     3. set up known conditions by manipulating the PROM address during single-pulse operations, and test for known predicted results by sampling the maintenance bus display (DPLY) and the data simulate (DSIM) lines. 
     SINGLE-PULSE MODE 
     After the Maintenance Card has localized the data link processor (DLP), it can place the DLP in a single-pulse mode by driving the signal SP/ . . . 0 into &#34;low&#34;. This action forces the logic term CLKEN into &#34;low&#34; because the NOR gate A3-2 of FIG. 4C is disabled by the following logic: 
     1. Signal SP/ . . . 0 is inverted into &#34;high&#34; and disables the top input line of gate A3-2. 
     2. The signal LOCAL/.1 is low because the DLP is not Maintenanced Card addressed. This action disables the middle input line of gate A3-2 because of the high signal output of gate A3-1. The signal START/.0 is high and disables the third input of gate A3-2. When gate A3-2 is disabled, a low signal is set into the data register C3 at CLOCK/time. The output signal CLKEN, when low, turns off NAND gates A3-4 and 13-1, and causes their outputs to go high and to disable the NAND gates H4-2 and H4-13. When these two gates are disabled, the DLP clocks are turned off. 
     Once the DLP is placed into a single-pulse mode, the Maintenance Card can issue from 1 to 4,096 clocks by driving the START/.0 low. START/.0 is a pulse which effectively is a window around a desired number of clocks that the Maintenance Card wants issued. During the time frame when START/.0 is low, the high signal output of gate A3-2 is clocked into the data register C3 by the always-present clock signal CLOCK/. The CLKEN now becomes a pulse rather than a level, and the DLP clocks are enabled only during the low active time span of START/.0. 
     After the data link processor (DLP) has been placed into the single-pulse mode, the Maintenance Card can take the DLP out of it by driving SP/ . . . 0 high. The signal SP/ . . . 0, when high, forces a high level input into register C3 as follows (FIG. 4C): The signal SP/ . . . 0 is inverted low by the inverter B4. This low signal is fed into NOR gate A3-2 and causes its output to go high. This high level is clocked into data register C3 with CLOCK/ and becomes CLKEN. The signal CLKEN, when high, enables all the data link processor&#39;s clocks. 
     PROM MICRO-CODE VERIFICATION 
     If the Maintenance Card has localized and placed the data link processor into the single-pulse mode, it can read any Common Front End PROM micro-code word by driving the signal SWH.1/.0 low. The signal SWH.1/.0 is routed through the buffer N5-1 (FIG. 4C) to become the logic term SW1/. This signal SW1/ is sent to the peripheral dependent board PDB 80 and is used in the bus interface directional control logic in order to develop the logic term SIMRCV/, (Simulate Receive, FIG. 4A, line 33). When SIMRCV/ is low, the maintenance bus lines 10 dm  DSIMnn/0 of FIG. 5B are gated into the inputs of PROM address multiplexor chips 12 of FIG. 4A. The SW1/ also forces the PROM address multiplexors 12 to select the DSIMnn/0 data as the actual PROM address. This way the Maintenance Card controls the current PROM address. SW1/ is also tied to one of the inputs of NAND gate 13-1, FIG. 4C, and is used to keep the clock signals CLK8/, CLK8, and the clock-enable signal CLKEN/ (to the peripheral dependent board 80) turned off. 
     When the Maintenance Card issues a single pulse clock, the PROMCLK/ latches the addressed PROM data into the PROM registers 14 (FIG. 4A) because SW1/ is not tied to the NAND gate A3. The Maintenance Card now has the current PROM word available for sampling. The Maintenance Card can now read the PROM data (8-bits at a time) by driving the maintenance bus lines DSELn/.0. The DSELn/.0 lines 34, FIG. 4A, are used on the CFE 10 c  to select which 8 data bits will be enabled onto the maintenance bus lines DPLYnn/0, at 40 of FIG. 4A. Seven reads are required to sample one entire micro-code address. 
     LOCAL MODE WITH BASE IN LOCAL 
     Under certain conditions the maintenance mode is operatively in effect; these conditions are: 
     1. BASLCL/0 is low (the base is in local). 
     2. ADRVLD/0 is low (that is, the LOCnn/.0 lines on the maintenance bus are valid). 
     3. The LOCnn/.0 lines equal the local address jumpers on the CFE 10 c . This equal comparison, along with ADRVLD/0 low, forces LOCAL/.1 low also. LOCAL/.1, when low, means that the data link processor DLP is being Maintenance Card addressed. This mode of operation causes the addressed data link processor to function in exactly the same manner as the standard local mode. All unaddressed DLPs disable their own clocks. 
     BASE MODULE IN LOCAL MODE AND DLP CLOCKS DISABLED 
     This is a mode of operation which takes place when the following conditions are satisfied: 
     1. BASLCL/0 is &#34;low&#34; (the base is in local). 
     2. ADRVLD/0 is low (the LOCnn/.0 lines are valid). 
     3. The LOCnn/.0 lines do not equal the CFE local address jumpers. This &#34;unequaled&#34; comparison along with ADRVLD/0 being low, forces LOCAL/.1 high and ADLOC/ low. The LOCAL/.1 when high, means that the DLP is not Maintenanced Card addressed. 
     In this mode all the DLP clocks are disabled because the BASLCL/0 low level is inverted by the inverter B4-1 (FIG. 4C). This level &#34;high&#34; is NANDed by gate M3 with the inverted ADLOC/ signal (inverted by P4). When both inputs to gate M3 are &#34;high&#34;, the output goes low and disables the NAND gate A3-4 and 13-1. The outputs of these gates both go high and disable the gate H4-1 and H4-13. When these two gates are disabled, all the DLP clocks are inhibited. BASE SINGLE-PULSE: The single-pulse maintenance mode is in effect when the following conditions occur: 
     1. BASLCL/0 is &#34;low&#34; (the base is in local). 
     2. ADRVLD/0 is high (that is, the LOCnn/.0 lines on the maintenance bus are invalid). 
     In this mode of operation the Maintenance Card can place the entire base in a single-pule mode by driving the logic as follows: 
     1. SP/ . . . 0 low--this action disables the top input of the NOR gate A3-2. 
     2. BASLCL/0 is low. This action puts the base in local and causes the output of NOR gate A3-1 to go high. This high level disables the middle input line of NOR gate A3-2. 
     3. START/.0 high disables the bottom input of NOR gate A3-2. 
     When NOR gate A3-2 (FIG. 4C) is disabled, its output goes low. This low output is latched into the data register C3 and becomes the logic term CLKEN. This clock enable line, when low, disables NAND gates A3-4 and 13-1. The outputs of these NAND gates go high and disable the gates H4-1 and H4-13. When the gates H4-1 and H4-13 are disabled, then all DLP clocks are inhibited. 
     The &#34;base single-pulse&#34; maintenance mode of operation will make all data link processors respond to single-pulse because ADRVLD/0 is high and BASLCL/0 is low. The signal ADRVLD/0 when high, disables the tri-state 8-2-1 multiplexor chip (12, FIG. 4A) on the CFE that is used to generate the logic terms LOCAL/.1 and ADLOC/. When this multiplexor chip is disabled, both signals are pulled up high with 1100 ohm resistors to +5 volts. ADLOC/, when high, (after inversion to &#34;low &#34; by the inverter P4, FIG. 4) disables the NAND gate M3. The high level output of gate M3 allows the single-pulse clock to be enabled when the Maintenance Card drives the signal START/.0 low. Further description of the single-pulse operation has been previously described in the &#34;standard local mode&#34; of operation. 
     STACK REGISTER OPERATION 
     The operation of the stack register 11 (FIG. 4A) is controlled by the state of a PROM 13 output signal line #LDSTCK/. The signal #LDSTCK/ is held low during non-subroutine micro-code instructions. This low level is applied to the low-enable inputs of the 3 stack register chips 11. This action causes the stack register 11 to load and hold the current PROM micro-code address. 
     All micro-code programs have the ability to enter one level of subroutine by driving and holding the signal #LDSTCK/ high. With #LDSTCK/ going high the following actions will occur: 
     1. The stack register load function is disabled. 
     2. The stack register count function is enabled. 
     The signal #LDSTCK/ of FIG. 4A is also routed through a Schottky data register chip to become the logic term STCLKEN/. This signal is then inverted and used to count up the stack register plus one. This address is held in the register until the subroutine is completed. This updated address is used when the subroutine does a stack branch to return to the main body of the micro-code instructions. The non-subroutine code is then re-entered and the signal #LDSTCK/ goes low. 
     MAINTENANCE DISPLAY LINES OF COMMON FRONT END (CFE) 
     The CFE 10 c  contains standard items such as inverting tri-state buffers and eight-to-one multiplexor chips that are used to enable the maintenance display lines 40 of FIG. 4A (DPLY01/0-DPLY10/0). The lines 40 are shown with a /10 to indicate the 10 individual lines which compose this group of lines. Table IX lists the various individual display lines (DPLY01/0-DPLY10/0). 
     
                                           TABLE IX                                
__________________________________________________________________________
CFE Signals Displayed With Given                                          
Display Select Codes                                                      
Maintenance                                                               
       DSEL8/,DSEL4/,DSEL2/,DSEL1/Values*                                 
Display                                                                   
       (GRPO/=0)                                                          
Lines***                                                                  
       1111** 0111 0110   0101                                            
                             0100   0011  0010  0001                      
                                                   0000                   
__________________________________________________________________________
DPLY01/0                                                                  
       STCKA9 *    *      *  *      *     *     *  *                      
DPLY02/0                                                                  
       STCKA8 *    *      *  *      *     *     *  *                      
DPLY03/0                                                                  
       STCKA7 AF   #LDINT/                                                
                          #K1                                             
                             #NEXT8 PER   #CONST7                         
                                                #I1                       
                                                   #NEXT7                 
DPLY04/0                                                                  
       STCKA6 TERMF                                                       
                   #LDSTCK/                                               
                          #K2                                             
                             #BRANCH1                                     
                                    PERF  #CONST6                         
                                                #I2                       
                                                   #NEXT6                 
DPLY05/0                                                                  
       STCKA5 REQ  #G3    #K3                                             
                             #BRANCH2                                     
                                    SCLR  #CONST5                         
                                                #I3                       
                                                   #NEXT5                 
DPLY06/0                                                                  
       STCKA4 EMREQ                                                       
                   #G4    #K4                                             
                             #BRANCH3                                     
                                    CLKEN #CONST4                         
                                                #I4                       
                                                   #NEXT4                 
DPLY07/0                                                                  
       STCKA3 A3   #H1    #L1                                             
                             #BRANCH4                                     
                                    NEXT9 #CONST3                         
                                                #J1                       
                                                   #NEXT3                 
DPLY08/0                                                                  
       STCKA2 A2   #H2    #L2                                             
                             #BRANCH5                                     
                                    LCPADF                                
                                          #CONST2                         
                                                #J2                       
                                                   #NEXT2                 
DPLY09/0                                                                  
       STCKA1 A1   #H3    #L3                                             
                             #LCPSTL/                                     
                                    STOPB #CONST1                         
                                                #J3                       
                                                   #NEXT1                 
DPLY10/0                                                                  
       STCKA0 A0   #H4    #L4                                             
                             #PARITY                                      
                                    CLKST #CONST0                         
                                                #J4                       
                                                   #NEXT0                 
__________________________________________________________________________
 NOTES:-                                                                  
 1 = HIGH                                                                 
 0 = LOW                                                                  
 *Maintenance display line not driven by CFE with this display select code
 **With this display select code, the CFE also drives DSTAT8/0DSTAT1/0,   
 DLCPST/0, and DIOSND/0 (MC Group 24).                                    
 ***Signals are low true on backplane.                                    
 
    
     The Common Front End also has the ability to display signals on the lines DSTAT8/0-DSTAT1/0 (Glossary Table I), which are the data link processor &#34;status lines&#34; to the Maintenance Card. The CFE also can display DLCPST/0 (the data link processor strobe to the Maintenance Card) and also the DIOSND/0 (which is the SEND level of I/O to the Maintenance Card). All of these lines are used by the Maintenance Card (as 20 om ) to test and verify the operation of the CFE card. The DSEL8/0 lines (multiplexor chip select line for input to the display lines)--DSEL1/0 (multiplexor address line for data selection to display lines) are sent to the Common Front End card from the Maintenance Card and are used to select the CFE internal signals that are enabled on to the display lines after the data link processor has been addressed by the Maintenance Card. The Table IX shows a list of the displayable CFE signals and also the DSELn/0 line codes that are required to enable them on to the maintenance display bus. 
     PROM ADDRESS SELECTION 
     Referring to Tables Xa and Xb, the selection of micro-code addresses is implemented on the Common Front End card 10 c  which uses circuitry that employs standard logic gates, multiplexor chips, register chips and a decoder chip. The Multiplexors 12 drive PROM address lines A0-A9 (FIG. 4A). This causes the micro-code data to be latched into the registers 14 with every 8-megahertz clock pulse. 
     
                       TABLE Xa                                                    
______________________________________                                    
CFE PROM Address Selection                                                
PROM Output PROM Input                                                    
Branch Lines                                                              
            Address Lines                                                 
(#BRANCHn)  (An)                  See                                     
BR6    54321    9-4       3-1     0       Note                            
______________________________________                                    
0      00000    #NEXTn    #NEXTn  #NEXT0  1                               
0      00001    #NEXTn    #NEXTn  AF      2                               
0      00010    #NEXTn    #NEXTn  TERMF   2                               
0      00011    #NEXTn    #NEXTn  LCPADF  2                               
0      00100    #NEXTn    #NEXTn  STOPF   2                               
0      00101    #NEXTn    #NEXTn  TEST5   2                               
0      00110    #NEXTn    #NEXTn  TEST6   2                               
0      00111    #NEXTn    OPDECn  OPDEC1  3                               
0      01000    #NEXTn    #NEXTn  TEST8   2                               
0      01001    #NEXTn    #NEXTn  TEST9   2                               
0      01010    #NEXTn    #NEXTn  TEST10  2                               
0      01011    #NEXTn    #NEXTn  TEST11  2                               
0      01100    #NEXTn    #NEXTn  TEST12  2                               
0      01101    #NEXTn    #NEXTn  TEST12  2                               
0      01110    #NEXTn    #NEXTn  TEST14  2                               
0      01111    STCKAn    STCKAn  STCKA0  4                               
X      1XXXX    #NEXTn    #NEXTn  PDB MX  5                               
1      XXXXX    #NEXTn    #NEXTn  PDB MX  5                               
SW1/ = 0    Maintenance Data Bus                                          
                                      6                                   
CLEAR       0         0         0       7                                 
______________________________________                                    
 NOTES-                                                                   
 1 = Unconditional branch.                                                
 2 = 2way branch (TEST5-TEST14 from PDB).                                 
 3 = 16way branch (OPDEC from PDB).                                       
 4 = Stack branch.                                                        
 5 = 2way branch (A0 is driven directly from the PDB).                    
 6 = Maintenance Card PROM read mode.                                     
 7 = Clear forces zeroes onto the PROM address lines.                     
 1 = high.                                                                
 0 = low.                                                                 
 X = don&#39;t care.                                                          
 
    
     
                       TABLE Xb                                                    
______________________________________                                    
Enable Lines For Expanded A0 Selection                                    
PROM output        Enable line to                                         
branch lines       Peripheral-Dependent                                   
(#BRANCHn)         board for expanded                                     
BR6    5         4         selection of A0                                
______________________________________                                    
0      0         0                                                        
0      0         1                                                        
0      1         0         SEL2                                           
0      1         1         SEL3                                           
1      0         0         SEL4                                           
1      0         1         SEL5                                           
1      1         0         SEL6                                           
______________________________________                                    
 
    
     With reference to Table Xa showing the CFE signals, the primary multiplexor input-select lines are the current latched PROM outputs of #BRANCH1-#BRANCH5, Table IX and the signal term BR6 (branch line from PDB 80 for PROM address selection). 
     In order to accomplish Address Bit A0 Selection (at line 36, FIG. 4A): #BRANCH1-#BRANCH3 will select the input lines to two separate multiplexor chips. #BRANCH4, #BRANCH5, and BR6 will select which one of the multiplexors to use. BR6 is peripheral-type dependent and may be driven either by the PDB logic or by one of the latched PROM outputs. If a PROM output line is used to drive BR6, (Table Xb), it is sent over to the peripheral dependent board PDB 80 and then is returned as the BR6 line. There are 22 such lines, #G3-#L4, Table IX, which are available to the peripheral dependent board, PDB 80. These lines are primarily used for controlling the peripheral dependent logic on the PDB 80. 
     The #BRANCHn bits and BR6 are also used for the generation of PROM address lines A0-A9 (FIG. 4A). This is accomplished by the use of the term BROP (PROM address branch) and BRST (use stack register for PROM address). BROP is &#34;high&#34; whenever the #BRANCHn bits and the BR6 select a branch to one of 16 addresses (16-way branch). BRST is high when the #BRANCHn bits and BR6 select a branch that uses the stack register 11 as the address. If neither a 16-way or a stack register branch is selected, then either an unconditional or a two-way branch is performed. 
     PROM address selection can also be achieved by the use of the maintenance term SW1/, (FIG. 4C). 
     When the CFE 10 c  is under the control of the Maintenance Card and the MC drives SW1/ &#34;low&#34;, the MC can address and read any micro-code address. This feature is utilized to read and verify the contents of the PROM matrix and to test the integrity of its associated registers. 
     The #BRANCHn bits and the BR6 are overridden whenever a master, selective, or local clear is issued to the CFE 10 c . Any clear condition forces the PROM address but (A0-A9, FIG. 4A) to equal 0. The address bus A0-A9 is defined in Glossary Table I. Address 0 is the starting point of all micro-code programs. 
     As will be seen in Table Xa, there is a listing of all the possible branching conditions that can be used for the generation of the next PROM address. Table Xb shows a list of five CFE generated enable lines that can be used for an expanded selection of PROM address bit A0. These enable lines are activated when the PDB 80 requires additional two-way branching capabilities. 
     PROM PARITY CHECKING 
     The CFE card 10 c  performs an odd parity check on every 49-bit PROM micro-code word. The total word length is actually 52-bits but the three most significant bits are not used by the micro-code program. These three bits are not parity checked. 
     The currently addressed micro-code word is latched into the register chips 14 and from these register chips is fed into cascaded parity checking chips 18 of FIG. 4A. Thus, this circuitry allows the CFE 10 c  to examine the entire 49-bit word at one time. If the sum total of the word equals &#34;even&#34; parity, then the logic term PER (parity error) goes high. PER going &#34;high&#34; causes the parity error flip-flop of 18 to be set on the next 8-megahertz clock pulse. The setting of the parity error flip-flop causes the logic term PERF/ to go &#34;low&#34;. 
     PROM PARITY ERROR 
     If &#34;even&#34; PROM parity is detected by the CFE card, the logic term PER goes &#34;high&#34; and causes the PROM parity error flip-flop to set. At the same time that PER goes high, then PER/ goes low. PER/ going low causes the logic term CLKST (clock stop) to go high. CLKST going &#34;high&#34; disables the clock that controls the latching of the PROM register chips (PROMCLK/). The disabling of PROMCLK/ causes the data (that caused the parity error) to remain in the registers 14. The disabling of PROMCLK/ also locks the stack register 11 to its current value. If the parity error flip-flop gets set, then the logic term PERF/ goes low on line 41 of FIG. 4A. The PERF/ going &#34;low&#34; causes the following actions to occur. 
     1. PERF/ is routed to the peripheral dependent board PDB 80 to turn off the peripheral device. 
     2. The request latch 19 on the CFE card is cleared. This action causes the I/O SEND flip-flop, REQ (request), and EMRREQ (emergency request) signals from Drivers 20 to turn &#34;off&#34;. 
     3. The logic term CONECT/ is forced high. This action prevents any distribution card (DC) connection and severs any DC connection that may have been in progress at the time of the error. 
     4. The peripheral dependent board 8-megahertz clock is turned off. PERF/ going low forces the logic term CLKEN/ high. CLKEN/ is used by the PDB 80 to permit 8-megahertz clocks. 
     5. The CFE 8-megahertz clocks (CLK8 and CLK8/) are turned off. 
     If a PROM parity error disables (locks up) a data link processor, the error condition can be removed through the use of a base power-up clear, a Maintenance Card base clear, a Distribution Card Master Clear or a Maintenance Card initiated DLP local clear. 
     PERIPHERAL DEPENDENT BOARD (PDB) 
     The Peripheral Dependent Board (PDB) 80, is the unit which in combination with the Common Front-End Card (CFE) 10 c  constitutes the peripheralcontroller (data link processor, DLP) for interfacing the magnetic tape units to the main host computer. 
     The magnetic tape data link processor (MT-DLP) forms an I/O subsystem (to the main system) which includes the Master Electronic Control 50 M  (MEC), the several DLP&#39;s 20 00 , 20 01 , etc., and the Message Level Interface 15 i . As previously indicated, the MT-DLP consists of two cards--a Common Front-End Card (CFE) and a Peripheral Dependent Card which is called the Peripheral Dependent Board. 
     Basically, the Common Front-End Card 10 c  provides: 1024 52-bit words of PROM with which to execute micro-code; 1024 17-bit words of RAM for data storage; the maintenance control logic; and a portion of the data link processor interface. 
     The Peripheral Dependent Board 80 provides: the bi-directional data path 101 (FIG. 5B) to the MEC 50 M  and associated control lines; address logic for the RAM 22; RAM access logic; longitudinal parity word (LPW) parity and vertical parity generation and checking logic; the identification jumpers; a 16-bit shift register for the fabrication of DLP generated words; and associated flags and control logic. 
     Table I, previously cited, gives a glossary of terms for the Common Front-End Card signal lines while Table II gives a glossary of terms and signal lines associated with the Peripheral Dependent Board, PDB 80. Together these two glossaries cover the logic terms and signal lines which are used in the data link processor for the magnetic tapehost system configuration. 
     A general functional block diagram of a typical Peripheral Dependent Board is shown in FIG. 5A. The system Write bus is seen at the top left of FIG. 5A connecting to the RAM input data bus 52 and thence to the device data register 82 (also in FIG. 5B). The device data register 82 has a bi-directional connection to the driver receiver 84 which connects to a peripheral, such as the MEC 50 M  (FIG. 1) and its associated tape units 53. 
     The RAM input data bus 52 (which connects to RAM 22 of FIG. 4A) also receives signals from: the longitudinal parity word (LPW) generator 88; from the vertical parity generator checker (part of element 83); and from a software operated code translator function circuit 83 a . A result descriptor generator 13 a , which is generated from the micro-code in PROM 13, FIG. 4A, also connects to the RAM input data bus 52. 
     In FIG. 4A the micro-code or micro-state bus 44 (PDB usage) is continued on FIG. 5A along bus 44 where micro-code inputs are directed: to the OP code decoders 83 a  and 83 b  ; to the RAM address units 90, 91; to the code translator 83 a  ; and to the device control register 105 onto bus 103. 
     As further seen in FIG. 5A, the micro-code words (from PROM 13, FIG. 4A-2) on the micro-state bus 44 are connected: to the result descriptor generator 13 a  to the vertical parity generator checker 83; to the longitudinal parity word generator 88; and to the device data register 82. 
     From FIG. 4A-2, the RAM data output bus 22 a  is again see in FIG. 5A as the RAM output data bus 22 a . This bus connects to the host and maintenance drivers 86 which form a system read bus 10 h  which connects to the Distribution Card (DC) and the message level interface (MLI). The RAM output data bus 22 a  (from RAM 22) also provides an output channel to: the RAM address units 90, 91; to the code translator 83 a  ; to the vertical parity generator checker 83; to the LPW checker 88 and to the device data register 82. 
     The RAM address unit 90, 91 connects to the address bus 53 for memory selection in the random access memory (RAM) storage 22 of FIG. 4A. 
     Referring to FIG. 5B which shows the peripheralcontroller data link processor for magnetic tape peripherals, the data line 10 h  to/from the host system feeds to receiver 16 (part of CFE) which includes a tri-state inverter in the Common Front-End Card (dotted square). Line 10 dm  from the Maintenance Card feeds data to a receiver 17 which includes a tri-state inverter also on the Common Front-End Card 10 c . The information bus 101 from the Master Electronic Control (50 M ) feeds to a tri-state buffer 15. 
     The outputs of these units are connected to a Data Register flip-flop 81 which forms the receive register 81 and which feeds its output to the dual 4-1 multiplexor 83. 
     The jumper signals and the jumper-select signal (JMPSEL) feeds an 8-1 multiplexor 92 p  having an output which is fed to shift register 92. The output of shift register 92 is fed to the imput of the dual 4-1 multiplexor 83. The longitudinal parity word LPW generator 88 constitutes a 512×4 PROM and a Data Register flip-flop having outputs which are fed to the dual 4-1 multiplexor 83. 
     The RAM 22 (located on the Common Front-End Card 10 c ) has one input which constitutes a chip select bus 50 and a data bus 52 in addition to a RAM address (RAMAD) bus 53 and a WE/ input line. The RAM 22 has 1024 locations of 17-bits each and its output is fed to the dual 4-1 multiplexor 83. 
     The output of dual multiplexor 83 is bussed: to the system register (Data Register flip-flop) 85; to a Device Register flip-flop 82; to the OP decoders 83 a , 83 b  (which constitute a 512×4 PROM), and also to dual 8-1 multiplexor 83 c , which provides the A0 signal (low order address bit on line 36 of CFE in FIG. 4A). 
     The output of the system register 85 is fed to tri-state inverters 86 and 87 which have output buses 10 h  (host system) and 10 dm  (maintenance card). The output of flip-flop 82 is fed to driver TQN 84 while the output of PROM 83 a  is fed to a quad 2-1 multiplexor 83 b  to provide the OP decoder bus signal OPDECn. 
     The queue address (QAD) is developed from a comparator 89 while the system address and the peripheral address are developed from 4-bit up/down binary counters 90 and 91, respectively. The queue address data from 89 is fed to dual 4-1 multiplexor 89 s , the output of the system address counter 90 is fed to PROM 90 s  which is a 512×4 capacity PROM; another output line of the system address counter 90 is fed to comparator 90 c . The peripheral address data from counter 91 is fed: to PROM 91 p  (512×4 PROM); to comparator 90 c  ; to multiplexor 89 s  ; and to the shift register 92. The outputs of 90 s , 90 c  and 91 p  connect to the Common Front End Card 10 c . 
     The CONST (8 lines from micro-code to load MEC registers and control functions of shift register 89 s ) signals are fed into a 4-bit binary counter 105 which provides one output as an interface control bus and another output to driver TQN 105 a , having its output line 103 to the peripheral device involved. A bus 102 connects from the peripheral device to the tri-state buffer 106 a . Likewise, bus 104 (PRIFnn/0) comes from the peripheral to the input of the tri-state inverter 106 b . The outputs of 106 a  and 106 b  are fed to a Data Register flip-flop 106 c  which outputs the interface status lines to the peripheral. 
     In FIG. 5B it will be seen that the LPW generator 88 has other inputs, such as the LPW clock, the LPW preset, the LPW word load. These signals are defined in Table IB. The receive register 81 has an input for the receiving register load line in addition to the input from the receivers 15, 16, 17. The inverter 16 has a receive (RCV/) line while inverter 17 has a simulate-receive (SIMRCV/) input line. Buffer 15 has an input designated PRCV/ for peripheral receive. 
     The CONST bus input to comparator 89 represents the micro-code constant lines 0-7. These originate in the micro-code and are used to determine the values to be driven when loading the MEC interface register; they also choose the function to be performed by the shift register and are used for certain QADn (individual queue address bits (n)) for representing the tape drive unit number and for assigning a queue to a magnetic tape unit. 
     A group of signals inputting to the up/down counter 90 are all prefixed with SAD and refer to the system address signals. The PAD inputs to counter 91 refer to the peripheral address clear and peripheral address countdown (CTD). The top input to system register flip-flop 85 is the system register load signal (SREGLD). 
     The operation decode (OPDEC) inputs to PROM 83 a  and multiplexor 83 b  represent the operation decode signal which reflects the decoded outputs of the ongoing present operation as presented by the output of the 16-way operation decode micro-code branch. 
     The CONST n  lines feeding the binary counter 105 originate in the micro-code and are used to determine the values to be driven when loading the MEC interface register; they are also used to choose the function to be performed by the shift register 89 s  in addition to control usage for queue address bits. 
     The inputs GPRIF connecting to buffer 106 a  and tri-state inverter 106 b  refer to the &#34;gated peripheral&#34; signals and are used in peripheral simulation. When this signal is driven high, it causes the MEC-related signals to be diverted to the Maintenance Card control. 
     The XMIT signal to flip-flop 82 is the transmit signal which enables the transmit register-driver 84 to load and drive the INFOnn lines to the MEC 50 M  and the mag-tape peripherals. 
     The inputs to shift register 92 designated with the prefix SH refer to shift signals such as shift clock, shift-down bit (SHDBIT), to shift up (SHU) and shift down (SHD). 
     The MUXSEL lines to multiplexor 83 are the multiplexor select lines such as &#34;multiplexor select A&#34;. This signal is used, in combination with &#34;MUXSLB&#34;, to select the RAM 22, the shift register 89 s , the receive register 16, or the LPW register 88, or to drive the INRAM bus 52. 
     RECEIVE REGISTER 
     Referring to FIG. 5B the receive register 81 can accept a 17-bit word (via receivers 15, 16, 17) from: the host system 10 on bus 10 h  (DATAnn/0); the peripheral unit on peripheral bus 101 (INFOnn), or from the Maintenance Card 20 om  (DSIMnn/0) on bus 10 dm . 
     Source selection is accomplished by activating one, and only one of the tri-state receivers 15, 16, 17 at a time via the PRCV/ (Peripheral Receive) RCV/ (Receive Enable), or SIMRCV (Simulate Receive) lines, respectively. In the receive mode (RCV), data from the host system 10 is loaded into the receive register 81 by the leading edge of an AF pulse (Table I). In the receive-from -peripheral mode (PRCV), the receive register 81 is loaded by the leading edge of a synchronized peripheral clock pulse (Q2). In the maintenance mode (SIMRCV), the receive register 81 is loaded by either the leading edge of AF (if the Maintenance Card MC is simulating the MLI), or the leading edge of Q2 (if the MC is simulating the peripheral). The receive register 81 remains valid until clocked again, unless cleared by a data link processor (DLP &#34;clear&#34; command). 
     TRANSMIT REGISTER 
     The transmit register 82, in FIG. 5B is a 17-bit wide register used to deliver words to the peripheral unit through the Master Electronic Control MEC 50 M . The transmit data register 82 is loaded from the multiplexor 83 (MUX) output by the rising edge of the micro-code signal #TREGLD (#L2), the Table VIb. The outputs of the transmit register 82 drive the transistors 84 (TQN) which provide a non-inverting transistor-type-logic (TTL) to CTL (complementary transistor logic) conversion. The transmit data register 82 is cleared to all zeros when the XMIT signal goes &#34;low&#34;, allowing the peripheral via MEC 50 M  to drive the INFOnn lines 101. When the signal XMIT is low, no loading of the register 82 can occur. New outputs are valid on the INFOnn lines, within 55 nanoseconds, following the active edge of the DLP clock. 
     SYSTEM REGISTER 
     In FIG. 5B the system register 85 is a 17-bit wide register used to deliver words to the message level interface 15 i  (FIG. 1) and/or the Maintenance Card (MC) 20 om , with selection controlled by the SND (DLP driving INFO lines to MEC 50 M ) and SIMSND (simulate send) lines. As mentioned in the glossary of Table IB, the SND is the &#34;send enable&#34; signal while the SIMSND is the &#34;simulate send&#34; signal. Gates 86 and 87, respectively, control the use of these signal lines to the host system (DATAnn/0) or the Maintenance Card units (DSIMnn/0). The output of the system register 85 will also be seen to provide data to the longitudinal parity word generator 88. 
     The system register 85 is loaded on the falling edge of the micro-code PROM output signal #SREGLD (#H4) of Table VIb or, during the burst mode (with #SREGLD held &#34;high&#34;) by the leading edge of AF. The worst case elapsed time from clock edge to &#34;data valid&#34; on the backplane is 81 nanoseconds for the #SREGLD command and is 89 nanoseconds for the &#34;read burst &#34; mode. The system register 85 is cleared to all zeros by a DLP clear command. 
     QUEUE STORAGE 
     As will be seen in FIGS. 5B and 5C, and FIG. 4A, a random access memory (RAM 22) is used to provide 16 different areas of memory which are utilized as magnetic tape unit scratchpads, that is to say, one memory scratchpad for each one of 16 possible magnetic tape transport units 53 of FIG. 1. These memory areas have been designated as &#34;queues&#34; and are used to store the I/O descriptors, the result descriptors, the descriptor links and various logical flags for each individual tape transport unit. Each of these queues is addressed by special logic called the queue-address register QAD 89. The queue address register 89 is loaded by the 4-bit &#34;C&#34; field and cleared by the queue address clear line QADCLR. The unit number (0-15) of the magnetic tape transport drive is used to address the correct queue. 
     FIG. 5C illustrates the organization of a queue. Each queue is divided into 16 parts labeled Q0 through Q15 and consisting of 17-bits each, as shown horizontally across the top of the diagram with the first or lowest order bit designated P as the parity bit. Q0-Q15 are used to store information as indicated by the horizontal lines opposite each Q position. 
     LONGITUDINAL PARITY WORD GENERATOR 
     The longitudinal parity word (LPW) generator 88 maintains a running longitudinal parity word on all messages transferred between the data link processor (DLP) and the message level interface 15 i  (MLI) of the host system. In the send mode (IOSF=high), the resulting LPW word is also transmitted. In the receive mode (IOSF=low), the last word from the host system 10 (the host system generated LPW) is clocked in and, if there are no transmission errors, an &#34;all zeros&#34; word (16-bits) results, which may then be detected by the 0 test bit. 
     With #FLAGCLR/ (#L1) Table VIb, held &#34;high&#34;, the output of the PROM state 13 (FIG. 4A) is the logical Exclusive OR of the transmit register 83 and the LPW register 88 outputs. With #L1 low, the PROM 13 output is forced to 0. The LPW register 88 is loaded from the PROM outputs on the falling edge of #SREGLD (rising edge of #H4, Table VIb). 
     The LPW register 88 must be pre-set to all ones prior to commencing a message transfer in order to attain a valid LPW tally. This is done by raising #LPWPRE (#L-3), which forces the LPW PROM outputs high, and clocking the LPW register via the #SREGLD/ (#H4) signal. As will be noted in Table II, the signal SREGLD represents the &#34;system register load&#34;. The signal #L1 (Table VIb) must be held &#34;high&#34; throughout the message transfer. 
     The LPW and the Vertical Parity Error Flags are associated with the LPW generator 88 and are defined as follows: 
     LPWERR: this is the longitudinal parity word error signal flag. It is loaded by the falling edge of #LPWPRE (#L3) in the receive RCV mode only. When &#34;high&#34;, it indicates a non-zero LPW check caused by a longitudinal parity error on a message from the host system message level interface 15 i . 
     SYSVPAR: this signal is the system vertical parity error signal flag. It is loaded on the same edge as the system register 85, in the receive mode only. When &#34;high&#34;, it indicates detection of a vertical parity error in a word received from the host system&#39;s message level interface 15 i . 
     PERVPAR: this is the peripheral vertical parity error signal flag. It is loaded on the falling edge of #DATAVK (#J2), in the PRCV mode only. When &#34;high&#34;, it indicates detection of vertical parity error in a word received from the peripheral device or the Master Electronic Control. 
     All three flags are cumulative until cleared to zero by a low level signal on #FLGCLR/ (#L1). 
     RAM ADDRESSING 
     The RAM 22 is located on the Common Front End card 10 c . Its timing, usage and partitioning are completely dependent on the peripheral dependent board (PDB) 80, and the particular micro-code requirements. The write-enable (WE/) pulse is generated from the micro-code signal #RAMWE (#H1), although it is driven low only during the last half of the cycle. This allows the address and input to become stable prior to the write command. The RAM 22 is enabled for &#34;read&#34; and &#34;write&#34; functions by setting the chip select (CS/) low. 
     As seen in FIG. 5B there are three address registers, the queue register 89, the system address register 90, and the peripheral address register 91. These three registers are designated as QAD, SAD and PAD. Any one of these three address registers may be selected for addressing the random access memory RAM 22. These addresses are selected by using various combinations of two microword signals: #ADDSLA (#K1) and #ADDSLB (#K2) in Table VIb. The address selection using these two signals is shown in Table XI. 
     
         ______________________________________                                    
#ADDSLB   #ADDSLA     Selected Address Register                           
______________________________________                                    
0         0           System address (SAD)                                
0         1           Peripheral address (PAD)                            
1         0           Queue Address (QAD)                                 
1         1           Invalid                                             
______________________________________                                    
 
    
     The RAM address is valid within 37 nanoseconds following the clock edge if the selected address was already stable. Table XII shows several functions regarding the address registers which are encoded into the shown function-select bits. Only one of the seven active functions shown in Table XII may be selected during a micro-word cycle. Each function output is asserted only during the last half of each cycle and can, therefore, be repeated on consecutive cycles. 
     
                       TABLE XII                                                   
______________________________________                                    
Encoded Functions                                                         
#FSEL4    FSEL2      FSEL1      Output                                    
______________________________________                                    
0         0          0          Inactive                                  
0         0          1          QUECLR                                    
0         1          0          SHFTCLR                                   
0         1          1          PADCTD                                    
1         0          0          PUSH                                      
1         0          1          SADLD                                     
1         1          0          SADCTD                                    
1         1          1          ADDCLR                                    
ADDCLR clears SAD and PAD.                                                
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     The system address register 90 may be used for accessing the RAM buffers 22 and/or be loaded into the C and D digits of the word generator 92 (8 least significant bits only). The SAD 90 is a nine-bit presettable up/down binary counter. It may be cleared to all zeros using the ADDCLR signal. It may be loaded from the A and the B digits of the INRAM bus 100 via the ADDLD/ signal (address load). A &#34;count up&#34; results from the falling edge of #SADCTU (#H3, Table VIb) or, during burst mode, with #H3 held high, from the leading edge of AF (strobe I/O has been received). The system address register SAD 90 has &#34;up counts&#34; which cannot be executed on consecutive micro-word cycles. Consecutive down-counts, however, are allowed using the SADCTD/ signal (system address count down). Since only the least significant 8-bits are loaded, an ADDCLR/ function must precede the loading of any byte to be used as an address, to avoid possible ambiguity in the SAD 8 (most significant counting) bit. 
     The output of the system address register SAD 90 is compared with the output of the peripheral address register PAD 91 to generate the ADDEQ (address equal) flag, indicating equal counts in the two registers. Another set of flags, collectively called BUFLIM, are assigned to specific addresses. BUFLIM may be tested directly, and/or used to stop a burst mode transfer via the BUFFEND/ signal to the Common Front-End Card (CFE). The RAMAD 9 signal is equal to the BUFSEL (buffer select) bit, stored in the peripheral interface while the SAD 90 is selected. 
     The peripheral address register PAD 91 may be used for accessing the RAM buffers 22 and/or loaded into the A and B digits of the word generator 92 (8 least significant digits only). The peripheral address register 91 may be seen as a 9-bit presettable up/down counter. It may be cleared to all zeros using the ADDCLR/ signal. It may be loaded from the C and D digits of the INRAM bus 100 via the PADLD/ signal. Up-counts or down-counts can be executed using the #PADCTU (#G4) or #PADCTD signals, respectively. As only the least significant 8-bits are loaded, an ADDCLR/ function must precede the loading of any byte to be used as an address, as in the SAD 90. The output of the PAD 91 is compared with the output of the SAD 90 to detect the ADDEQ situation, (address equal). The BUFDONE (buffer limit) flag group is assigned to specific peripheral addresses and, when detected causes a &#34;one&#34; to be shifted up into the poll register 90 s . RAMAD 9 is equal to the BUFSEL bit while the PAD is selected. 
     The queue address (QADn, representing the tape drive unit number for assigning a queue to a magnetic tape unit) consists of a 4-bit binary counter 89 and 6-bits from the #CONSTn field. When the QAD is selected, RAMAD 9 and RAMAD 8 are driven by #CONST 5 and #CONST 4, respectively selecting one of the four RAM quadrants. At the end of a cycle where #SHFTOP (#I1) is high and #CONST 7 is low, the most significant four of the remaining 8-bits are loaded from the C digits of the INRAM bus 100. This same digit is incremented at the end of a cycle where #I1 and #CONST 6 are both high. The digit is cleared to zero via the QUECLR/ signal. Consecutive up-counts are allowed. The lower digit of the QAD is dictated from the micro-code using #CONST 3 (most significant bit), #CONST 2, #CONST 1, and #CONST 0 (least significant bit). 
     The selection of any of the 16 queue areas can be loaded or counted into the &#34;queue register&#34; while the selection of any of 16 specific locations within the area is made via the micro-code. 
     WORD GENERATOR 
     The word generator register 92 employs bit and byte manipulation to produce custom words within the data link processor. The word generator is composed of several stages of shift register. 
     The various words are formed by loading and/or shifting the bits to get the required word format. The word generator register 92 is clocked at the end of each cycle for which the shift operation #SHFTOP (#I1) is held high. Consecutive operations are allowed. The register is 16-bits wide and is divided into two individually controlled 8-bit bytes. The #CONSTn field is used to select the operation to be performed. This is shown in Table XIII. A byte-swap function is performed on each word loaded from the PAD/SAD registers. This operation can be repeated to obtain the original word, it required. The first-installed jumpers can be assigned, as required, for each data link processor ID. Each jumper can be selected for loading into the word generator as part of a custom word or for bit testing. 
     
                       TABLE XIII                                                  
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Shift Register Control                                                    
#CONST                                                                    
7 6 5 4 3 2 1 0                                                           
           Register   Function    Input                                   
______________________________________                                    
1 0 0 0 0 0 0 0                                                           
           All 3      No change                                           
0 - - - - - - -                                                           
           Queue      Load        INRAMCn                                 
1 1 - - - - - -                                                           
           Queue      Increment                                           
- - 0 0 - - - -                                                           
           Upper byte No change                                           
- - 0 1 0 - - -                                                           
           Upper byte Shift down  0                                       
- - 0 1 1 - - -                                                           
           Upper byte Shift down  1                                       
- - 1 0 - - - -                                                           
           Upper byte Shift up    WDC8                                    
- - 1 1 - - - -                                                           
           Upper byte Load        PAD                                     
- - - - - 0 0 -                                                           
           Lower byte No change                                           
- - - - -  0 1 1                                                          
           Lower byte Shift down  SHDBIT                                  
- - - - - 1 0 0                                                           
           Lower byte Shift up    0                                       
- - - - - 1 0 1                                                           
           Lower byte Shift up    1                                       
- - - - - 1 1 -                                                           
           Lower byte Load        SAD                                     
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 NOTES                                                                    
 1. - = Don&#39;t care. The bit is not used.                                  
 2. Upper byte = A and B digits of the shift register.                    
 3. Lower byte = C and D digits of the shift register.                    
 4. Queue = Queue address register, four bits.                            
 5. INRAMCn = C digit of INRAM bus.                                       
 6. SHDBIT = Selected jumper input bits.                                  
 7. PAD, SAD = Least significant eight bits only.                         
 8. Up = Toward the most significant bit (A8).                            
 9. Down = Toward the least significant bit (D1).                         
 
    
     MULTIPLEXOR (DATA PATH) 
     The data path multiplexor 83 operates to select one of four possible sources to drive the INRAM bus 100. The following Table XIV indicates the four sources which may feed the bus 100. These sources are the LPW generator register 88, the receive register 81, the shift register and word assembler 92, and the RAM 22 output. The INRAM 100 lines are valid within 43 nanoseconds following the active clock edge, if the selected source is already stable. 
     
                       TABLE XIV                                                   
______________________________________                                    
INRAM Bus Source Selection                                                
#K3        #K4          Source                                            
______________________________________                                    
0          0            LPW register                                      
0          1            Receive register                                  
1          0            Shift register                                    
1          1            RAM output                                        
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     MICRO-CODE BRANCH TESTING 
     Testing is done in the magnetic tape data link processor to provide micro-code program branch selection. Testing can be performed on words, bytes, bits, or flags. Branching can be 2-way or 16-way. Branching is accomplished by using the symbolic #BRANCHn field to select a bit (or set a bit) which is used, in turn, to modify the next address field. The branching assignments for the magnetic tape data link processor set are shown in Table XV on test bit selection. 
     
                                           TABLE XV                                
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Test Bit Selection (A0)                                                   
# # #                                                                     
B B B                                                                     
R R R                                                                     
A A A                                                                     
N N N (SELB1)                                                             
            (SELB2)                                                       
                   (SEL2)                                                 
                         (SEL3)   (SEL4)                                  
                                        (SEL5)                            
                                              #BRANCH6                    
C C C 0     0      0     0        1     1     (#L4)                       
H H H 0     0      1     1        0     0     #BRANCH5                    
3 2 1 0     1      0     1        0     1     #BRANCH4                    
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0 0 0 #NEXT0                                                              
            SELECT ZERO  RDYST    INRAMC8                                 
                                        INRAMA8                           
0 0 1 AF    ADDEQ  TRMBRST                                                
                         READY    INRAMC4                                 
                                        INRAMA4                           
0 1 0 TERMF SADZERO                                                       
                   PERVPAR                                                
                         BUSY     INRAMC2                                 
                                        INRAMA2                           
0 1 1 LCPADF                                                              
            SADBRW SYSVPAR                                                
                         SPARE    INRAMC1                                 
                                        INRAMA1                           
1 0 0 STOPF FLAGC  LPWERR                                                 
                         Q2       INRAMD8                                 
                                        INRAMB8                           
1 0 1 BUFLIM/                                                             
            PARERR/                                                       
                   DATAV P4       INRAMD4                                 
                                        INRAMB4                           
1 1 0 EQ+LIM                                                              
            SEMREQ FLAGB (P3@SELECT)/                                     
                                  INRAMD2                                 
                                        INRAMB2                           
1 1 1 OPDEC1                                                              
            STACKA0                                                       
                   FLAGA P1       INRAMD1                                 
                                        INRAMB1                           
__________________________________________________________________________
 
    
     Word testing is performed on the 16 data bits of INRAM bus 100 to generate the ZERO flag for the LPW checking and the new vertical parity (NVP) which is also used for parity checking (PARERR). 
     The magnetic tape data link processor uses FLAGA to detect non-binary coded digits (non-BCD) in the space count (of space operation) when binary coded decimal (BCD) values are required. The various testing of bits of INRAM bus 100 and the various flags are defined in Table XV above. The OP code set allows a 16-way branch on the A and B digits of the INRAM bus when (#OPDEC) (#J1) is held high. With #OPDEC low, a 16-way branch is produced by the combination of AF, STOPB/, DATAV, and P3SELECT, which is necessary for handling both interfaces concurrently. 
     PERIPHERAL INTERFACE 
     The peripheral interface of the peripheral dependent board (PDB) is DTL (diode-type logic) compatible. This peripheral interface consists of 25 lines: 17 bi-directional lines 101, 5 receive-only lines 102, and 3 transmit-only lines 103. In FIG. 5B, the 17 bi-directional lines are designated 101, the 5 receive-only lines are designated 102, and the 3 transmit-only lines are designated 103. 
     The magnetic tape data link processor uses all 17 bi-directional lines for data words together with parity, 4 receive lines (RDYST, CLOCK . . . 1, BUSY and READY) and 2 transmit lines (SELECT and SEND). One receive line and one transmit line are not used. 
     The four receive-only lines carry status and/or timing information from the peripheral 53 and MEC 50 M  to the data link processor as 20 03 .via buffer 106 a  and inverter 106 b . Each line can be simulated for maintenance purposes via the PRIFnn/0 line 104 while GPRIF is held high by the maintenance card (MC). Because the interface itself is asynchronous, each of the 5 incoming signals is synchronized through two stages of a data register 106 c  prior to appearing as a test flag bit. In addition, the synchronized peripheral clock signal Q2 is used to load the receive register 81 when the data link processor is in the PRCV mode (peripheral receive mode). Further, detection of the trailing (falling) edge of the peripheral clock pulse sets the DATAV (data available) flag. DATAV can be reset by raising #DATAVK (#J2) listed on Table VIb. 
     The two transmit-only lines 103 (SEND and SELECT) along with XMIT, ABANDON, CS/, BUFSEL and IGNORE are loaded into the peripheral interface driver register 105 a   from the #CONSTn field at the end of each cycle where #PINTLD/ (#J3) is held low. With ABANDON loaded &#34;high&#34;, the detection of a BUSY* READY/ condition automatically clears the SEND, SELECT and XMIT lines to zero. Loading the IGNORE bit &#34;low&#34;, allows a host-system connect to override the PRCV mode and enter the RCV mode in anticipation of receiving a strobe (STIOL) and therefore a word from the host system 10. With the IGNORE bit set &#34;high&#34;, the data link processor (DLP) ignores a host system connect while at the same time receiving PRCV mode from the peripherals 53, 50 M . Loading the chip select signal CS/ &#34;high&#34; disables the RAM 22. 
     POLL REGISTER 
     The poll register 83 p  and its associated poll flags P1, P3, and P4 are provided to monitor the buffer RAM 22 status in determining when to set &#34;poll request&#34; or to set &#34;emergency request&#34; and/or when to flag &#34;access error&#34;. (There is no logical P2 flag). 
     During a &#34;read&#34; operation each time a buffer of RAM 22 is filled by the peripheral (53 via 50 M ) a &#34;one&#34; (high) is automatically shifted, or pushed, into the poll register 83 p . Concurrently, as each buffer of the RAM 22 is unloaded by the host system 10, a zero (low is shifted down (using #H2) into the poll register, popping a &#34;one&#34; out. Thus, when the bottom bit P1 is tested and found &#34;high&#34;, a buffer of the RAM 22 is ready for the host system 10 and a poll request is generated. 
     If P3 is found &#34;high&#34;, indicating that 3 of the 4 buffers are full, an emergency request is generated if the tape is still active (that is, the magnetic tape data link processor is receiving information). If P4 is found &#34;high&#34;, it indicates all four buffers were filled before the host system 10 responded and an &#34;access error&#34; is indicated in the result descriptor. This logical scheme results in the peripheral (53 and 50 M ) trying to keep the buffers full and the host system trying to empty them. 
     In a &#34;write&#34; operation, the magnetic tape data link processor fills all four RAM buffers with data from the host system 10 (unless terminated) before starting the tape motion. Then each time the MEC 50 M  takes a buffer full of data for a selected magnetic tape unit, a &#34;one&#34; is pushed in the poll register. Each time the host system 10 fills another buffer, a &#34;one&#34; is popped out. Thus, the same technique is used for &#34;read&#34; operations as is used for write operations to update and test the poll flags. This logical scheme results in the host system trying to keep the buffer full (having a head start) and the peripheral trying to empty the data from the buffer. 
     There has thus been described a data transfer network using a modularized I/O subsystem which can intercommunicate with a variety of different host computer systems while operating and maintaining itself with its own architecture and circuitry. 
     The data link processors, which form one building block of the subsystem, can connect and disconnect to a host system and perform data transfer operations to/from peripherals and host, while freeing the host to perform other operations during I/O subsystem operation. Each data link processor has been modularized into basically two printed circuit cards, one card for functions common to all types of data link processors, and one card for handling the specific requirements and protocol for a particular type of peripheral terminal.