Patent Publication Number: US-4546429-A

Title: Interactive communication channel

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of application Ser. No. 387,581, filed June 11, 1982, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to automatic digital switching/digital processing telecommunications systems and in particular to an interactive communication channel that provides line interface between the duplex serial data channels and the computer in such systems. 
     Systems of the type to which the invention applies generally comprise a common storage unit storing data and programs and processing units inter-operating with multiple duplex channel devices. U.S. Pat. No. 3,906,151 of Gunter Grossmann et al entitled Method and Apparatus of Signal Conversion in Program Controlled Automatic Data Exchanges,  issued Sept. 16, 1975 discloses a system that is typical of the state of the art. 
     Such systems, however, have various limitations that the present invention seeks to overcome For example, current systems are sometimes: limited in the number of channels they can handle effectively; limited in the speed at which it can process data; transmit undetected errors; require additional processing hardware and memory to accommodate bit stuffing for code transparancy; and require special programs and processing techniques to accommodate different data transmission speeds on different data channels. Individually these limitations have been overcome to varying degrees in various existing systems For instance, the data transmission speed problem is solved by De Ver C. Finch et al in their U.S. Pat. No. 3,676,858 entitled Method, Apparatus and Computer Program for Determining the Transmission Rate and Coding Configuration of Remote Terminals, filed July 11, 1972. However, like current solutions to the various other limitations the solution requires the addition of complex techniques, programs and apparatus. 
     In view of the foregoing discussion, it is apparent that there currently exists the need for an improved I/O interface for automatic digital telecommunications systems that is not subject to the above itemized limitations. The present invention is directed toward satisfying that need. 
     SUMMARY OF THE INVENTION 
     The Interactive Communication Channel (ICC) of the invention provides termination for frame formatted synchronous communication lines in an automatic data processing telecommunications system. Line interface between the system computer and duplex data channels is made via the Channel Buffer System. Character exchange with the Channel Buffer System is on a byte basis. The ICC provides frame start and frame end procedures as well as bit stuffing requirements. Data is stored in memory in block format The ICC maintains byte address information and block byte count. Data exchange with memory is made by cycle steals. 
     Information exchange with the systems computer central processing unit (CPU)) for character processing data is accomplished by memory transfers. Designated memory locations are used for this purpose. When directed to do so by the CPU, the ICC accesses the designated location to read control information. At the conclusion of frame transfer, the ICC writes termination information into memory followed by a termination interrupt. 
     The CPU controls ICC operation by the use of command indicators. These permit the CPU to initiate ICC operations. 
     Channel servicing in the ICC is provided in a channel service period. In this period the transmit and receive lines of a full duplex channel are serviced. All data exchanges in a period are related to the particular channel being serviced. The channel service sequence or scan pattern is programmable and depends upon the number of channels and their speed. 
     Operation of the input function of the ICC is as follows: 
     When the start of a frame is recognized, the ICC extracts a Buffer Area List Entry (BALE) from a designated location in memory. The BALE indicates if the buffer area is available, the starting half word boundary, and the block size in bytes. The ICC transfers data and cyclic redundancy check CRC characters (excluding stuffed bits) into memory via memory steal cycles. With each memory transfer (two bytes of data) the memory address is incremented and the byte count reduced by two. If the byte count reaches zero before the frame is completed, the Bale pointer is incremented and a second BALE read. A second data block is started and memory address and block size maintained as in the first block. The process continues until a Flag character is detected, this indicates completion of the frame. 
     A CRC accumulation is made for all received bytes except Flag characters. At frame completion a CRC test is made to determine if any transmission errors had occurred. 
     A Bale is written into memory at frame completion and provides a last block indicator and the block size remainder. The block size remainder is the byte count reduced by one for each data byte received and stored. CRC characters not included. 
     A Termination Entry is also written into memory. The location of this entry is based upon a rotating counter maintained by the ICC. It provides information necessary to process the frame; channel number, last block BALE pointer and error data. 
     Finally, an interrupt is generated after the Bale and Termination Entry are in memory to inform the CPU that a received frame requires processing. 
     Operation of the output function of the ICC is as follows: 
     Each channel has a timer which indicates when a new message test is to be made. The Bale of each inactive send channel is accessed and an Act bit tested. All channels for which a reset act bit is found become active. The Bale provides additional information for activated channels, the data block starting double word boundary, a last block indicator, block byte count, and the last byte bit count. 
     The ICC reads data from memory for transfer to the Channel Buffer. It provides bit stuffing when required and accumulates the CRC character. Two data bytes are read at a time and with each transfer the byte count is reduced by two and memory address incremented. At the completion of the first data block and assuming it was not indicated as the last block the Bale counter is incremented and a second Bale access is made. A second block is started and data continues to be read. New blocks will continue to be accessed until one which contains a last block indicator is found. When this block is completed, the CRC characters are appended to the transmitted message. At least two Flag characters will also be transmitted. A new message may start at this time, but if none are available the channel buffer will be provided with flag characters. 
     The ICC informs the CPU of the completion of frame transmission by writing a termination entry into memory and raising an interrupt. The termination entry contains the same information as that of input function. 
     An abort sequence may occur for an active output channel. This will result if a memory access is not honored. The message will be aborted and a termination entry and interrupt generated. The first bale of the aborted message will be tested for a reset act bit to resume transmission on the channel. 
     The ICC hardware comprises a random access memory (RAM) and a variety of registers, shifter registers, multiplexers and counters. Information for any channel is transferred from the RAM to the registers and counters during scanning of that channel and returned if no channel securing is required. If channel securing is required the updated data is read back into the RAM. 
     It is a principal object of the invention to provide a new and improved interactive communications channel for use in digital telecommunication systems. 
     It is another object of the invention to provide an interactive communications channel capable of interfacing up to 16 full duplex data channels. 
     It is another object of the invention to provide an interactive communications channel that can process digital data with a maximum total throughput of two megabits per second. 
     It is another object of the invention to provide an interactive communications channel having an improved error checking function. 
     It is another object of the invention to provide an interactive communications channel capable of automatically providing bit stuffing and stripping. 
     It is another object of the inventon to provide interactive communications channel that automatically accommodates different data transmission speeds on different data channels. 
     These, together with other objects, features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the illustrative embodiment in the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the interactive communications channel of the present invention; 
     FIG. 2 is a block diagram of the interaction communications channel process units; 
     FIG. 3 is a block diagram of the memory means that compose the random access memroy subsystem; 
     FIG. 4 is a block diagram of the central processor unit interface; 
     FIGS. 5 illustrates the interactive communications channel device port timing waveforms; 
     FIG. 6 illustrates the interactive communications channel memory timing waveforms; 
     FIG. 7 illustrates the timing waveform for the interactive communications channel; 
     FIGS. 8 and 9 and 10 comprise the transmit memory process flowcharts of the interactive communications channel; 
     FIGS. 11 and 12 comprise the transmit memory process flowcharts of the interactive communications channel; 
     FIGS. 13 and 14 comprise the receive process flowcharts of the interactive communications channel; and 
     FIGS. 15 and 16 comprise the receive memory process flowcharts of the interactive communications channel. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The interactive communications channel (ICC) of the present invention is an input-output multiplexer system that provides an interface between the central processing unit (CPU) of a telecommunications system, and a channel buffer which terminates sixteen full duplex serial data links. The information exchange between the CPU and the channel buffer is accomplished by the ICC on a byte basis. The ICC provides frame start and frame end procedures as well as bit stuffing requirements. Data is stored in memory in block format. The ICC maintains byte address information and block byte count Data exchange with memory is made by cycle steals. 
     The ICC is divided into five subsystems to accomplish the functions described above FIG. 1 is a block diagram of the five sybsystems of the ICC. The five subsystems are: The CPU interface 101, the device port interface 102, the ICC random access memory subsystem 103, the ICC process units 104, and the ICC display panel 105. 
     The device port interface 102 sends and receives data from the sixteen duplex channels by interfacing with a channel buffer 120. The channel buffer 120 itself is not part of the ICC, and may be any one of a number of commercially available communication data buffers that can receive signals from duplex serial data links, and pass on that information to the device port interface 102 in the form of a bit serial data stream. As shown in FIG. 1, the device port interface 102 is simply a bus that transmits and receives data between the channel buffer 120 and the ICC process units 104. 
     The ICC process units 104 contain the logic elements to process, receive, and transmit messages. The ICC process units 104 perform certain key functions so that the Advanced Digital Communications Control Protocol (ADCCP) can be employed on these synchronous data links. These functions include bit stuffing and destuffing, generation and recognition of &#34;Flag&#34; character and Cyclic Redundancy Check (CRC) generation and validation for each simplex termination. 
     The ICC random recess memory (RAM) subsystem 103 provides storage for the data necessary to process all channel messages. The RAM subsystem 103 contains: a random access memory which provides 256 bits of RAM storage for each of the 16 duplex channels, four multiplexers, which conduct all received incoming messages and processing data from the ICC process units to the RAM, and five output registers for passing messages and processing data out of the RAM subsystem to the CPU interface 101 and the ICC process units 104 as required. 
     The CPU interface 101 is an interface between the ICC RAM subsystem 103 and the CPU 110 of the telecommunication system. This interface conducts all message traffic between the ICC RAM the CPU memory, as well as control signals from the CPU into the ICC. Information exchange between the ICC and the Central Processing Unit 110 of the telecommunicatons system computer is accomplished via Direct Memory Access techniques for both data transfers and control transfers. Fixed memory locations are accessed by the ICC to read control information when appropriate conditions occur. Data is stored in CPU memory in frame or packet format. The ICC enters or extracts data packets by maintaining byte address and byte count information for each simplex data channel. 
     The ICC display panel 105 provides a means to perform initial ICC checkout and maintanance. A display of all time slots, the selection of operational modes, and the selection of timing modes are among the functions available on the display panel 105. 
     FIG. 2 is a detailed block diagram of the ICC process units 104 subsystem of the present invention; FIG. 3 is a detailed block diagram of the ICC random access memory subsystem 103; and FIG. 4 is a detailed block diagram of the CPU interface 101. The function and operation of the above subsystems are perhaps best described by the following description of the operation of the ICC, which identifies each step of the input and output activities with each element of the ICC subsystems which perform those steps. 
     The operation of the ICC in the input mode begins when a non-flag character is received by the device port interface 102 from the channel buffer 120. A flag character is the binary sequence: 01111110, which has some specific functions which will be discussed. Note that the presence of a flag character is provided by all users of a system which incorporates ADCCP protocol. Hence the flag character will automatically accompany incoming message traffic, and need not be provided by the channel buffer. 
     An incoming message is conducted from the port interface 102 into the message multiplexer 200, of the device port interface in FIG. 2, on the receive data (RD) terminal. The serial data signal is then conducted by the input shift register (INSR) 201, which provides a path to a bit stuffing unit (S) 202. 
     The bit stuffing unit 202 is a device that provides bit stuffing and unstuffing. It removes stuffed bits from all incoming messages, and provides stuffed bits to all outgoing messages. The determination of the presence of stuffed bits as well as the presence of a flag character, is made by the elements enumerated as 203-206, which are in a loop that feeds back into the bit stuffing unit 202. 
     The output shift register (OUTSR) 203 and output register 204 initially provide a data path for input data into control bit center 205. The control bit counter 205 is used to detect flags and stuffed bits by simply counting bits on all data The bit count and data are then sent to the CBC decode 206, which tests the data for six consecutive binary 1&#39;s in a sequence (which indicates a flag character). When five consecutive binary 1&#39;s are detected, the next binary 0 is a stuffed bit, which will be removed by the bit stuffing unit 202. 
     Once the stuffed bits have been removed from incoming data by the unit 202, the data is conducted by the output shift register 203 and output register 204 to the ICC random access memory subsystem 103. 
     The ICC process units also perform a cyclic redundancy check (CRC) run for all message traffic which enters or leaves the ICC. The CRC run is performed by the CRC register 210, which receives data from the input shift register 201. The CRC register simply makes an accumulation or CRC sum for all received bytes which is also conducted through the unit 202, output shift register 203, and output register 204 to the ICC RAM subsystem. If and when the ICC is directed to retransmit that message, a CRC test run will be performed by that register 210 to check the message for errors. The CRC is delivered with the data as part of the message. 
     FIG. 3 is a detailed block diagram of all the elements that make up the ICC random access memory subsystem 103 of FIG. 1. The RAM subsystem contains: four 16 bit multiplexers 321-324, a RAM memory 325 which provides 256 bits of RAM storage for each of the 16 duplex channels, and five output registers 326-330, for passing messages and data out of the RAM subsystem to the CPU interface 101 and the ICC process units 104 as required. 
     The actual message data of incoming messages is conducted from the ICC process units at the output register 204, and enters the RAM subsystem over bus 370 over the R1 and R2 terminals, through MUX E, 323, into the RAM 325 itself. With each memory transfer, two bytes of data are exchanged. Therefore, the data will eventually be transferred out register one 327 and register two 328 through the CPU interface to the CPU memory. 
     At this point, in addition to preparing to transferring the data itself to the CPU memory, the ICC also extracts some &#34;traffic management&#34; type data from the ICC RAM 325, which is associated with that particular message. The particular &#34;traffic management&#34; datas is known as a Buffer Area List Entry (BALE). The BALE indicates if the buffer area is available, the starting halfword boundary, and the block size, in bytes. If the Buffer Area is available, the ICC transfers data and part of the Cyclic Redundancy Check (CRC) Characters into the CPU memory via memory steal cycles. 
     As mentioned in the discussion of the ICC process units 103, before storing this data, the ICC strips out any &#34;stuffed&#34; zeros inserted in the data stream to maintain code transparency. Therefore, stuffed zeros and flags are not stored as part of the data in the CPU memory. With each memory transfer two bytes of data are exchanged, the memory address is incremented and the byte count is reduced by two. If the byte count reaches zero before the frame is completed, a Bale pointer is incremented and a second BALE is read. 
     Each data block requires a Buffer Address List Entry (BALE). A BALE consists of two half words. The half words of a BALE contain the following information: 
     
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1st BALE Halfword -                                                       
              Act Bit         1      bit                                  
              Last block indicator                                        
                              1      bit                                  
              Byte count      8      bits                                 
              Bit count       3      bits                                 
              Channel out of service                                      
                              1      bit                                  
              Spare           2      bits                                 
2nd BALE Halfword -                                                       
              Block address   16     bits                                 
            (Address is on a half word boundary.)                         
            (Buffer address capability is 64E half                        
            words.)                                                       
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     The BALE information, as shown in FIG. 3, is located in and extracted from the ICC RAM 325 where it will be conducted with the data through the CPU interface 110 to the CPU memory as follows: 
     The data address (DTAD) is conducted out of the RAM 325, through the cycle redundancy check (CRC) register 326, to the CPU interface, along with the last block indicator (LBI), byre count (BYC), bit count (XBC). The other elements of the BALE word will be discussed later. 
     Additionally, some elements of the BALE word are also input back into the ICC process units 104 for updating. For example, since two bytes of data are exchanged with each memory transfer, the byte count number in the RAM needs to be reduced by two each time a message is input into the CPU memory. Also, the byte count needs to be increased by two when a message is removed from the CPU memory. The updating of the byte count is accomplished by: having the old byte count leave the ICC RAM 325 by register two 328; the byte count enters the ICC process unit 104 with a signal from the memory bus (MB), which indicates whether the message is being input or output, by multiplexer 243; and the new byte count is incremented as required by the counter 253 and is input back into the ICC RAM by bus 370. Similarly, the last bit count LBC is incremented by register 254 and returned to the ICC RAM by bus 380. 
     FIG. 4 is a block diagram of the CPU interface 101. The CPU interface contains two busses. The first bus 401 provides an interface with the CPU control to conduct control messages from the CPU into the ICC. The second bus provides an interface with the CPU memory to pass the actual messages and &#34;traffic management&#34; signals back and forth between the ICC RAM and the CPU memory. 
     The message data and associated BALE information enters the CPU memory interface 410 via multiplexer 421. Multiplexer 421 has four input terminals C, D, E and F which are connected as follows: 
     Terminal F receives all the labeled messages from the registers three and four 329 and 330; 
     terminal E receives all labeled messages from registers one and two 327 and 328; and 
     Terminals C and D receive all labeled outputs from the 32 bit CRC register 326. 
     As mentioned above, with each memory transfer, two bytes of data are exchanged, and the byte count is reduced by two. If the byte count reaches zero before the frame is completed, a Bale Pointer (BP) is incremented and a second BALE is read. The BP is sent from the CPU register 326 through multiplexer 420 into the CPU memory interface 410. 
     When second data block is started, the memory address and block size are maintained as in the first block. The process continues until a Flag character or an ABORT sequence is detected. This indicates completion of the frame. As many as 8 BALES (and therefore buffers) can be linked together to store a single frame of information. This does not imply that the same 8 memory buffers are always used for a channel as a BALE can contain any buffer starting address. It is true that a channel will sequence through the same 8 designated memory BALE locations to find where its next assigned buffer begins and now large it is. All buffers except the last one of a frame must be an even number of bytes long. 
     A Bale is written into memory at frame completion providing a Last Block Indicator and the block size remainder. The block size remainder is the byte count reduced by one for each data byte received and stored (CRC characters not included). 
     A Termination Entry is also written into the CPU memory at frame completion by multiplexer 420. 
     The CPU control interface 401 serves to conduct two control signals, Channel Memory Write (CMW), and execute (EX) into the ICC and conducts an interrupt (INT) signal from the ICC into the CPU. A termination is written into the CPU memory at every frame completion. Simultaneously an interrupt is generated by the flip flop 405 which feeds the INT signal into the CPU control interface 401 to inform the CPU that a received frame requires processing. The INT flip flop 405 is set by a signal from the conditions unit (CND) 406 which contains the ACT bit of the Bale word. The ACT bits function is described below is the discussion relating to ICC output operation. 
     ICC operational status is determined by an On/Off indicator (ICCON) 403. The state of this indicator is under CPU control through a control signal which enters the decoder 402. Normal channel service proceeds when the indicator is On. An On-to-Off indicator transition will cause the ICC to become nonfunctioning but it will complete the current channel service period. The ICC remains Off until the indicator is altered by the CPU. 
     The CPU can gain access to the ICC channel RAM by setting the ICC channel RAM Read/Write indicator bit 404. This indicator is enabled only when the ICC is Off. As a function of the bit being set, the ICC accessed a fixed half word location in memory to extract the channel RAM address and the operation request (Read or Write). The ICC than transfers a half word of data to or from the next half word location in memory depending on whether a read or write operation was specified. 
     The initiation of the output of a message from the ICC to the channel buffer can be started by either the CPU or a channel requesting a message. Inactive channels are listed once every n scans (n=a programmable number) to see if, during the time slot alloted to that channel, the ACT bit was reset. When the ACT bit is found to be reset, a new message has been queued for transmission. 
     When an output channel is not active, the bit byte count (BYC) in the transmit BYC location of the ICC channel word serves as an elapsed time counter. Each time the channel is scanned the count is downcounted. When the count reaches zero, the ACT bit in that channel BALE is accessed If the ACT bit is set, the channel is still inactive. The zero count is then replaced by a byte count location in the same BALE as the ACT bit. 
     In this way inactive channels are tested only once every n scans. (n=a programmable number) If, however, the ACT bit in the CND 406 tested is found to be reset, a new message has been queued for transmission on the channel and the channel is now active. The total BALE information is now transferred into the ICC RAM location for that channel. The BALE information of an activated channel, includes the block starting address (on a word boundary), a Last Block Indicator bit, and the block byte count. The block byte count now occupies the transmit BYC location and serves to keep track of bytes transmitted so that the ICC can determine when the buffer has emptied and when the frame transmission has been completed. The ACT is then set by the ICC. 
     The ICC reads data from memory for transfer to the Device Port by transferring both the data and BALE information from the CPU memory interface 410 in the WD (write data) signal into the memory bus 340 back through the four multiplexers 321-324 into the ICC RAM where the labeled signals are output out the registers 326-330 into the multiplexer 200 of the ICC process units 104. 
     The ICC reads the data from the ICC RAM to the Device Port interface 102 through the path described below. From multiplexer 200 and the INSR 201 a cyclic redundancy check is run by CRC 210. The bit stuffing unit 202 returns stuffed bits and the OUTSR 203 and output register 204 sends the channel address and transmit data to the device port interface. 
     Two data bytes are read each time the memory is accessed. With each transfer the byte count is reduced by two and the memory halfword address is incremented by one by counter 253. At the completion of the first data block, the Last Block Indicator in the BALE is checked. If it is not set, the BALE counter is incremented and a second BALE access is made. A second block is started and data continues to be read. New blocks will continue to be accessed until one is encountered which contains a Last Blockn Indicator. When this block is completed, the CRC characters are appended to the transmitted message prior to bit stuffing. At least two Flag characters will also be transmitted. A new message may start at this time, but if none are available the Device Port will be provided with Flag characters. It should be noted that all buffers except the last one of a frame must contain an even number of bytes. Also as many as 8 BALES per output channel may be linked to form a single frame. The ICC informs the CPU of the completion of frame transmission by writing a termination entry into the cpu memory through multiplexer 420 and raising an interrupt through the INT flip flop 405. 
     The ICC will service up to 16 full duplex channels. It utilizes a scan table that will accommodate any mix of channel speeds. Span sequence can be modified under program control. 
     Channel servicing in the ICC is provided during a channel service period. During this period, the transmit and receive lines of a full duplex channel are serviced. All data exchanges during a period are related to the particular channel being serviced. The channel service sequence or scan pattern is programmable and depends upon the number of channels and their speed. Command indicators are used by the CPU to initiate ICC operations. 
     Line procedure conform with Proposed American National Standard for Advanced Data Communication Control Procedures (ADCCP). The ICC will for each channel: generate/detect ABORT sequence and Flag character; generate/check CRC sequence; and will insert/delete stuffed zero bits. 
     With regard to the ICC data format, a frame may be composed of one or more data blocks. The last data block in a frame is recognized by inspecting the Last Nlock Indicator bit in the respective BALE. 
     Data block size is not fized. It may be any number of half words but cannot exceed 128 (256 bytes). The Bale contains a byte count filed which is used to indicate data block size. Block size is established by program control. Error detection is provided by cyclic redundancy check (CRC) technique. The Generator polynominal P (X) is hard-wire programmable and at present has been set to X 32  +X 26  +X 23  +X 22  +X 16  +X 12  +X 11  +X 10  +X 8  +X 7  +X 5  +X 4  +X 2  +X+1. This is a CRC character length of 4 bytes, or 32 bits. 
     Operation of the ICC with the telecommunications system CPU is as follows: 
     Initialization is required after the ICC is powered up. The CPU must provide a procedure which loads correct initial information for each channel into the ICC channel RAM memory. The contents of channel memory for each simplex channel is tabulated below in Table I. 
     
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CRC Word               32     bits                                        
Data Address           16                                                 
Data Store (3 bytes)   24                                                 
X-Reg                  8                                                  
X bit count            4                                                  
Bale Register          3                                                  
Channel Mode           1                                                  
Memory Process State   5                                                  
First Block Indicator  1                                                  
Control bit Count      3                                                  
Reg Status Indicator   3                                                  
Character Process State                                                   
                       3                                                  
Packet Errors          5                                                  
Bale Pointer           3                                                  
Last Block Indicator   1                                                  
Byte Count             8                                                  
Last bit Count         3                                                  
Good CRC               1                                                  
Next Channel Adr./Aux. 4                                                  
Byte Count                                                                
                       128    bits                                        
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     Data exchange with the ICC channel RAM is accomplished by memory steal cycles. The CPU places in a fixed fullword Iocation in memory, the channel RAM address, a halfword of data, and a write request indication. The ICC uses this information to perform the requested operation. The format of this fullword is shown below. 
     
                                           TABLE I                                 
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CHANNEL MEMORY CONFIGURATION                                              
FIELD DESIGNATION                                                         
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Bit                                                                       
  000    001  010 011  100                                                
                          101                                             
                             110  111                                     
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1 BRO    BYC 0                                                            
              LBI DAD O                                                   
                       XR 1                                               
                          R2 1                                            
                             CRC O                                        
                                  CRC 16                                  
2 1      1    BPO 1    2  2  1    17                                      
3 2      2    1   2    3  3  2    18                                      
4 MPS 1  3    2   3    4  4  3    19                                      
5 22     4    FBK 4    5  5  4    20                                      
6 3      5    RSI 1                                                       
                  5    6  6  5    21                                      
7 4      6    2   6    7  7  6    22                                      
8 5      7    3   7    8  8  7    23                                      
9 PE 1   NCA/                                                             
  (MAE)  ABYC 1                                                           
              CM  8    R3 1                                               
                          R1 1                                            
                             8    24                                      
  2                                                                       
10                                                                        
  (BHR)  2    CBC O                                                       
                  9    2  2  9    25                                      
11                                                                        
  3      3    1   10   3  3  10   26                                      
  (B CRC)                                                                 
12                                                                        
  4                                                                       
  (SH MSG)                                                                
         4    2   11   4  4  11   27                                      
13                                                                        
  5                                                                       
  (ABT MSG)                                                               
         CPS 1                                                            
              XBC O                                                       
                  12   5  5  12   28                                      
14                                                                        
  LBC O  2    1   13   6  6  13   29                                      
15                                                                        
  1      3    2   14   7  7  14   30                                      
16                                                                        
  2      GCRC 3   15   8  8  15   31                                      
         SECTION A                                                        
                SECTION B                                                 
Memory Location Address: (1FA00).sub.16                                   
Fullword Data Format:                                                     
1st HW: R/W O O C C C C T/R O O O O O F F F                               
2nd HW: (Two bytes of data or two bytes of data storage)                  
 ##STR1##                                                                 
is the Read/Write Indicator bit.                                          
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     Read is indicated by a one. 
     CCCC is the channel address. Range of values are 0-F. 
     T/R indicates either the transmit or receive section of channel CCCC. Receive is indicated by a zero. 
     FFF is the field or segment of the receive or transmit section under read/write consideration. Range of values are 0-7 (hexadecimal). 
     The CPU can initialize 16 full duplex channels. Each duplex channel contains 256 bits of RAM storage in which 128 bits are allocated to each of the receive and transmit sections of one channel. Since the CPU can only transfer one halfword of data at a time, 16 such transfers are required to load the channel RAM per duplex channel. An entire channel RAM load encompasses 256 CPU initiated transfers. 
     Proper ICC initialization requires that the transmit section of the in-service channels contain flag [(7E) 16  ] the X-Register location and the appropriate NCA value in the NCA/ABYC location of channel memory. The remaining channel memory bits should be initialized to zero. 
     Channel scanning in the ICC is designed to accommodate different data rate on different data duplex channels. 
     The NCA or Next-Channel Address value determines the channel scan sequence. 
     The channel scan sequence is developed such that every participating channel is serviced in a sufficient amount of time so that no overun condition is experienced. 
     The mechanics of establishing a scan sequence is illustrated by the examples given below. 
     Certain ICC hardware restrictions must be satisifed. They are, 
     1. Channel 0 must appear once in the primary scan cycle. 
     2. The primary scan cycle must be a linear sequence of the form 0 to X. 
     Examples of a primary scan sequence are: 0, 1, 2, 3: 0, 1, 2; 0, 1, 2, 3, 4, 5; etc. How this would be encoded in the NCA of the channels is shown below for the examples given. 
     
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           Example 2                                                      
Example 1    Channel          Example 3                                   
Channel No.                                                               
         NCA     No.      NCA   Channel No.                               
                                         NCA                              
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0        3       0        2     0        5                                
1        0       1        0     1        0                                
2        0       2        0     2        0                                
3        0                      3        0                                
                                4        0                                
                                5        0                                
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     Channel zero&#39;s NCA value is always the last channel of a given primary scan. All other ohannels of a primary scan always have the NCA value of channel zero. 
     In operation, examples 1, 2 and 3 will be processed as follows: 
     
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Example 1 3       2     1     3   etc.                                    
          0       0     0     0                                           
Example 2 2       1     2         etc.                                    
          0       0     0                                                 
Example 3 5       4     3     2   1     5   etc.                          
          0       0     0     0   0     0                                 
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     3. In the event that the primary scan will be the only scan, example 3 can be modified as follows: 
     
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       Channel No.                                                        
                NCA                                                       
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       0        1                                                         
       1        2                                                         
       2        3                                                         
       3        4                                                         
       4        5                                                         
       5        0                                                         
______________________________________                                    
 
    
     In operation, this would be processed as follows: 
     
         ______________________________________                                    
       1     2       3   4     5   1     2   etc.                         
0                              0                                          
______________________________________                                    
 
    
     This type of scan dictates that all the channels present in the scan sequence have the same baud rate; if optimization is under consideration. 
     4. In the event that secondary scan sequences are required, a primary scan sequence of the type illustrated in item 2 above must be established. 
     A secondary scan sequence would be used if a mix of baud rates over the number of channels utilized in an operational system exists and optimization is desired. Such a sequence is illustrated below. 
     
         ______________________________________                                    
             Channel No.                                                  
                      NCA Value                                           
______________________________________                                    
PRIMARY        0          3                                               
               1          8                                               
               2          8                                               
               3          8                                               
SECONDARY      8          9                                               
               9          A                                               
               A          B                                               
               B          C                                               
               C          D                                               
               D          0                                               
______________________________________                                    
 
    
     The NCA value of 8 replaces the value of 0 in the primary scan sequence. This is done in this example so that the secondary scan sequence is performed every time one of the primary scan channels are serviced. The secondary scan sequence always ends with the NCA value of 0, thus completing the initial primary scan requirement of having all channels return to zero. 
     In operation, the channels will be processed as follows: 
     
         ______________________________________                                    
             3     2       1   3                                          
             8     8       8   8                                          
             9     9       9   9                                          
             A     A       A   A                                          
             B     B       B   B                                          
             C     C       C   C     PRIMARY                              
SECONDARY    D     D       D   D     SCAN                                 
SCANS        0     0       0   0                                          
______________________________________                                    
 
    
     This completes the discussion on the form that a scan sequence can take and the mechanics of encoding the NCA values. 
     In order to determine the exact pattern a scan sequence can take, the baud rates of the individual channels must be taken into consideration. 
     The ICC requires a duration of 4 usecs per byte per simplex channel for processing. Since the smallest processing operation is a duplex channel (receive and transmit sections) 8  usecs must be allotted. In this time, a receive as well as a transmit byte may be processed. 
     Given the processing time of 8 usec: per duplex channel and the required baud rates that must be sustained on each channel, a scan sequence is developed to meet all the criteria. 
     GIVEN: 
     3 channels at 32K baud for subscriber interface 
     7 channels at 80K baud for 6 trunks and 1 high speed channel subscriber. 
     (These are channel addresses 8, 9, A, B, C and D for the trunks and channel 0 for the high speed subscriber) 
     
         ______________________________________                                    
DERIVED:      Channel Number                                              
                           NCA Value                                      
______________________________________                                    
PRIMARY       0            3                                              
SCAN          1            8                                              
              2            8                                              
              3            8                                              
SECONDARY     8            9                                              
              9            A                                              
              A            B                                              
              B            C                                              
              C            D                                              
              D            0                                              
______________________________________                                    
 
    
     Referring to Example 4 above, any given channel of the secondary scan and channel 0 is processed every eighth time. This is a time delay of 64 usec. Since a byte on any given secondary scan needs to be processed in 100 usec, a processing delay of 64 usec is satisfactory. Therefore, channels 8, 9, A, B, C, and D channel can be assigned to the 80K baud trunks. 
     In a similar fashion, the primary scan sequence can be analyzed. Here each channel excluding channel zero is processed every 24th time (see processing scan in Example 4 above). This works out to a time delay of 192 usec. The byte rate on any primary channel is 250 usec. (32K baud) Therefore, the 3 low speed subscribers will be channels 1, 2, and 3. Once again, more than enough time is allowed for processing. 
     Channel zero gets processed at the secondary scan rate. Therefore its baud rate can be similar to the secondary channel baud rate. Channel zero may be assigned to the high speed subscriber. 
     Each data block requires a Buffer Address List Entry (BALE). A BALE consists of two half words. The half words of a BALE contain the following information. (See FIG. 5) 
     
         ______________________________________                                    
1st BALE Halfword -                                                       
              Act Bit         1      bit                                  
              Last block indicator                                        
                              1      bit                                  
              Byte count      8      bits                                 
              Bit count       3      bits                                 
              Channel out of service                                      
                              1      bit                                  
              Spare           2      bits                                 
2nd BALE Halfword -                                                       
              Block start address                                         
                              16     bits                                 
            (Address is on a half word boundary.)                         
            (Buffer address capability is 64K half                        
            words.)                                                       
______________________________________                                    
 
    
     A channel can address 8 different BALES. The BALE address is composed as follows: 
     
         ______________________________________                                    
Hardwired constant                                                        
                  9 bits                                                  
Channel number    5 bits                                                  
Bale pointer      3 bits                                                  
Bale halfwords    1 bit.sup.                                              
______________________________________                                    
 
    
     A Termination Queue Entry is written to memory after a message frame is transmitted or received. The Termination Queue Entry contains the following information. This is tabulated in the following Table II 
     
         ______________________________________                                    
Channel Number     5 bits                                                 
Last block Bale pointer                                                   
                   2 bits                                                 
Errors             5 bits                                                 
Spare              3 bits                                                 
______________________________________                                    
 
    
     Errors include ABORT (ABT), SHORT MESSAGE (SHMSHG), BAD CRC (BCRC), BUFFER NOT READY (BNR), MEMORY ACCESS ERROR (MAE). 
     The Termination Queue Entry address if composed as follows: 
     
         ______________________________________                                    
Hardwired constant  10 bits                                               
Term queue entry pointer                                                  
                     8 bits                                               
Address Format 1 F 1 .0. 1 Term Queue                                     
Entry Pointer .0. hexadecimal                                             
______________________________________                                    
 
    
     
                       TABLE II                                                    
______________________________________                                    
BALE AND TERMINATION ENTRY CONFIGURATIONS                                 
         BALE DATA                                                        
BIT        2nd HW    1st HW   TERM ENTRY                                  
______________________________________                                    
LSB    1       BSA    1    BYC  0   CAD    1                              
       2              2         1          2                              
       3              3         2          3                              
       4              4         3          4                              
       5              5         4          5                              
       6              6         5   LBP    0                              
       7              7         6          1                              
       8              8         7          2                              
       9              9    BTC  0   PE     1                              
                                    (MAE)                                 
       10             10        1   2                                     
                                    (BNR)                                 
       11             11        2   3                                     
                                    (B CRC)                               
       12             12            4                                     
                                    (SH MSG)                              
       13             13   ACT      5                                     
                                    (ABT MSG)                             
       14             14   LBI                                            
       15             15   COS                                            
       16             16                                                  
       17             --   --       --                                    
MSB    18             --   --       --                                    
______________________________________                                    
 
    
     A CPU interrupt will be generated after a Termination Queue Entry is written to memory. 
     Operation of the ICC with data memory is as follows: 
     All data and information exchanges between the ICC and CPU are provided by memory cycle steal or Direct Memory Access (DMA) procedures. When the ICC mode is ICCON, data exchanges are message data, BALES and Termination Queue Entries. When the mode is ICCOFF, data exchanges are ICC Channel RAM Field information. 
     Data storage capacity in the ICC is limited. In processing channel messages, a minimum number of halfword memory accesses must be available or data will be lost. 
     The ICC can have as many as 1 cycle steal every 4 usec. for data transfers and will average 1 cycle steal every 8 usec. for data even when it is fully loaded. From the time the ICC raises a Memory Break Request it must receive a memory cycle within 2.5 usec. or data will be lost. 
     The following represents interfacing between the ICC and device ports (DP). ICC-DP timing waveforms are illustrated by FIG. 5. Signal identification and signal levels are as follows: 
     
         ______________________________________                                    
Signal Identification                                                     
                   Number of                                              
                           Signal                                         
                   Lines   Sources                                        
______________________________________                                    
Signal Name                                                               
Address Information  4         ICC                                        
Receive Service Request                                                   
                     1         DP                                         
Transmit Service Request                                                  
                     1         DP                                         
Transmit Data        8         ICC                                        
Transmit Strobe      1         ICC                                        
Receive Data         8         DP                                         
Receive Strobe       1         ICC                                        
Signal Levels                                                             
Logical 0, +5 V                                                           
Logical 1, 0 V                                                            
Twisted pair lines                                                        
Receiver termination, 1k ohm to +5 V                                      
______________________________________                                    
 
    
     The signal identification and signal levels for CPU are: 
     
         ______________________________________                                    
Signal Identification                                                     
                   Number of                                              
                           Signal                                         
                   Lines   Source                                         
______________________________________                                    
Signal Name                                                               
Indicators                                                                
Indicator Lines      2         CPU                                        
Control Lines        1         CPU                                        
Indicator Settings   2         ICC                                        
Interrupt            1         ICC                                        
Signal Levels                                                             
Logical 0, +5 V                                                           
Logical 1, 0 V                                                            
Twisted pair lines                                                        
Receiver Termination, 1K ohm to +5 V                                      
______________________________________                                    
 
    
     Control signals between ICC and CPU are asynchronous. Except for the ICC On/Off signal, control signals are enabled for the state of the ICC On/Off indicator shown. Pulse width, 
     1/2 us to 1 us. 
     ICC On/Off--No conditions 
     Channel RAM Access--ICC Off 
     Interrupt--ICC On 
     The following represents signal identification and signal levels for the ICC memory. Memory timing waveforms are shown in FIG. 6. 
     
         ______________________________________                                    
Signal Identification                                                     
                   Number of                                              
                           Signal                                         
                   Lines   Source                                         
______________________________________                                    
Signal Name                                                               
Memory Address       18        ICC                                        
Memory Break Request 1         ICC                                        
Read/Write Command   1         ICC                                        
Write Data           16        ICC                                        
Read Data            16        Memory                                     
Memory Break Req. Honor                                                   
                     1         Memory                                     
Execute              1         Memory                                     
Signal Levels                                                             
Logical 0, +5 V                                                           
Logical 1, 0 V                                                            
Twisted pair lines                                                        
Receiver termination, 1K ohm to +5 V                                      
______________________________________                                    
 
    
     The ICC includes a display panel the layout of which is shown in tabular form in the following Table III. The panel provides a means to perform initial ICC checkout and maintenance. A group of toggle switches provide memory read data and latches with LED indicators provide a means to observe memory write data. The following operational modes are available and each may be made on one of three possible timing modes. The operational modes are: Single Channel--The device address does not change for repeated channel service periods; and, Multichannel--The channel buffer address changes based upon the scan table. The timing modes are: Single Pulse--Timing periods are advanced by pushbutton depressions; Single channel service period--The channel service period occurs once for each push button depression; and, Continuous channel service period--The channel service period occurs at the normal rate. 
     Three additional features are provided by the ICC panel. Display of time slots, display of channel RAM by fields and display of operational data and registers. 
     1. Time slots--16 time slots and transmit/receive bit. 
     2. Channel memory--16 bits of selected RAM field. 
     3. Operation data and registers--2 groups of information can be displayed. See FIG. 5 for displayed data. 
     
                                           TABLE III                               
__________________________________________________________________________
ICC PANEL DISPLAY                                                         
BITS    18                                                                
          17 16                                                           
               15                                                         
                 14                                                       
                   13                                                     
                     12                                                   
                       11                                                 
                         10                                               
                           9 8 7 6 5 4 3 2 1                              
__________________________________________________________________________
TIME SLOTS                                                                
        --                                                                
          T/R                                                             
             T T T T T T T T T T T T T T T T                              
             S S S S S S S S S S S S S S S S                              
             16                                                           
               15                                                         
                 14                                                       
                   13                                                     
                     12                                                   
                       11                                                 
                         10                                               
                           9 8 7 6 5 4 3 2 1                              
CHANNEL --                                                                
          -- B B B B B B B B B B B B B B B B                              
MEMORY       I I I I I I I I I I I I I I I I                              
             T T T T T T T T T T T T T T T T                              
             16                                                           
               15                                                         
                 14                                                       
                   13                                                     
                     12                                                   
                       11                                                 
                         10                                               
                           9 8 7 6 5 4 3 2 1                              
OPERATION                                                                 
        --                                                                
          -- --                                                           
               --                                                         
                 --                                                       
                   X X X C C C F M M M M M L                              
                   B B B B B B B P P P P P B                              
                   C C C C C C K S S S S S I                              
                   2 1 0 2 1 0   5 4 3 2 1 1                              
REGISTERS                                                                 
        --                                                                
          S3 S2                                                           
               S1                                                         
                 V0                                                       
                   C C C A A B B B B B B B B                              
                   P P P B B B Y Y Y Y Y Y Y                              
          =  = = =                                                        
                   S S S Y Y C C C C C C C C                              
          1  1 1 1 3 2 1 C C 7 6 5 4 3 2 1 0                              
                         1 0                                              
__________________________________________________________________________
 
    
     The following is a glossary of terms and abbreviations used to describe ICC operations in the remainder of this document. 
     ABT: ABORT 
     ABYC: Auxiliary byte counter; used to count the first four characters of a data buffer for receive channels. 
     AFF: A Flip-Flop; used to address channel RAM 
     BNR: &#34;Buffer Not Ready&#34; Error Condition 
     BP: Bale Pointer 
     BR: Bale Register 
     BYC: Byte Counter; used for data buffer control. Decremented for each byte transferred. 
     CBC: Control Bit Counter; used to detect flag and abort characters, also for bit stuffing. 
     CBS: Channel Buffer System 
     CM: Channel Mode; ADCCP or other (e.g. ARPA) 
     CMAC: Channel Memory Access 
     CMO: Channel Memory Output 
     CMW: Channel Memory Write 
     CPS: Character Process State 
     CPU: Central Processing Unit 
     CRC: Cyclic Redundancy Check (Register) 
     DLE: &#34;Delte&#34; Character (ARPA) 
     DRC: DLE Received (ARPA) 
     DTAD: Data Address 
     ETX: &#34;End of Text&#34; Character (ARPA) 
     EX: Execute Signal from CPU or Memory 
     FBK: First Block of Buffer Area 
     FL: &#34;Flag&#34; Sequence 
     FLD: Field of Channel Memory 
     GCRC: Good Cyclic Redundancy Check 
     ICCON: Inter Communications Channel ON Indicator 
     INT: Interrupt 
     IR: Input Shift Register 
     LBC: Last Bit Count 
     LBI: Last Block Indicator 
     MAG: Memory Access Gate 
     MAE: Memory Access Error 
     MB: Memory Bus (Data from Core Stored in Register) 
     MBH: Memory Break Honored (from memory) 
     MBR: Memory Break Request (to memory) 
     MPS: Memory Process State 
     MS: Memory Sequence 
     NCA: Next Channel Address 
     NMP: Next Memory Process 
     OR: Output SHift Register 
     PE: Error Flags 
     PB: Panel Bus (Data from Panel) 
     R1: Data Registers (R2 or R3) 
     RD: Read Data From Memory (becomes MB) 
     RSI: Register Status Indicator 
     S1: Parts of RSI (S2 or S3) 
     SOS: Start of Scan 
     SR: Service Request from Device Port 
     STX: &#34;Start of Text&#34; Character (ARPA) 
     SYNC: &#34;SYNC&#34; Character (ARPA) 
     TP: Termination Queue Pointer 
     TE: Termination Entry 
     TSq: Time Slot 
     VO: Bit Being Shifted Out of Input Shift Register 
     WD: Write Data (Data to Memory) 
     XBC: X Bit Counter-used for data control. Receive--as the bits are shifted into OUTSR the XBC is incremented. When XBC=8 a character is complete and must be stored in channel memory. Transmit--as the bits are shifted out of the INSR the XBC is incremented. When XBC=8 a new character is required from channel memory. 
     XR: X Register, used to store a part of a byte for continued processing in next cycle. 
     YBC: Y Bit Counter, used for data control. Receive--as the bits are shifted out of the INSR the YBC is incremented. When YBC=8 all of the received bits have been processed. Transmit--as the bits are shifted into the OUTSR the YBC is incremented. When YBC=8 a character is ready to go to the character buffer. 
     The ICC hardware centers around the ICC Channel Memory. There are 256 fields of 16 bits in each field. Information in this memory is partitioned so that each simplex channel uses 8 fields in this Random Access Memory (RAM). Each time a device is scanned by the ICC, all the information for that channel is accessed from the memory and stored in a variety of registers, shift registers and counters. At the end of the 4 usecs dedicated to that device all the information is restored in the Channel Memory. If the device had required servicing, updated information is stored back in the memory. If the device did not require servicing, the old information will remain in the Channel Memory. The manipulation of this memory, registers, shift registers and counters to perform the required ICC functions and hereinafter described in detail. 
     The ICC goes through 16 time slots per simplex device and services first a transmit device and then a receive device. Each time slot has a 250 nsec duration which is divided into 3 timing pulses. Therefore, 8 usecs is expended to service a full duplex device. FIG. 7 shows the ICC timing waveforms. 
     There are two processes that occur simultaneously within the ICC. One is character processing and the other is CPU memory processing. 
     For each simplex device there are 16 time slots to perform the necessary function. Seven of the 16 time slots (1, 2, 3, 3, 14, 15 and 16) have fixed functions. The other 9 time slots encompass 8 register shifts and 1 register store with the register shift occuring anywhere within the 9 time slots, since it is dependent on the data being received or transmitted. Table IV shows the General Operation Timing. In this table, it is noted that the channel memory word for simplex channel is divided into two parts, Section A and Section B as shown in Table 1. In Table IV, the abbreviations are as follows: 
     CPS--Character Process State 
     MPS--Memory Process State 
     NMP--Next Memory Process State 
     It should be noted that the Character processing has an effect on the Memory Processing and vice versa. 
     
                       TABLE IV                                                    
______________________________________                                    
GENERAL OPERATION TIMING                                                  
Time Service  RAM                                                         
Slot Period   Access                                                      
______________________________________                                    
                       Character Process Operation                        
1    Trans    Read A   Read A word from RAM, load                         
                       registers.                                         
2                      Change RAM address to B section.                   
3             Read B   Make CPS decision, Read B word                     
                       from RAM                                           
5                                                                         
6                                                                         
7                      Character Process                                  
8                      8 shifts and 1 register load                       
9                      Under control of sequence                          
10                     Control Generator                                  
11                     (TS 4- 12)                                         
12                     CPS sets next CPS &amp; Initiates NMP                  
13                     MPS modify next CPS &amp; sets NMP                     
14            Write B  Write B word to RAM                                
15                     Change RAM address to A section                    
16   Trans    Write A  Write A word to RAM                                
1    Rec      Read A   Read A word from RAM, load                         
                       registers                                          
2                      Change RAM address to B section                    
3             Read B   Make CPS decision, Read B word                     
                       from RAM                                           
4                                                                         
5                      Character Process                                  
6                                                                         
7                      8 shifts and 1 register store                      
8                                                                         
9                      Under control of sequence                          
10                     Control Generator                                  
11                     (TS 4-12)                                          
12                     CPS sets next CPS &amp; Initiates NMP                  
13                     MPS modify next CPS &amp; sets NMP                     
14            Write B  Write B word to RAM                                
15                     Change RAM address to A section                    
16   Rec      Write A  Write A word to RAM, change                        
                       Chan. Address                                      
______________________________________                                    
 
    
     Aside from ICC memory cycle steals to CPU core memory for transferring data, the ICC must access memory to read or write control information. The ICC must read from and write to the 1st halfword of a given BALE, read the 2nd halfword of the same BALE and write Termination Entries at the completion of a good or bad frame. The following is a list of memory accesses for control information when they occur and the functions performed during each state. 
     Transmit Cycle 
     A. Start of Buffer 1: 
     1. Read BALE, 1st Hw--test ACT bit, store byte count and Last Block Indicator. 
     2. Read BALE, 2nd Hw--store data buffer start address. 
     3. Write BALE, 1st Hw--set ACT bit. 
     B. Start of Buffer N: 
     1. Read BALE, 1st Hw--same as A1. 
     2. Read BALE, 2nd Hw--same as A2. 
     C. End of Last Buffer (Normal): 
     1. Write Term Entry--write channel no., BALE pointer of last buffer and errors. 
     2. Read BALE, 1st Hw--test ACT bit of next BALE for possible start of new message. 
     D. End of Last Buffer (Error) 
     1. Write Term Entry--write channel no., BALE pointer of first buffer, and errors. 
     2. Read BALE, 1st Hw--same as A1. 
     A. Start of Buffer 1: 
     1. Read BALE, 1st Hw--test ACT bit, test COS (Channel Out of Service) bit and store byte count. 
     2. Read BALE, 2nd Hw--store data buffer start address. 
     3. Write BALE, 1st Hw--set ACT bit. 
     B Start of Buffer N: 
     1. Read BALE, 1st Hw--same as A1. 
     2. Read BALE, 2nd Hw--same as A2. 
     C. End of Message (Normal): 
     1. Write BALE, 1st Hw--write last block indicator bit, byte count (remaining in buffer), and last bit count. 
     2. Write Term Entry--write channel no., BALE pointer of last buffer, and errors. 
     D. End of Message (Buffer Recovery): 
     1. Write BALE, 1st Hw--reset ACT bit of first buffer used for lost message. 
     2. Write Term Entry--Write channel no., BALE point of first buffer, and errors. 
     These memory accesses along with Read Data (RD) for Transmit and Write Data (WD) for Receive have been grouped together into Memory Sequences. The following is a list of these Memory Sequences as they appear in ICC documentation. 
     I. Transmit 
     MS1=RB2, RD, WB1S (part of start buffer) 
     MS2=TER, RB1 (error termination) 
     MS3=TE, RB1 (normal termination) 
     II. Receive 
     MS1=RB1, RB0, WD, WB1S (start of message) 
     MS2=WB1S, TER (error termination) 
     MS3=RB1, RB2 (new buffer) 
     MS4=WB1S, TE (normal termination) 
     Abbreviations 
     MS--Memory Sequence 
     WB1S--Write BALE 1st halfword (start) 
     RB2--read BALE 2nd halfword 
     RD--read data 
     WD--write data 
     TE--term entry (normal) 
     TER--term entry (error) 
     FIGS. 5-16 disclose flow diagrams for ICC operation for transmit and receive functions of the ICC when using ADCCP procedures. It should be noted that the ICC is a bit-oriented unit. For every bit that is transferred to the Device Port the ICC must perform some function and make some decision. The following Tables delineate the various memory and processing states and their affiliated mnemonics, diagram key and code. This information will be useful in understanding the ICC Flow Diagrams. 
     
                       TABLE V                                                     
______________________________________                                    
RECEIVE MEMORY PROCESSING STATE CODES                                     
State Reason  Mnemonic  Diagram Key                                       
                                   Code 5432                              
______________________________________                                    
Read - Bale (2nd Hw)                                                      
              MS11      A          1000                                   
Read - Bale (1st Hw)                                                      
              MS12      B          1001                                   
Write - Bale (2nd Hw)                                                     
              MS2       D          1010                                   
Read - Bale (2nd Hw)                                                      
              MS3       E          1011                                   
Write - Bale (2nd Hw)                                                     
              MS4       F          1100                                   
Bale (1st Hw) RBO       G          1101                                   
Write - Normal TE                                                         
              Te        J          1110                                   
Write - Error TE                                                          
              TER       K          1111                                   
Data Access   MS13      C          0001                                   
Write - Bale (2nd Hw)                                                     
              WB1S      I          0010                                   
______________________________________                                    
 
    
     
                       TABLE VI                                                    
______________________________________                                    
TRANSIT MEMORY PROCESSING STATE CODES                                     
State Reason  Mnemonic  Diagram Key                                       
                                   Code 432                               
______________________________________                                    
Read - Bale (1st Hw)                                                      
              MS11      A          001                                    
Data          MS12      B          010                                    
Write - Error TE                                                          
              MS21      C          011                                    
Write - Normal TE                                                         
              MS3       D          100                                    
Bale (2nd Hw) RB1       E          101                                    
Bale (1st Hw) RBO       F          110                                    
Write - Bale (2nd Hw)                                                     
              WB1S      H          111                                    
______________________________________                                    
 
    
     
                       TABLE VII                                                   
______________________________________                                    
RECEIVE CHARACTER PROCESSING STATE CODES                                  
State Reason   Diagram Key                                                
                          Code 321                                        
______________________________________                                    
Idle           A          000                                             
Flag           B          001                                             
Data           C          010                                             
______________________________________                                    
 
    
     
                       TABLE VIII                                                  
______________________________________                                    
TRANSMIT CHARACTER PROCESSING STATE CODES                                 
State Reason   Diagram Key                                                
                          Code 321                                        
______________________________________                                    
Data           A          010                                             
CRC            B          011                                             
E Flag         C          000                                             
Flag           D          001                                             
Abort          E          100                                             
______________________________________                                    
 
    
     While the invention has been described in its presently preferred embodiment it is understood that the words which have been used are words of description rather than words of limitation and that changes within the purview of the appended claims may be made without departing from the scope and spirit of the invention in its broader aspects.