Patent Publication Number: US-4254473-A

Title: Rack adapter for serially connected I/O interface racks

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The field of the invention is digital controllers, and more particularly programmable controllers in which I/O interface racks are remotely located from the central processor of the controller to control industrial machines and processes. 
     2. Description of the Prior Art 
     A programmable controller with a central processing station and remotely located interface racks is disclosed in Markley et al, U.S. Pat. No. 3,997,879, issued Dec. 14, 1976, and entitled &#34;Fault Processor for Programmable Controller with Remote I/O Interface Racks.&#34; The controller includes a central processor, a memory, and an I/O scanner which are interconnected with one another and located at a central processing station. I/O interface racks, such as disclosed in Springer et al, U.S. Pat. No. 3,992,654, issued Nov. 21, 1977, are each connected by a separate cable to the I/O scanner circuit at the central processing station. Each I/O rack has a capacity of 128 input/output circuits which are arranged in eight rack slots having sixteen I/O circuits each. Each I/O circuit can be monitored or controlled by a single logic voltage signal, which is generated in response to a single bit of data. With the I/O racks connected in &#34;parallel&#34; communication channels to the I/O scanner circuit, and with proper connection through distribution panels and to remote power supplies, it is possible to locate each of the eight I/O racks up to 5,000 feet from the central processing station. 
     Each communication channel in this prior system is formed by a cable, which is connected on its opposite ends to a pair of universal asynchronous receiver/transmitters (UAR/T&#39;s). There is one UAR/T in the I/O scanner for each communication channel, and there is one UAR/T in a rack adapter in each I/O rack. The UAR/T&#39;s couple bytes of data from the central processor to the I/O rack slots by converting the data to serial data for transmission through the communication channels and by reassembling the serial data into data bytes at the I/O rack adapters. 
     The programmable controller described above is contemplated for use in an industrial environment where electrical noise can produce erroneous transmissions over the transmission cables. It has been observed in industry that electrical noise generally produces only a single transmission error within the time represented by several scans of the I/O scanner, which is typically less than 0.1 second. Thus, in the system above, the detection of a major fault requires two error signals on a single scan, or on two successive scans of an I/O interface rack by the I/O scanner. If a major fault is detected, the operating devices on a machine controlled by the faulted rack are decontrolled. 
     The monitoring of transmission errors is performed at each end of the transmission cables by the I/O scanner circuit and the rack adapter circuits in each of the I/O interface racks. Besides its UAR/T, the rack adapter circuit at each I/O rack is comprised of discrete components such as storage registers, data selectors, flip-flops, gates, counters, and others, which are wired together to perform the logic functions required in monitoring transmission errors. 
     There is need for greater flexibility and efficiency in the application of programmable controllers to industrial control tasks. It is desirable in a programmable controller to be able to place I/O interface racks at greater distances from the central processing station. It is also desirable to subdivide I/O interface racks into partial racks that can be spaced apart to perform a number of control tasks, none of which require all of the 128 I/O control circuits that are available in a full I/O interface rack. 
     SUMMARY OF THE INVENTION 
     The invention is embodied in a rack adapter for a digital controller in which I/O interface racks are connected in series through two-way communication channels to a central processor means. The I/O interface racks are interconnected by two-way serial communication cables that connect to serial I/O data ports on the rack adapters. Such adapters enable a single central processor means to control a distributed network of I/O interface racks. The configuration of the network is flexible, so that different numbers and different sizes of I/O interface racks can be combined in stringing the racks together to form communication channels. 
     Data flows back and forth through the communication channel between the central processor means and the serially connected I/O interface racks, with intermediate I/O interface racks coupling data between the central processor means and the last I/O interface rack in the communication channel. Each rack adapter receives data from the central processor means through a first serial data input, and each rack adapter transmits serial data back towards the central processor means through a first serial data output. Each rack adapter, except the last, also receives serial data from a next I/O interface rack through a second serial data input and transmits data to the next I/O interface rack through a second serial data output. 
     The rack adapter of the invention has first logic circuit means for coupling data received from the central processor means to the second serial data output, so that serial data is relayed to the next I/O interface rack. The rack adapter also has second logic circuit means for coupling data received from the next I/O interface rack to the first serial data output, so that serial data is relayed back to the central processor means. The rack adapter also includes rack interface processor means which is connected to both serial data inputs and to both logic circuit means to control the flow of data through the communication channel. The rack interface processor means is also connected through an address bus and a data bus to the input/output circuits in its I/O interface rack to couple multibit data words between these circuits and the central processor means. The rack interface processor means is also connected to clock means, and is coupled to memory means to execute program instructions which are read sequentially from the memory means in response to timing signals received from the clock means. 
     The rack interface processor means is responsive to selected program instructions to perform many processing and control tasks. It receives bits of data, serially, through the first serial data input and enables the coupling of these bits through the first logic circuit means to the second serial data output. It couples multibit data words between groups of input/output circuits and the memory means on the rack adapter. It selectively couples serial data bits through the second logic circuit to the first serial data output, and it detects faults associated with the receipt and transmission of data bits through the communication channel. Still others of these processing functions will be described in the detailed description of the invention. 
     While providing for remote, serially connected, I/O interface racks, the invention retains and expands the fault-detecting capability of previous remote I/O rack adapters. The rack interface processor means is responsive to certain program instructions to detect faults associated with the coupling of data between its I/O interface rack and the next I/O interface rack. Termination sensing circuit means have also been provided to sense the proper connection of the rack adapter to a rack adapter on another I/O interface rack. 
     To provide further flexibility in the arrangement of I/O interface racks, the invention allows the serial connection of a group of I/O interface racks. In place of a single I/O rack of maximum capacity, a number of I/O racks of smaller capacity can be distributed to remote locations and connected to one another through a serial communication channel. The rack interface processor means is responsive to certain program instructions and the setting of a group of DIP switches to set up an absolute byte area in the memory means, where the address locations for the I/O modules in that particular I/O rack are identified. The rack interface processor means is then further responsive to still other program instructions to control the second logic circuit means and to select data from either its I/O interface rack, or the second serial data input, according to adddressed I/O location, to send back to the central processor means in the digital controller. 
     The invention also provides a clock and a flip-flop which are connected together to provide dual frequency clock means. This dual frequency clock means can be connected to the rack interface processor means to control the bit rate at which serial data is received and transmitted. Input circuits which can be switched to receive data at varying data bit rates are also provided for connection to the first and second serial data inputs. This allows the communication channel between the central processing station and an I/O interface rack to extend anywhere in the range of 0-10,000 feet. 
     It is one object of the invention to provide a digital controller with remote I/O interface racks that can be connected in series. 
     It is another object of the invention to provide a digital controller with remote, serially connected, I/O interface racks, each having a capacity which is less than the capacity of the largest I/O rack that can be employed in the controller. 
     It is another object of the invention to expand the fault detection capability of a rack adapter circuit. 
     It is another object of the invention to permit the extension of each serial communication channel to anywhere in the range of 0-10,000 feet. 
     The foregoing and other objects and advantages of the invention will appear from the following description. In the description reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is therefore made to the claims herein for interpreting the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a programmable controller which includes a rack adapter that incorporates the present invention; 
     FIG. 2 is a block diagram of the programmable controller of FIG. 1 with I/O interface racks connected in four communication channels; 
     FIG. 3 is a combination electrical schematic and block diagram of one of the rack adapter circuits of FIG. 1, each of which incorporates the present invention; 
     FIG. 4 is an electrical schematic diagram of an input circuit which forms a part of the rack adapter circuit of FIG. 3; 
     FIG. 5 is an electrical schematic diagram of a data shaper circuit which forms part of the rack adapter circuit of FIG. 3; 
     FIG. 6 is an electrical schematic diagram of a select data out circuit which forms part of the rack adapter circuit of FIG. 3; 
     FIG. 7 is an electrical schematic diagram of a termination sensing logic circuit which forms part of the rack adapter circuit of FIG. 3; 
     FIG. 8 is an electrical schematic diagram of a reset circuit which forms a part of the rack adapter circuit of FIG. 3; 
     FIGS. 9a-9h are flow charts of the program which controls the operation of the microprocessor that forms a part of the rack adapter circuit of FIG. 3; 
     FIG. 10 is a memory map diagram of the on-board RAM memory which is coupled to the microprocessor and which forms a part of the rack adapter circuit of FIG. 3; 
     FIG. 11 is a more detailed map diagram of the working registers of the memory in FIG. 10; and 
     FIG. 12 is a format diagram of an absolute data byte which is stored in selected registers of the memory mapped in FIG. 10. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a programmable controller which incorporates the present invention also includes a central memory 1, a central processor 2, and a remote I/O scanner circuit 3, which are located at a central processing station and coupled together for operation. This tripartite central processor means in the programmable controller is the same as that disclosed in Markley et al, U.S. Pat. No. 3,997,879, mentioned above. 
     The central memory 1 is a random-access read/write memory which stores words of eighteen bits each in 4,096 to 12,288 separately addressable lines depending on the size of the control program which is stored. Two bits of each word are used for parity checking and sixteen bits of each word are data bits. An output image table 4 is stored on the first 64 lines of the central memory 1 and is addressable with the octal addresses 000-077. An input image table 5 is stored on the next 64 lines of the central memory 1 and is addressable with the octal addresses 100-177. Preset and accumulated values of counters and timers are stored in a contiguous area 6 of the central memory 1, which has 128 lines or 384 lines or 896 lines that are addressable with the octal addresses 200-377 or 200-777 or 200-1777. The remaining area 7 of the control memory 1 contains a control program in which program instructions are addressable with octal addresses 400  or 1000 or 2000 or greater. 
     Selected data is read from the central memory 1 by applying the binary equivalent of the octal address of the line containing the data to a memory address bus 8, by applying a logic low voltage to a read/write line 9, and by applying a logic high voltage to the memory cycle line 10. Selected data is written into a selected line of the central memory 1 by applying the address of that line to the memory address bus 8, by applying a logic high voltage to the read/write line 9, and by applying a logic high voltage to the memory cycle line 10. The 16-bit data word appearing on the memory data bus 11 during a one-microsecond cycle time is written into the selected line of the central memory 1. 
     The control program is executed by the central processor 2 which is connected to the memory buses 8, 11 and to the control lines 9, 10. In response to clock pulses generated at a frequency of one megahertz by a polyphase clock 12 connected to it, the central processor 2 continuously and sequentially reads instructions of the control program from its program storage area 7 in the central memory 1. The central processor 2 decodes an operation code in each program instruction to carry out various control operations. One control operation is the examination of status bits in the input image table 5. Each of the status bits corresponds to a sensing device on a controlled machine such as a limit switch or a photoelectric cell. Another control operation is the setting of status bits in the output image table 4. Each of these status bits corresponds to a control device on the machine such as a motor starter or a solenoid. Output status bits are set according to the status of bits in the input image table 5 and the particular logical processing instructions contained in the control program. Further information on the operation of the central processor 2, the central memory 1, and the polyphase clock 12 is disclosed in Dummermuth, U.S. Pat. No. 3,942,158, issued Mar. 2, 1976, and entitled &#34;Programmable Logic Controller.&#34; 
     The I/O scanner circuit 3 is connected to the central processor 2 and to the central memory 1 through the memory address bus 8, the memory data bus 11, and the read/write line 9. It is also connected to the central processor 2 through an interrupt request line 13, a grant line 14, and a disable line 15. The I/O scanner circuit 3 periodically steals a memory cycle from the central processor 2. During this memory cycle the I/O scanner circuit 3 either writes a 16-bit data word into the input image table 5, or reads a 16-bit data word from the output image table 4. 
     The I/O scanner circuit 3 couples data between the image tables 4, 5 in the central memory 1 and four remotely located I/O interface racks 16-19 shown in FIG. 1. In U.S. Pat. No. 3,997,879, mentioned above, the prior I/O scanner circuit is connected to four I/O interface racks through four separate, shielded cables, each having four conductors. With this arrangement the I/O interface racks can be located up to 5,000 feet away from the central processing station. In FIG. 1 of the drawings herein, the four I/O interface racks 16-19 are connected by cables 20-23 of the same kind, but the I/O interface racks 16-19 are connected in series along a single communication channel 24 rather than in parallel. This communication channel 24 extends from the I/O scanner circuit 3 to the most remotely located interface rack 19, which can be located up to 10,000 feet from the central processing station. Three other communication channels 25-27 also extend from the I/O scanner circuit 3, and up to four separate I/O interface racks can be connected in series along each of these channels 25-27 to provide a maximum capacity of sixteen I/O interface racks in the controller. The I/O interface racks 16-19 are distributed so that each rack is positioned alongside a respective one of a corresponding number of controlled machines 28-31. 
     The interface racks 16-19 each contain four I/O modules, which are physically inserted in slots in their respective racks 16-19. The I/O modules are printed circuit boards having either eight input circuits (i.e., an input module) or eight output circuits (i.e., an output module). The output circuits, such as those disclosed in Struger et al, U.S. Pat. No. 3,745,546, issued July 10, 1973, are each connected to drive operating devices on an associated controlled machine 28-31, while the input circuits, such as those disclosed in Kiffmeyer, U.S. Pat. No. 3,643,115, issued Feb. 15, 1972, are each connected to sensing devices on an associated controlled machine 28-31. Thus, the I/O interface racks 16-19 have the capacity to monitor and control 128 separate devices on an associated controlled machine 28-31. Word-oriented I/O modules for controlling analog devices such as those described in Grudowski et al, U.S. Pat. No. 4,104,731, issued Aug. 1, 1978, can also be employed in the I/O racks 16-19. 
     For convenience of illustration in FIG. 1, the communication channel 24 loops back toward the central processing station, however, it should be understood that in many situations the I/O interface racks 16-19 are located at successively greater distances from the central processing station. In FIG. 2 an arrangement is shown wherein I/O interface racks 16-19 and 32-38 are distributed in all four of the communication channels 24-27 that extend from the I/O scanner 3. 
     Whereas, in U.S. Pat. No. 3,997,879, mentioned above, the I/O interface racks have sixteen slots, the present invention contemplates the use of a number of interface racks of smaller capacity in one or more of the serial communication channels 25-27. The rack slots from the racks in each communication channel are grouped together for addressing purposes, so they shall be identified by consecutive numbers &#34;0-15.&#34; The central processor 2, through the I/O scanner circuit 3, scans sixteen I/O slots in each communication channel 24-27, irrespective of the division of these slots among the interface racks in the channels 24-27. Thus, for example, in FIGS. 1 and 2, in the first communication channel 24, slots &#34;0-3&#34; are located in the first rack 16; slots &#34;4-7&#34; are located in the second rack 17; slots &#34;8-11&#34; are located in the third rack 18; and slots &#34;12-15&#34; are located in the fourth rack 19. As exemplified in FIG. 2, other combinations are possible, or if desired, a rack with up to sixteen I/O modules can be connected to any of the communication channels 24-27 in place of a serial group. 
     The present invention resides in the rack adapters 39-42 which are each physically mounted in a respective I/O interface rack 16-19 in FIG. 1, and which are each connected to the I/O modules in its respective rack 16-19 through a slot address bus 43 and an I/O data bus 44. Communication between the I/O scanner circuit 3 and the rack adapters 39-42 is accomplished by sending and receiving data bits serially through the transmission cables 20-23. Data in the output image table 4, which is stored in 16-bit words, is first divided into two eight-bit words or bytes, and is then converted to serial data that is transmitted from the I/O scanner circuit 3 to the I/O interface racks 16-19. The rack adapters 39-42 receive the serial data and couple bytes of &#34;parallel&#34; output data to the I/O modules in its own I/O interface rack. Input data is also received from each I/O module and is converted from bytes of parallel data to eight bits of serial data by the rack adapters 39-42. This serial data is then transmitted to the I/O scanner circuit 3, which assembles the data into 16-bit words for storage in the input image table 5. 
     The connection of the rack adapters 39-42 in the communication channel 24 is shown more particularly in FIG. 3, where two conductors in the first transmission cable 20 form an input line 20&#39; coming to the first rack adapter 39 from the I/O scanner circuit 3, and the other two conductors in that transmission cable 20 form an output line 20&#34; going to the I/O scanner circuit 3 from the first rack adapter 39. Similarly, two conductors in the second transmission cable 21 form an input line 21&#39; coming to the first rack adapter 39 from the second rack adapter 40, and the other two conductors in the second transmission cable 21 form an output line 21&#34; going to the second rack adapter 40 from the first rack adapter 39. 
     The serial data on input line 20&#39; coming from the central processing station is amplified, filtered and shaped before being applied to the rack adapter circuitry. More specifically, the input line 20&#39; is connected to a pair of inputs 45, 46 on a first input circuit 47. The first input circuit 47 in turn has a pair of outputs 48 and 49 that are connected to the inputs 50 and 51 on an optical coupling circuit 52 and to the inputs 53 and 54 on another optical coupling circuit 55. Outputs 56 and 57 on the respective coupling circuits 52 and 55 are connected to a pair of inputs 58 and 59 on a first data shaper circuit 60. The data shaper circuit 60 has a pair of tap terminals 61 and 62 the connection of which will be explained further below. The data shaper circuit 60 also has a single output 63 that is connected to the first of two inputs 64 and 65 on a receiving line NAND gate 66. The NAND gate 66 has an output 67 connected to an input 68 on a first driver circuit 69 and the line driver 69 has a pair of outputs 70 and 71 that are connected to the conductors of the output line 21&#34; going to the second rack adapter 40. 
     Similarly, the input line 21&#39; coming from the second rack adapter 40 is connected to a pair of inputs 72 and 73 on a second input circuit 74. The second input circuit 74 has a pair of outputs 75 and 76 that are connected to four inputs 77-80 on a second pair of optical coupling circuits 81 and 82 , in the same manner as described above for the first input circuit 47 and the first pair of optical coupling circuits 52 and 55. The second pair of optical coupling circuits 81 and 82 have a pair of outputs 83 and 84, respectively, that are connected to a pair of inputs 85 and 86, respectively, on a second data shaper circuit 87. This data shaper circuit 87 also has a pair of tap terminals 88 and 89, and it also has an output 90 which is connected to the first of three input terminals 91-93 on a select data out circuit 94. The select data out circuit 94 has an output 95 connected to an input 96 on a second line driver circuit 97. The second line driver circuit 97 has a pair of outputs 98 and 99 that are connected to the output line 20&#34; going back to the I/O scanner circuit 3. 
     This receiving and transmitting circuitry performs several functions. Input circuits 47 and 74 limit current and allow for the receiving of serial data at two different bit transmission rates. The optical coupling circuits 52, 55, 81 and 82 are standard coupling circuits which are listed in Appendix B, and which provide optical isolation between the input lines 20&#39;, 21&#39; and the communication interface circuitry to be described below. The data shaper circuits 60 and 87 each have a data latch as a primary component which shapes the bit pulses and further immunizes the rack adapter circuitry from electrical noise in the transmission cables 20 and 21. The line driver circuits 69 and 97 are standard circuits for amplifying data bit pulses that are transmitted serially through transmission lines. 
     As seen more particularly in FIG. 4, the first input circuit 47, which is like the second input circuit 74, has a first current-limiting resistor 100 and a first capacitor 101 connected in parallel between the first input 45 and a first intermediate junction point 102. The input circuit 47 also has a second current-limiting resistor 103 and a second capacitor 104 connected in parallel between the second input 46 and a second intermediate junction point 105. A first diode 106 has an anode connected to the first junction 102 and a cathode connected to a collector of an NPN transistor 107. The emitter of this NPN transistor 107 is connected to an emitter of a PNP transistor 108, and to the second junction 105. The PNP transistor 108 has its collector connected to the anode of a second diode 109, which has its cathode connected to the first output 48. The first junction point 102 is also directly connected to the anode of the second diode 109 and to the first output 48 to provide an alternate current path. A biasing resistor 110 is connected between the second junction 105 and a triple connection to the second output 49 and to the bases of the two transistors 107 and 108. A pair of shorting terminals 111 are connected to opposite sides of the biasing resistor 110. 
     The input circuit 47 in FIG. 4 is shown for operation with a 12,500 bits/second transmission rate, where the length of the communication channels 24-27 may extend to 10,000 feet. In this circuit a current flows through the biasing resistor 110 to cause both transistors 107 and 108 to conduct. To convert the circuit for operation at a 25,000 bit/second rate, the shorting terminals 111 are connected to short the base/emitter junctions of both transistors 107 and 108 and to prevent them from conducting any appreciable current. The capacitors 101 and 104 are open circuited or removed from the input circuit 47 leaving only the current-limiting resistors 100 and 103 in series with the incoming line 20&#39;. 
     The first data shaper circuit 60, which is like the second data shaper circuit 87, is seen in more detail in FIG. 5. The first input 58 is connected through an inverter 112 to a first input 113 on a first NAND gate 114, and is also directly connected to a first input 115 on a second NAND gate 116. The second input 59 is directly connected to a second input 117 on the first NAND gate 114, and is also connected through an inverter 118 to a second input 119 on the second NAND gate 116. An output 120 on the first NAND gate 114 is connected to a first input 121 on a third NAND gate 122 and an output 123 on the second NAND gate 116 is connected to a second input 124 on a fourth NAND gate 125. The tap terminals 61 and 62 are takan from the outputs of the first and second NAND gates 114 and 116. An output 126 on the third NAND gate 122 is connected to a first input 127 on the fourth NAND gate 125, and an output 128 on the fourth NAND gate 125 is connected to a second input 129 on the third NAND gate 122 to form a data latch that is connected to the output 63 of the data shaper circuit 60. 
     The data shaper circuit 60 gates a data bit from its input terminals 58 and 59 to the data latch NAND gates 122 and 125, where the bit is held until the next data bit is received. The latched data bit is then sent out through the output terminal 63. The transmission of bit pulses through the data shaper circuit 60 reestabishes the waveform shape of the bit pulse to counteract the effects of noise and distortion in the input line 20&#39;. The second data shaper circuit 87 operates in the same manner for data bit pulses received on the input line 21&#39; coming from the second rack adapter 33. 
     Referring again to FIG. 3, the output terminal 63 of the first data shaper circuit 60 is also connected to a T1 input on a rack interface processor 130. Data bits coming to the rack adapter 39 are received by the rack interface processor 130 at the T1 input, and are also coupled to the second transmission cable 21 through the receiving line NAND gate 66. The rack interface processor 130 includes the processing, control and other portions, except memory, of an Intel 8048 Microcomputer manufactured by the Intel Corporation, Santa Clara, California. The internal structure and operation of this microcomputer is described in the MCS-48® Microcomputers Users Manual, which was published by the Intel Corporation in 1976. Some of the information contained in this manual is repeated herein, however, the manual should be consulted for full details on the use and operation of this microcomputer. In the present invention, the rack interface processor 130 controls the flow of data between the central processing station and the I/O modules in the first I/O interface rack 16, and also controls the transmission of data between the other racks 17-19 and the central processing station. 
     The rack interface processor 130 also has a T0 input, an INT input, and positive and negative clock inputs 131 and 132. The T0 input is connected to the output terminal 90 of the second data shaper circuit 94 to monitor data coming from the next rack adapter 40 through the input line 21&#39;. The INT input controls the resetting of the rack interface processor 130, which will be explained more fully below. A six-megahertz external clock 133 has an output 134 connected to the two processor clock inputs 131 and 132 through alternate pairs of jumper terminals 135-136 and 137-138. An inverter 139 is connected in the line going to the negative clock input 132. A D-type flip-flop 140 has a clock input 141 connected to the output 134 of the external clock 133 and a D input 142 connected to a Q output 143. A Q output 144 of this flip-flop 140 is connected to a terminal 135 from the first pair of jumper terminals 135 and 136. 
     The connection of the D-type flip-flop 140 in the clock circuit provides for alternate timing signals for the rack interface processor 130 so that it can be adjusted to receive and transmit data at either of two bit transmission rates. The six-megahertz clock frequency is divided by three, by a state counter (not shown) in the rack interface processor 130, to obtain a frequency of two megahertz at a cycle time of 0.5 microseconds. A five-cycle counter (not shown) in the rack interface processor 130 counts five of these cycles to produce a 2.5 microsecond instruction period that is used by the rack interface processor 130, provided that the jumper terminals 137 and 138 are connected. If instead, however, the jumper terminals 135 and 136 are connected, and the jumper terminals 137 and 138 are not connected, a pulse is output from the Q output terminal 144 of the D-type flip-flop 140 for every two pulses that are input to its clock input terminal 141. This reduces the clock frequency received at the processor clock input terminals 131 and 132 to three megahertz and increases the instruction cycle to five microseconds. The six-megahertz timing signal is used in conjunction with the 25,000 bits/second transmission rate, while the three-megahertz timing signal is used in conjunction with the 12,500 bits/second transmission rate. 
     The rack interface processor 130 is coupled to an on-board programmable read-only memory (PROM) 145, which is another portion of the Intel 8048 microcomputer. The PROM 145 has a storage capacity of 1,000 eight-bit words or bytes and is mask programmable. The rack interface processor 130 is also coupled to an on-board random-access memory (RAM) 146, which is included in the Intel 8048 microcomputer. The random-access memory (RAM) 146 is arranged as sixty-four eight-bit registers. These registers provide a means for storing data during the execution of the control program, which is stored in the PROM 145. During execution of the control program, data is coupled between an accumulator (not shown) and certain registers in the RAM 146 called working registers. In addition data is coupled between the accumulator and three input/output ports P1, P2 and DB on the rack processor 130. Microprocessors with external program memories can, of course, also be used as the rack interface processor 130. For details of the internal coupling of the PROM 145 and the RAM 146 to the processing and control portions of the Intel 8048 microcomputer, reference is made to the Users Manual cited above. 
     The first I/O port, P1, on the rack interface processor 130, has eight terminals P10-P17 through which eight data bits are input or output. The least significant terminals P10-P13 are connected through a four-line slot address bus 147 to four inputs 148 on a four-line-to-sixteen-line decoder 149. The decoder 149 has sixteen outputs 150 for connection to sixteen corresponding slots &#34;0-15&#34; in the I/O interface rack 16 to form the slot enable bus 43. In this embodiment, outputs 150 would be connected only to slots &#34;0-3&#34; in the first I/O interface rack 16. The four-bit code generated at output terminals P10-P13 causes one of the sixteen leads in the slot enable bus 43 to go low, thereby enabling one of the sixteen I/O modules in the I/O interface rack. 
     Terminal P14 on the rack processor 130 is connected through a rack fault line 151 to a first input 152 on a rack fault NAND gate 153. This NAND gate 153 has a second input 154 which is connected through a pull-up resistor 155 to a +5-volt voltage source 156. An output 157 on the rack fault NAND gate 153 is connected through a light-emitting diode (LED) 158 and a pull-up resistor 159 to a +5-volt voltage source 160. The output 157 of the rack fault NAND gate 153 is also connected to an input 161 on a reset circuit 162, and an output 163 on the reset circuit 162 is connected to the INT input on the rack interface processor 130. A reset pushbutton 164 is connected to the reset circuit 162. A logic high voltage signal on the rack fault line 151 indicates a rack fault and causes a logic low voltage signal at the output 163 of the NAND gate 153. This signal activates the rack fault LED 158, which is part of a rack fault status indicator. 
     The rack fault indication signal is also applied to the reset circuit 160 which is shown in more detail in FIG. 8. The input 161 on the reset circuit 162 is connected through an inverter 165 to a first input 166 on a reset NAND gate 167 of the open-collector type. An output 168 on this NAND gate 167 is connected through a pull-up resistor 169 to a +5-volt voltage source 170 and a second input 171 on the reset NAND gate 167 is connected to a collector of an NPN transistor 172. This collector is also connected through a pull-up resistor 173 to a +5-volt voltage source 174. The emitter of the NPN transistor 172 is connected to a circuit ground and the base of the NPN transistor 172 is connected through two resistors 175 and 176 to the anode of a diode 177. A biasing resistor 178 is also connected between the base of the NPN transistor 172 and ground to provide a bias voltage across the base-emitter junction of the NPN transistor 172. A grounded capacitor 179 is connected to the junction of the two resistors 175 and 176. Connected to the grounded plate of this capacitor 179 is a resistor 180 that is connected at its other end to one side of the pushbutton 164. The other side of the pushbutton 164 is connected to the cathode of the diode 177. The anode of the diode 177 is also connected through a pull-up resistor 181 to a +15-volt voltage source 182. 
     To reset the rack interface processor 130 following the detection of a rack fault, a logic low voltage signal is applied to its INT input terminal by the NAND gate 167. One input of the NAND gate 167 is held high by the rack fault signal at input 161 and the other input 171 is driven high when the pushbutton 164 is depressed and then released. When the reset pushbutton 164 is depressed, the capacitor 179 discharges through the resistor 176, the diode 177, the reset pushbutton 164 and resistor 180 to ground. The NPN transistor 172 is thus turned off and its collector is pulled high to enable the NAND gate 167. Once reset is accomplished the output 163 of the reset circuit 162 goes to a logic high voltage due to the release of the pushbutton 164. 
     Returning to FIG. 3, terminal P15 on the rack processor 130 is connected through a slot enable line 183 and a pair of inverters 184 and 185 to a strobe input terminal 186 on the four-line-to-sixteen-line decoder 149. A second strobe input terminal 187 on the decoder 149 is grounded. Terminal P16 is connected through an output strobe line 188 to the slots &#34;0-3&#34; in the I/O interface rack 16, and terminal P17 is connected through a read/write line 189 and a second pair of inverters 190 and 191 to a first input 192 on a read NAND gate 193. The output of the inverter 184 is connected to a second input 194 on the read NAND gate 193, and is also connected to a first input 195 on a write NAND gate 196. The output of the inverter 190 is connected to a second input 197 on the write NAND gate 196. 
     The read NAND gate 193 has an output 198, and the write NAND gate 196 has an output 199, which are connected to enable read and write operations for the slots &#34;0-3&#34; in the I/O interface rack 16. When a logic low voltage signal is output on the slot enable line 183 and an address is present on the slot address bus 147, one of the slots &#34;0-3&#34; is enabled. If a logic high voltage signal is output on the read/write line 189, a logic low voltage signal is present at the output 198 of the read NAND gate 193, and data is read from the addressed slot &#34;0-3.&#34; During a read operation the output 199 of the write NAND gate 196 is at a logic high voltage. If, on the other hand, a logic low voltage signal is output on the read/write line 189, a logic low voltage signal is present at the output 199 of the write NAND gate 196, and data is written into the addressed slot &#34;0-3&#34; as signals are sent out on the output strobe line 188. During a write operation the output of the read NAND gate 193 is at a logic high voltage. 
     The rack interface processor 130 also has a second I/O port, P2, with eight terminals P20-P27 through which eight data bits are input or output. Terminal P20 is connected through a transmit data out line 200 to the second terminal 92 on the select data out circuit 94 and terminal P21 is connected to an &#34;active&#34; monitoring line 201. Terminal P22 is connected to an output reset line 202 that is connected to the I/O interface rack 16 to deenergize the I/O devices on the controlled machine by providing a logic high voltage reset pulse. Terminal P23 is connected to a module I/O line and terminal P24 is connected through a select data out line 204 to the third input 93 on the select data out circuit 94. 
     The select data out circuit 94 selects either serial data coming from the next I/O interface rack to its first input 91, or serial data coming from its closely coupled I/O interface rack 16 to its second input 92, for transmission back to the central processor means through its third input 93, which is connected to the select data out line 204. As seen in more detail in FIG. 6, the first input 91 is connected to a first input 205 on a first NAND gate 206. The second input 92 is connected to a first input 207 on a second NAND gate 208. The third input 93 is connected through a first inverter 209 to a second input 210 on the second NAND gate 208, and through inverters 209 and 211 to a second input 212 on the first NAND gate 206. The outputs 213 and 214 of the first and second NAND gates 206 and 208, respectively, are connected together in a &#34;wired OR&#34; connection. The NAND gates 206 and 208 are open collector gates. A pull-up resistor 215 (shown in phantom), included in the second line driver circuit 97 (not shown in FIG. 6), is connected to a +5-volt voltage source 216. When one of the NAND gates 206 or 208 is enabled, it outputs a logic low voltage signal, while the other NAND gate output remains at a logic high voltage. When a logic low voltage pulse is output on the select data out line 204 to the third input terminal 93 on the select data out circuit 94, the second NAND gate 208 is enabled and data from the first I/O interface rack is sent back to the central processor 2. When a logic high signal is output on the select data out line 204, the first NAND gate 206 is enabled and data coming from one of the other I/O interface racks 17-19 is sent back to the I/O scanner circuit 3. 
     While the flow of data back to the central processor 2 is controlled by the select data out circuit 94, the flow of data to the I/O interface racks 16-19 is controlled through the receiving line NAND gate 66. Referring again to FIG. 3, terminal P25 of the rack interface processor 130 is connected through a received data out line 217 to the second input 65 on the receiving line NAND gate 66. When a logic high voltage is maintained on the received data out line 217, the receiving line NAND gate 66 is enabled. When a logic low voltage is maintained, the receiving line NAND gate 66 is disabled, which prevents data coming from the central processor 2 from being transmitted further to more remotely located I/O interface racks 17-19. 
     Terminal P26 of the rack interface processor 130 is connected to an input module sensing line 218. This line is connected to those I/O rack slots &#34;0-3&#34; on the first rack 16 to sense whether or not an input module is present. A logic high voltage signal received by the rack interface processor 130 through this line 218 indicates that an input module is present in the enabled slot &#34;0-3,&#34; while a logic low voltage signal indicates the absence of an input module. Finally, terminal P27 is connected to the T1 input on the rack interface processor 130 to provide another input for receiving serial data bits coming from the I/O scanner circuit 3. 
     The respective functions of the terminals P10-P17 and P20-P27 in the first two I/O ports are summarized in Table 1 below for convenient reference. The I/O ports P1 and P2 are quasi-bidirectional ports in which input terminals and output terminals can be mixed. Single bits can be tested or output using certain program instructions as illustrated in Appendix A. 
     
                                           TABLE 1                                 
__________________________________________________________________________
Port                                                                      
   Bit                                                                    
     I/O     Description                                                  
                    High (1) Low (0)                                      
__________________________________________________________________________
1  0 O  1.sub.10    1        0                                            
1  1 O  2.sub.10    1        0                                            
             Slot Address                                                 
1  2 O  4.sub.10    1        0                                            
1  3 O  8.sub.10    1        0                                            
1  4 O  Rack fault indicator/reset                                        
                    On/PB inhibited                                       
                             Off/PB enabled                               
        pushbutton (PB) enable                                            
1  5 O  I/O enable  Inhibited                                             
                             Enabled                                      
1  6 O  Output module strobe                                              
                    Off      On                                           
1  7 O  Read/write  Read     Write                                        
2  0 O  Transmit data out                                                 
                    1        0                                            
2  1 I  Active indicator                                                  
                    Active   Not active                                   
2  2 O  Output reset                                                      
                    Reset    No reset                                     
2  3 O  Mod ID line Not Used                                              
2  4 O  Select data out                                                   
                    This rack                                             
                             Another rack                                 
2  5 O  Receiving line gate enable                                        
                    Enabled  Inhibited                                    
2  6 I  Input module present                                              
                    Present  Not present                                  
2  7 I  Receive data input                                                
                    1        0                                            
__________________________________________________________________________
 
    
     Referring again to FIG. 3, a third I/O port DB on the rack interface processor 130 has its eight terminals DB0-DB7 connected to an eight-line adapter I/O data bus 219. This I/O port is a true bidirectional port, however, all of its terminals must operate simultaneously as either input terminals or output terminals, depending upon how the port is enabled. The adapter I/O data bus 219 is connected through a first set of buffers 220 to the rack I/O data bus 44 in the first I/O interface rack 16 to couple bytes of data between the I/O modules and the rack interface processor 130. When a slot &#34;0-3&#34; is addressed through the slot address bus 147, and when appropriate signals are transmitted on the read/write line 189, the first set of buffers 220 is enabled through a first enable line 221 and through a second I/O enable line 222 that is connected to the slot enable line 183. With the buffers 220 enabled, data is coupled between the I/O modules and the adapter I/O data bus 219. 
     The adapter I/O data bus 219 is also connected through a second set of buffers 223 to eight switches in a DIP switch circuit 224. The setting of certain DIP switches 224 signals the position of the I/O interface rack and the slots which are occupied by I/O modules. The rack interface processor 130 uses this information in setting up for communication with the central processor 2 and with the other I/O interface racks 17-19. The second set of buffers 223 is connected through the DIP switch enable line 188&#39; to the output of the inverter 184. When a voltage pulse is applied to the slot enable line 183, the second set of buffers 223 is enabled and the status of the DIP switches 224 can be read into the rack processor port DB. The DIP switches are listed below with the last number in each switch label corresponding to a respective DB port terminal that each is connected to. Also listed are the type of information recorded by each respective switch and the status indicated by an &#34;on&#34; or &#34;off&#34; setting for each switch. switch. 
     
                       TABLE 2                                                     
______________________________________                                    
Switch Description     Off         On                                     
______________________________________                                    
DSW 0  Not used (spare).                                                  
DSW 1  Last rack indicator.                                               
                       Last rack.  Not the last                           
                                   rack.                                  
DSW 2  Starting byte   10.sub.2    00.sub.2                               
       address, lower order.                                              
DSW 3  Starting byte   100.sub.2   00.sub.2                               
       address, higher order.                                             
DSW 4  Last state indicator.                                              
                       Deenergize  Retain I/O                             
                       I/O devices.                                       
                                   devices in                             
                                   last state.                            
DWS 5  Processor restart                                                  
                       Locked out. Not locked                             
       lockout.                    out.                                   
DSW 6  Rack size, lower                                                   
                       8.sub.10    4.sub.10                               
       order.                                                             
DSW 7  Rack size, higher                                                  
                       8.sub.10    0                                      
       order.                                                             
______________________________________                                    
 
    
     A termination sensing circuit 225 insures that the rack adapter 39 is properly connected in the communication channel 24. The last rack DIP switch DSW 1 is connected through a last rack line 226 to a single input 227 on the termination sensing circuit 225. The tap terminals 61 and 62 on the first data shaper circuit 60 are connected to a first pair of inputs 228 and 229 on the termination sensing circuit 225, and the tap terminals 88 and 89 on the second data shaper circuit 87 are connected to a second pair of inputs 230 and 231 on the termination sensing circuit 225. The output 232 of the termination sensing circuit 225 is connected through a light-emitting diode (LED) 233 and a resistor 234 to a +5-volt voltage source 235. The LED 233 is part of a status indicator that is illuminated when the output of the termination sensing circuit 225 is pulled low. This is an indication that the input lines 20&#39;, 21&#39; are properly connected and that the last rack DIP switch DSW 1 is properly set. The active monitoring line 201 is also connected to the output 232 of the termination sensing circuit 225 to report this result to the rack interface processor 130. 
     As shown in more detail in FIG. 7 the first pair of inputs 228 and 229 on the termination sensing circuit 225 are connected through a first NAND gate 236 to a first input 237 on a second NAND gate 238. The single input 227 is connected to one input 239 on an exclusive OR gate 240, and the second pair of inputs 230 and 231 are connected through a third NAND gate 241 to another input 242 on the exclusive OR gate 240. An output 243 on the exclusive OR gate 240 drives a second input 244 on the second NAND gate 238, and an output 245 on the second NAND gate 238 drives the output 232 of the termination sensing circuit 225. 
     Unlike much of the circuitry described above, the termination sensing circuit is not controlled by the rack interface processor 130, but instead, directly senses the status of the two incoming transmission lines 20&#39;, 21&#39;. The proper connection of the input line 20&#39; coming from the I/O scanner circuit 3 produces logic low voltage signals at the inputs 228 and 229, which in turn produces a logic high voltage signal at the first input 237 of the second NAND gate 238. The exclusive OR gate 240 generates a second logic high voltage signal when the last rack line 226 is carrying a logic low voltage signal (signaling that another rack is connected to the input line 21&#39;) and a pair of logic low voltage signals are present at the inputs 230 and 231 of the third NAND gate 241 or, when the last rack line 226 is carrying a logic high voltage signal (signaling that this is the last rack), and a pair of logic high voltage signals are present at the inputs 230 and 231 to the third NAND gate 241. The lines in the data shaper circuits 60, 87 switch between high and low when the lines are properly connected, and the termination circuit output switches between high and low logic voltage levels at the frequency of the bit transmission rate, which results in the illumination of the LED 233. 
     Having described the circuitry in one of the rack adapters 39-42, the manner of communication between the central processor 2 and the rack adapters 39-42 shall now be described. Referring again to FIG. 1, the I/O scanner circuit 3 scans the I/O interface racks 16-19 in the first communication channel during a 10.08 millisecond period in which it interrupts the execution of the control program being executed by the central processor 2. This scan time is divided into eighteen byte transmission times. During the first sixteen byte transmission times input and output data bytes are transferred between the image tables 4, 5 in the central memory 1 and the I/O modules coupled through the rack adapters 39-42. During the seventeenth byte transmission time the rack adapters 39-42 each transmit a status byte through the communication channel 24 towards the I/O scanner circuit 3 to report any occurrence of errors in the transmission of data. The eighteenth byte transmission time is a delay period, sometimes referred to as a dwell period, which allows the rack adapters 39-42 to be resynchronized with the I/O scanner circuit 3. If I/O interface racks are connected to more than one of the communication channels 24-27, the byte transmission times are staggered for the respective channels to stagger the interruptions of the central processor 2. 
     Each of the eighteen transmission times is divided into fourteen bit transmission times. The present invention contemplates at least two different bit transmission rates: 25,000 bits/second and 12,500 bits/second. If a bit transmission rate of 25,000 bits/second is selected, each bit transmission time becomes forty microseconds. If the scanner I/O scanner circuit 3 and the rack adapters 39-42 are switched to operate with a bit transmission rate of 12,500 bits/second, then each bit transmission time will be eighty microseconds and the total scan time will be increased to 20.16 milliseconds. The lower bit transmission rate allows the extension of each of the communication channels 24-27 from 5,000 feet to 10,000 feet. 
     The rack interface processor 130 sequentially fetches program instructions stored in its PROM 145 and executes them to communicate with the central processor 2 and to couple data between the central processor 2 and the other I/O interface racks 17-19. The listing of these porgram instructions is contained in Appendix A and a definition and description of the individual instructions in this listing is provided in the Users Manual referred to above. It should be noted that the operation code mnemonics in the instruction set in Appendix A do not exactly coincide with those given in the Users Manual, however, the instructions in Appendix A can nevertheless be correlated with those in the Users Manual. The hexadecimal equivalent of each instruction is listed in the &#34;Contents&#34; column in Appendix A. The hexadecimal instruction codes are listed in the Users Manual and the hexadecimal codes can be converted to the binary codes given in the Users Manual to completely identify and correlate instructions from the two sets. 
     Many of the program instructions move data between the accumulator (not shown) and registers in the RAM 146. As seen in FIG. 10, the RAM 146 includes 64 eight-bit registers. The first eight of these registers are designated register bank B0, and another bank of working registers, referred to as register bank B1, includes registers at addresses 18-1F (hexadecimal). Between the two banks of registers, B0 and B1, there is an eight-level stack of register pairs, in which the program counter can be stored during subroutine calls. A stack pointer (not shown), which is coupled to this stack of registers, points to a register pair in the stack to store the program counter. The pointer advances to store succeeding return addresses in higher levels in the stack as successive subroutines are nested. Registers with addresses 20-2F (hexadecimal) are available in the remaining area of the RAM 146 to form an absolute byte storage area 246. In a particular program to be described, registers 20-2F are used to store information on the status of slots 0-15 in the I/O interface rack 16, whether or not the slots are all active, i.e., occupied by I/O modules. This status information is stored in &#34;absolute bytes&#34; of data in registers 20-2F of the absolute byte area 246. 
     A diagram of an absolute byte of data 247 is shown in FIG. 12. Bits &#34;0-3&#34; contain the slot address in binary form, i.e., 0000-1111. Bit &#34;4&#34; is set to a logic high level in two adjoining absolute data bytes when at least one slot in an adjoining pair of slots has an I/O module present. This is necessary to maintain the status of the eight-bit I/O modules in the image tables 4 and 5 of the central memory 1, which store data words that are sixteen bits wide. Bit &#34;5&#34; indicates the state of the select data out line 204 and determines whether data from this rack 16 or from the other I/O interface racks 17-19, is to be transmitted back to the central processor 2. A logic &#34;0&#34; selects data from this rack 16; a logic &#34;1&#34; selects data from one of the other racks 17-19. Bit &#34;6&#34; stores an error in transmission detected on a previous I/O scan, which is indicated by a logic &#34; 1.&#34; Bit &#34;7&#34; indicates that the slot address in bits &#34;0-3&#34; is in this rack when a logic &#34;1&#34; is present, and that the slot address in bits &#34;0-3&#34; is not in this rack when a logic &#34;0&#34; is present. 
     It is convenient to indirectly address the registers in the absolute byte area 246 through pointer registers B0R0 and B1R0 in the working register banks B0 and B1. The other registers in banks 0 and 1 are shown in FIG. 11. B0R0 is a pointer register that is used as an absolute byte counter which can be incremented or decremented as the registers in the absolute byte area 246 are sequentially addressed. Registers B0R2 maintains the status of I/0 port P2 and B0R3 is not used. B0R4 is a register which maintains the status of I/O port P1. B0R6 is a starting byte address register that maintains the address of the first active slot in the I/O interface rack 16, and B0R5 is an ending byte address register that maintains the address of the last active slot in the rack 17. Register B0R7 temporarily stores the status of the DIP switches DSW 0-DSW 7. 
     Of the other registers in bank 1, B1R1 is a pointer register which is set to point to register B0R4. Register B1R2 maintains data to be output through the second I/O port P2. Register B1R3 stores data to be transmitted back to the central processor 2, and register B1R4 stores data received from the central processor 2. Register B1R5 stores the parity of data in B1R3 as it is transmitted back to the central processor 2. Register B1R6 stores a serial data out mask, and B1R7 is a general purpose register. 
     Referring now to FIGS. 1, 3, 10 and 11, the rack interface processor 130, under program control, prepares for communication with the central processor 2 by loading initial values into its I/O ports P1 and P2. A logic high voltage signal is sent out through terminal P25 to enable the receiving line NAND gate 66. The rack interface processor 130 then examines slot &#34;0&#34; in its I/O interface rack 16 to determine whether an input I/O module is present. If an input module is present, data is read into register B1R3 in the RAM 146. Then, the rack interface processor 130 is synchronized with the central processor 2 by delaying until a start bit is received at its T1 input to begin the first byte transmission in the I/O scan. 
     After the start bit is received and several system checks, the byte transmission occurs with the rack interface processor 130 alternately sending and receiving each of eight I/O data bits. Eight data bits are transmitted from register B1R2 of the RAM 146 through the second I/O port P2, the transmit data out line 204, the select data out circuit 94 and the first output line 20&#34;. Eight data bits are received in register B1R4 of the RAM 146 through the first input line 20&#39;, the receiving circuitry, and the T1 input. Thus, in each byte transmission time, one byte of data is received from the central processor 2, and one byte of data is transmitted through the communication channel 24 towards the central processor 2. 
     During the I/O data bit transmission times, the rack adapters 39-42 in FIG. 1 receive data bits nearly simultaneously from the central processor 2, because the receiving line NAND gate 66 (see FIG. 3) in each rack adapter 39-42 is enabled. The transmission of I/O data bits back to the central processor 2 from the I/O interface racks 39-42 is controlled by the select data out circuit 94 (see FIG. 7) in each rack adapter 39-42, which in turn is controlled by the rack interface processor 130. 
     As the I/O data bits are transmitted and received, the parity of the transmitted data bits is maintained in register B1R5 of the RAM 146. After receipt of the eighth I/O data bit in each byte transmission time, a parity bit is transmitted from the P2 output register B1R2. A parity bit for the received I/O data byte is then received by the rack interface processor 130 from the central processor 2. The rack interface processor 130 then transmits a first stop bit before examining stop bits received from the central processor 2, and from adapters 40-42 in the other I/O interface racks 17-19, for transmission errors. The rack interface processor 130 does not, however, write the data received in register B1R4 into slot &#34;0&#34; of its I/O interface rack 16, until the data has been examined for errors. 
     The rack interface processor 130 checks data coming from both the central processor 2, and from other I/O interface racks 16-19 for two valid stop bits following the parity bit. A rack fault is indicated by two errors in the same I/O scan for the addressed slot, or two errors in successive I/O scans of the addressed slot. An error is indicated by a parity error or an invalid stop bit. If a single error is detected, the rack interface processor 130 stores this result in the corresponding register with address &#34;20&#34; in the absolute byte area 246 of the RAM 146. In this case, the data in register B1R4 is not written into the addressed slot &#34;0.&#34; If a rack fault is detected, the rack interface processor 130 turns on the rack fault LED 158 and examines DIP switch DSW 4. According to the setting of the switch DSW 4 (see Table 2 above) the rack interface processor 130 either deenergizes the control devices on its associated controlled machine 28 or allows them to remain in their last state. The rack interface processor 130 then delays over a period sufficient to signal the I/O scanner circuit 3 that a fault has occurred. The reaction of the I/O scanner circuit 3 to a fault is explained in U.S. Pat. No. 3,997,879, cited above. The rack interface processor 130 then examines DIP switch DSW 5 and is locked out or not locked out from communicating with the I/O scanner circuit 3 according to the switch setting (see Table 2 above). When locked out, the rack interface processor 130 can be restarted by depressing the reset pushbutton 164. 
     If no transmission errors are detected during the byte transmission time, the rack interface processor 130 writes the received data in register B1R4 into the addressed slot and proceeds to the next byte transmission, to couple data between the I/O module in slot &#34;1&#34; to the central processor 2. At the end of sixteen byte transmissions, in which no fault has been detected, the rack interface processor 130 generates and transmits a status byte indicating a &#34;no fault&#34; condition to the central processor 2. The rack interface processor 130 then dwells for one transmission time, and then executes a set of instructions to prepare for the first byte transmission in the next I/O interrupt scan, which will commence when a start bit is received from the central processor 2. 
     The program listed in Appendix A will now be described in more detail with reference to the flow charts 9a-9h and FIGS. 1, 3, 10 and 11. Referring to FIG. 9a, after the start of the program indicated in block 248, I/O ports P1 and P2 are initialized by a set of instructions indicated by process block 249. This initialization enables the receiving line NANE gate 66. The DIP switches DSW 0-DSW 7 are then read as indicated by I/O block 250, and their value is stored in register B0R7. Next, the active bit present at terminal P16 is tested as indicated by decision block 251. If a logic high is present, the rack adapter 39 is not active due to the improper termination of its input lines 20&#39;, 21&#39;, and the rack interface processor 130 remains in a program loop 252 until the condition is corrected. The rack fault indicating LED 158 is turned on for an instant at the beginning of the program, and if the rack processor 130 enters this program loop 252, the rack fault indicator stays on. If a logic low voltage is present at terminal P16, the program continues and the rack fault indicator is turned off. 
     A rack size routine is then executed as indicated by process block 253 and the number of occupied slots in the I/O interface rack 16 is determined from the information read from the DIP switches DSW 0-DSW 7. The rack size information is provided by DIP switches DSW 6 and DSW 7. The rack size information can also be provided via circuits on the I/O interface rack 16 itself, which are preselected according to rack size (for racks with 4, 8, 12 or 16 slots) and which are connected to terminals DB6 and DB7 through the buffers 223. As indicated by process block 254, the addresses of all the slots in the I/O interface rack 16 are written into registers 20-2F of the absolute byte area 246, and a binary &#34;1&#34; is written into bit &#34;7&#34; of the bytes with addresses corresponding to occupied slots in the I/O interface rack 16. Bytes corresponding to empty slots contain a binary &#34;0&#34; in bit &#34;7.&#34; 
     Following this, a routine indicated by process block 255 is called up to set bit &#34;4&#34; in certain absolute byte registers 20-2F to prepare for transmissions when only one of two adjoining input modules is present. When this routine has been completed, the program enters a delay routine, indicated by process block 256, of 25.6 milliseconds which extends through the status byte transmission in two I/O scan times. This is a signal to the central processor 2 that the rack adapter 39 is ready to begin receiving and transmitting data. 
     The rack processor 130 then executes a REST portion of the program, indicated by process block 257, in which the registers in the second register bank, B1, are initialized. A PST portion of the program follows in which bit &#34;7&#34; of the absolute byte in register 20 is tested, as indicated by decision block 258, to determine whether the first slot is located in this I/O interface rack 16. For the controller shown in FIG. 1 the answer in the first rack adapter 39 will be in the affirmative, while the answer in the other rack adapters 40-42 will be negative. The rack interface processor 130 in the first rack adapter 39 then tests whether the I/O module in the first slot, &#34;0&#34; is an input module, as indicated in decision block 259, while the rack interface processors in the other rack adapters 40-42 call up a delay routine 260. If an input module is present in the first I/O interface rack 16, a read operation is performed, as indicated by I/O block 261 to read the input data into register B1R3. If an input module is not present or if no I/O module is present, as in the case of the other rack adapters 40-42, register B1R3 is loaded with zeros in all eight bits, as indicated by process block 262, so that all ones are transmitted when no input module is present. A &#34;1&#34; transmitted on the select data out line 204 will enable serial data coming through input line 21&#39; from the other I/O interface racks 17-19 to be coupled to the output line 20&#34; going towards the central processor 2. After this, a short process block of instructions 263 is executed to prepare the received data register B1R4 and to load the select data out mask into register B1R6. A test instruction in decision block 264 generates a single instruction program loop 265, where the rack interface processor 130 remains until a start bit is received. 
     The rack interface processor 130 then executes the main portion of the control program for each byte transmission time, which consists of a start bit time, eight data bit times, a parity bit time and four stop bit times. Each bit transmission time is 40 microseconds. At the 25,000 bits/second serial data rate a single-cycle program instruction requires 2.5 microseconds for execution. Therefore, this part of the program limits the number of machine instructions to execute the receipt and transmission of a single bit to sixteen single-cycle machine instructions or less. 
     During each byte transmission the rack interface processor 130, under program control, checks for faults in the transmission. One type of fault is a complete loss of data transmission. This type of fault can be tested by starting a timer at one point in the program and exiting to a fault routine ER1 unless a certain instruction in the program is reached to stop the timer. If the program is being executed in normal fashion, this instruction will be reached. If the timer times out the interrupt portion of the main program is ended, and the program is restarted at block 248. Another type of fault is indicated when a logic high signal is received on the active monitoring line 201, to indicate that the rack adapter 39 is not active. The active status of the rack adapter 39 is checked during the status byte transmission as well as during the sixteen data byte transmission times. Another type of fault occurs when the start bit that is received is not a valid start bit but an erroneous transmission pulse. A fault is also indicated when two consecutive logic low stop bits are received from the central processor 2. A fault is also indicated when a single low stop bit is received from the central processor 2, and an error has been recorded in the previous I/O scan for the same addressed I/O slot. Still another source of a fault is two low stop bits received at the T0 monitoring input, which is connected to monitor data received through the input line 21&#39; coming from the next rack adapter 40. 
     Referring now to FIG. 9b, and FIGS. 1, 3, 10 and 11, the start bit transmission portion of the program, SBT, begins with an instruction in block 266 to stop the transmission loss timer, which is started near the end of each I/O scan during the dwell transmission time. After a transmit start bit instruction in an I/O process block 267 is executed, the received start bit is checked to determine if it is a valid start bit as indicated in decision block 269. If the received start bit is not valid, the rack interface processor 130 exits to a fault routine ER2. If the received start bit is valid, the rack interface processor 130 determines, as indicated by another decision block 270, whether the rack adapter module is active, i.e., properly connected to the input lines 20&#39;, 21&#39;. If the rack adapter module is not active, the rack interface processor 130 exits to another fault routine ER3. Otherwise the rack interface processor 130 continues, as indicated by process block 271, where the first data bit is readied in register B1R2 for output through the second port P2. 
     The first data bit is then transmitted through the transmit data out line 200 to the select data out circuit 94, which is indicated by I/O block 272. The parity register B1R5 is updated, as indicated by process block 273, and the contents of the transmit data out register B1R3 are moved to the accumulator in preparing the next bit for transmission. The first data bit from the central processor 2 is received at the input T1, as indicated by decision block 274, where it is tested to determine whether it is a binary &#34;0&#34; or a binary &#34;1.&#34; If it is a binary &#34;0,&#34; the received data bit is stored in the received data register B1R4, as indicated by process block 275. If the received data bit is a binary &#34;1,&#34; the rack interface processor 130 branches, as indicated by process block 276, to update the parity flag F0 and to then store the received data bit in the received data register B1R4. The next data bit is then prepared for transmission, as indicated in process block 277, by rotating the data byte which is being held in the accumulator, one bit to the right, by masking the bit to be transmitted, and then storing that bit in the second port register B1R2. The serial data out mask is stored in register B1R6. Blocks 271-277 are then repeated for bits &#34;2-7&#34; of the received data byte and the transmitted data byte during the byte transmission time. 
     Referring to FIG. 9c, and FIGS. 1, 3, 10 and 11, the eighth data bit is then transmitted, as indicated in block 278, and the parity bit is readied for transmission, as indicated by process block 279, in the P2 output register B1R2. The eighth bit of data received from the central processor 2 is tested, as indicated by decision block 280, to determine if it is a binary &#34;1&#34; or a binary &#34;0.&#34; If it is a binary &#34;0&#34; it is stored, as shown by process block 281, in the received data register B1R4. If it is a binary &#34;1,&#34; the rack interface processor 130 branches to a process block 282, where the parity flag F0 is updated before the bit is stored in the receive data register B1R4. As represented in process block 283, the parity stored in register B1R5 is transferred into the accumulator, and the parity and transmit data out registers, B1R3 and B1R5, respectively, are cleared. 
     Eight data bits have been received and transmitted during eight bit transmission times. The rack interface processor 130 then executes the following blocks of instructions during a parity bit time and four stop bit times. Although there are four stop bit times, only two stop bits are tested during these four bit times. 
     Referring to FIG. 9d, and FIGS. 1, 3, and 10-12, the parity bit is then transmitted back to the central processor 2, as indicated in I/O block 284. As shown by decision block 285, the parity bit received from the central processor 2 through the I/O scanner circuit 3 is tested for its binary identity. If it is a binary &#34;1,&#34; the parity flag bit F0 is updated, as indicated in process block 286, before the next decision block 287 is reached. If it is a binary 0, the rack interface processor 130 continues, as indicated by decision block 287, and determines whether the next absolute byte has a slot address that is within the range of this I/O interface rack 16. As shown in decision block 288, if the slot is not within the range of this I/O interface rack 16, the rack interface processor 130 tests the select data out bit, which is the third highest bit in the absolute byte 247. If this bit is a binary &#34;1,&#34; a stop bit is transmitted as shown in I/O block 289. The next process block 290 indicates that the current slot address is stored in register B1R7. If the select data out bit is a binary &#34;0,&#34; the rack interface processor 130 branches to store a &#34;0&#34; in bit &#34;4&#34; of the second port register B1R2, as shown in a following process block 291. The select data out line 201 will then be at a logic high voltage when serial data coming from other I/O racks 17-19 is to be selected over the contents of register B1R2. A stop bit is then transmitted as shown in I/O block 292. 
     On the other hand, if the slot in the next absolute byte is within the range of this I/O interface rack 16 in decision block 287, then it must be determined via another decision block 293 whether one of an adjoining pair of even and odd input module slots is empty. This must be determined because the I/O scanner circuit 3 couples 16-bit data words to the I/O image tables 4, 5 in the central memory 1. If bit &#34;4&#34; is a binary &#34;1,&#34; one of an adjoining pair of even and odd slots is empty, and yet a transmission must be made for the empty slot. A first stop bit is transmitted as shown in I/O block 294, and the addressed slot is enabled, as shown in process block 295, so that it can be determined, as shown in decision block 296, whether an input module is present. If the input module is not present, a second stop bit is stored in register B1R7, as shown in process block 297, and register B1R3 is loaded with all 0&#39;s, as shown in process block 298, so that all 1&#39;s are transmitted back towards the central processor 2. If an input module is present, a second stop bit is stored in register B1R7, as shown in process block 299, and data from the addressed slot is read into the accumulator and then stored in the transmit data out register B1R3, as shown in I/O block 300. 
     Returning to the decision block 293, if bit &#34;4&#34; of the absolute byte is a binary &#34;0,&#34; a stop bit is transmitted to the central processor 2 during I/O block 301, and the addressed slot is enabled, as shown in the next process block 302, to prepare for an input module test in decision block 303. If an input module is present, a second stop bit is stored in register B1R7, as represented by process block 299 and the input data is read from the slot, as shown in I/O block 300, before the rack interface processor 130 continues to process block 290. If an input module is not present in decision block 303, a second stop bit is stored in register B1R7, as shown in process block 304, and the rack interface processor 130 returns to execute process block 290. 
     After a first stop bit has been transmitted and after the slot address and a second stop bit have been stored in register B1R7, the rack interface processor 130 checks data coming from both the central processor 2, and the other I/O rack adapters 40-42 for two consecutive stops bits at a logic high voltage level. Referring to FIG. 9e, and FIGS. 1, 3, 10 and 11, data coming through the input line 21&#39; from the next rack adapter 40 is checked first at monitoring input T0, as shown in decision block 305. If a logic low voltage signal indicating a bad stop bit is detected, this result is stored in flag F1, as shown in process block 306. This flag was cleared previously during process block 268 in FIG. 9b. If a logic high signal is detected at input T0, the data received at input T1 from the central processor 2 is checked for a first stop bit in decision block 307. If a logic low voltage signal or binary &#34;0&#34; is detected at input T1, the rack interface processor 130 branches to test for a second low stop bit in decision block 308. Incoming data from the central processor 2 can be checked at terminal P27 which is connected to input T1 on the rack interface processor 130. If a second low stop bit is detected at this terminal P27, the rack interface processor 130 exits to a fault routine ER4 for this particular type of fault in transmission. 
     If a logic high voltage signal or binary &#34;1&#34; is detected at terminal P27 during decision block 308, the rack interface processor 130 determines, as shown in decision block 309, whether this is a second transmission error in two successive I/O scans. This is determined by testing bit &#34;6&#34; in the absolute byte for the slot being scanned. If bit &#34;6&#34; is a binary &#34;1&#34; in decision block 309, the rack interface processor 130 exits to a fault routine ER5 for a fault caused by two transmission errors for the same slot on two successive I/O scans. If bit &#34;6&#34; in the absolute byte is a binary &#34;0,&#34; bit &#34;6&#34; is set, as shown in process block 310 to record the transmission error in this I/O scan. The rack interface processor 130 then bypasses an I/O block 311 in which a byte of data received during this transmission time would be written into the slot being scanned. In this way, faulty data which is received from the central processor 2 is not allowed to affect the status of the output devices on the controlled machine 28. 
     Returning to decision block 307, if the first stop bit tested at input T1 is a good stop bit as indicated by a logic high voltage signal, a second stop bit is tested at terminal P27, as shown in decision block 312 in the same manner as in decision block 308. If this second stop bit is at a logic low voltage level, the rack interface processor 130 again branches to decision block 309 to determine whether an error was recorded on a previous I/O scan. Depending upon the result of that test, the rack interface processor 130 either exits to the fault routine ER5 or sets bit &#34;6&#34; of the absolute byte in process block 310. 
     If the stop bit tested in decision block 312 is a logic high voltage signal, two good stop bits have been received. It is still necessary, however, to test for a parity error in decision block 313. If a parity error is detected, the rack interface processor 130 again branches to decision block 309 to determine whether this is the first or second transmission error that has occurred. If there is no parity error detected in decision block 313, the data that has been received from the central processor and stored in the received data register B1R4 is written into the slot being scanned during I/O block 311. 
     Continuing in FIG. 9f, the absolute byte counter in register B1R0 is incremented, as shown in process block 314, and a second stop bit coming through input line 21&#39; on its way to the central processor 2 is tested, as shown by decision block 315. If a logic low voltage signal is received at the T0 input of the rack interface processor 130, the processor 130 next examines flag F1 for a previous error. If a binary &#34;1&#34; is stored in flag F1 in decision block 316, the processor 130 exits to a fault routine ER7 which corresponds to this type of fault in transmission between the I/O interface racks 16-19 and the central processor 2. If the second stop bit tested at input T0 is a logic high signal in decision block 315, or if flag F1 contains a binary 0 in decision block 313, the rack interface processor 130 proceeds, as shown in decision block 317, to test for sixteen byte transmissions. If sixteen byte transmissions have not been completed, the rack interface processor 130 loops back to begin the SBT portion of the control program. If sixteen byte transmissions have been completed, the processor 130 proceeds to the status byte transmission. 
     Referring to FIG. 9g and FIGS. 1, 3, 10 and 11, the status byte transmission time, SW, begins, as shown in process block 318, in which a status byte start bit is generated and prepared for transmission. As seen in I/O block 319, the status byte start bit is transmitted to the I/O scanner circuit 3. The status byte is then generated and stored in register B1R3, as represented by process block 320. If no errors have been recorded, a 04 (hexadecimal) is loaded into B1R3. This places a binary &#34;1&#34; in bit &#34;2&#34; and binary &#34;0&#34; in bit &#34;1&#34; of the status byte, which will indicate to the I/O scanner circuit 3 that no errors have been recorded. A decision block 321 follows in which the active monitoring line 201 is checked. If the I/O module is not active, as indicated by the signal on the active monitoring line 201, the rack interface processor 130 exits to a fault routine ER6 which corresponds to this particular kind of transmission fault. Otherwise, the program continues, as shown in process block 322, in which the first bit of the status byte is readied for transmission to the I/O scanner circuit 3. 
     The first status bit is transmitted, as shown in I/O block 323 and the second bit is readied for transmission, as represented in process block 324. The second bit in the status byte is then transmitted, as represented in I/O block 325. As indicated in process block 326, the parity of the transmitted bit is stored in register B1R5, the next bit is readied for transmission, and a delay is introduced to fill out the bit transmission time. After each bit is transmitted, a count is decremented by one and tested, as indicated by decision block 327, to determine whether seven bit transmissions have occurred. The rack interface processor 130 follows a program loop 328 until seven transmissions have occurred, and then proceeds to I/O block 329 where the eighth bit is transmitted. As indicated by process block 330, the status byte parity bit is readied for transmission and as indicated by I/O block 331 the parity bit is transmitted. A delay process block 332 follows the parity bit transmission and precedes I/O instruction block 333 in which a stop bit is transmitted to the I/O scanner circuit 3. The status byte transmission is concluded with a process block 334 in which the rack interface processor 130 delays until the end of the fourth stop bit time. 
     After the status byte transmission, the rack interface processor 130 enters a dwell transmission time, during which it executes a delay routine before its resynchronization to the central processor 2 at the beginning of each I/O scan interrupt. As represented by the dwell transmission process block 335, the rack processor 130 delays through the tenth bit time, and then resets the transmission loss timer and the absolute byte counter B0R0. The rack processor 130 then returns to the PST portion of the program which determines whether an input module is present and which performs certain other initial preparation, before receiving the start bit for the first byte transmission in the next I/O scan interrupt. This start bit must be received before the transmission loss timer times out and causes an exit to the first fault routine ER1. 
     Referring to FIG. 9h and FIGS. 1, 3, 10 and 11, the fault routines ER1-ER7 have a common portion ERR. In process blocks 336-342 for the respective routines ER1-ER7 the slot address and the type of fault detected are latched in the first port P1. Each of the routines ER1-ER7 then proceeds to the common portion labeled ERR. As indicated by the first process block 343 of that routine the reset pushbutton 164 is enabled and the rack fault LED 158 is turned on. The status of the DIP switches DSW 0-DSW 7 is then read, as indicated in I/O block 344 and switch DSW 4 is examined to see whether the output devices on the controlled machine 28 are to be deenergized, before they are decontrolled. As represented by decision block 345, if the last state DIP switch DSW 4 is set at a logic low or &#34;on,&#34; the devices on the controlled machine are to remain in their last state before a fault was detected, when the machine is decontrolled. If the last state switch DSW 4 is set at a logic high state or &#34;off,&#34; a reset strobe is applied, as represented by process block 346, to deenergize the control devices on the controlled machine 28. This is followed by a delay, represented by process block 347, to signal the I/O scanner circuit 3 that a fault has occurred. As indicated in a following I/O instruction block 348, the DIP switches are read again to examine the processor restart lockout DIP switch DSW 5. As indicated by decision block 349, if this switch is set at a logic high or &#34;off,&#34; the rack interface processor 130 enters a program loop 350, where it remains until the condition causing the fault is corrected. If the PRL switch DSW 2 is set at a logic low or &#34;on,&#34; the rack processor 130 is locked out, and it returns to the RST portion of the program to reinitialize registers and to restart the execution of the program instructions during the next I/O scan interrupt of the central processor 2. 
     
                                           APPENDIX A                              
__________________________________________________________________________
Program Listing                                                           
               Processor                                                  
Location                                                                  
     Contents                                                             
          Label                                                           
               Instruction                                                
                       Comments                                           
__________________________________________________________________________
000  00        NOOP    No operation.                                      
001  00        NOOP                                                       
002  00        NOOP                                                       
003  54        CALL2,RT                                                   
                       External interrupt                                 
004  63                comes here.                                        
005  04        JMP0,GO                                                    
006  09                                                                   
007  44        JMP2,TOUT                                                  
                       Timer/counter interrupt                            
008  66                comes here.                                        
009  35   GO:  DISTI   Disable timer interrupts.                          
00A  05        ENI     Enable external interrupts.                        
00B  C5        SLRB0   Select register bank 0.                            
00C  23        MAI(F0H)*                                                  
                       Initialize ports.                                  
00D  F0                                                                   
00E  39        OUTP1                                                      
00F  BC        MR4I(C0H)                                                  
                       Preset R4 to select B7, read                       
010  80                bus; B6, no mod strobe; B5                         
011  23        MAI(FAH)                                                   
                       back plane bus; B4, RF indi-                       
012  FA                cator off.                                         
013  3A        OUTP2                                                      
014  80        MXA0    Float I/O data bus to                              
                       read inputs.                                       
015  A5        CLRF1   Clear flags F0, F1.                                
016  85        CLRF0                                                      
017  08        INBS    Input DIP switch status.                           
018  37        CPLA    Input data and store in                            
019  AF        MR7A    B0R7 (buffers invert data).                        
01A  0A        INP2    Check for rack adapter                             
                       module active.                                     
01B  53        NAI(02H)                                                   
01C  02                                                                   
01D  32        JB1,GO  B1=1, rack adapter module                          
01E  09                not active, return to go;                          
                       B1=0, rack adapter module                          
                       is active, continue.                               
01F  99   ACT: NP1I,0EFH                                                  
                       Turn off rack fault                                
                       indicator.                                         
020  EF                                                                   
 *(F0H) is F0 in hexadecimal notation.                                    
 
    
     
         Determine Rack Size                                                       
021  23        MAI(20H)                                                   
                       Load oF(H) in registers                            
                       20 to 2F.                                          
022  20                                                                   
023  A9        MR1A                                                       
024  BD        MR5I(10H)                                                  
025  10                                                                   
026  A8        MR0A                                                       
027  23        MAI(0FH)                                                   
028  0F                                                                   
029  A0   CAR: M0A                                                        
02A  18        INCR0                                                      
02B  ED        DJNZ5,CAR                                                  
02C  29                                                                   
02D  FF        MAR7    Read and mask starting                             
                       byte address.                                      
02D  FF        MAR7    Read and mask starting byte                        
02E  53        NAI(0CH)                                                   
                       address bits 2 and 3 and load                      
02F  0C                into starting byte address                         
030  AE        MR6A    register B0R6.                                     
031  FF        MAR7                                                       
032  F2        JB7,RSA Branch if bit 7=1.                                 
033  3E                                                                   
034  D2        JB6,RSB Bit 7=0, branch if bit                             
                       6=1.                                               
035  3A                                                                   
036  23        MAI(04H)                                                   
                       Bit 6 and 7=0.                                     
037  04                                                                   
038  04        JMP0,EBA                                                   
                       Rack size = 32 I/O circuits                        
039  48                (4 bytes), go to EBA.                              
03A  23   RSB: MAI(08H)                                                   
                       Bit 7=0 bit 6=1.                                   
03B  08                                                                   
03C  04        JMP0,EBA                                                   
                       Rack size = 64 I/O circuits                        
03D  48                (8 bytes), go to EBA.                              
03E  D2   RSA: JB6,RSC Bit 7=1, branch if bit 6=1.                        
03F  44                                                                   
040  23        MAI(0CH)                                                   
                       Bit 7=1 bit 6=0.                                   
041  0C                                                                   
042  04        JMP0,EBA                                                   
                       Rack size = 96 I/O circuits                        
043  48                (12 bytes), go to EBA.                             
044  23   RSC: MAI,(10H)                                                  
                       Bit 7=1 bit 6=1.                                   
045  10                Rack size = 128 I/O circuits                       
                       (16 bytes), go to EBA.                             
046  00        NOOP                                                       
047  00        NOOP                                                       
048  AD   EBA: MRSA    Move the number of slots                           
                       in this rack into B0R5.                            
049  23        MAI(20H)                                                   
                       Load the correct registers                         
04A  20                of the absolute byte area                          
04B  6E        AAR6    with a 1 in bit 7 and with                         
04C  A8        MR0A    the correct slot address                           
04D  23        MAI(80H)                                                   
                       for this rack in bits 0-3.                         
04E  80                                                                   
04F  A0   CBR: MOA                                                        
050  18        1NCR0                                                      
051  17        INCA                                                       
052  ED        DJNZ5,CBR                                                  
053  4F                                                                   
054  23   CAS: MAI(2FH)                                                   
                       Load the remaining slots                           
055  2F                in absolute byte area with                         
056  A0        M0A     2F(H) to select the correct                        
057  F8        MAR0    transmit data out line.                            
058  D3        XOAI(30H)                                                  
059  30                                                                   
05A  18        INCR0                                                      
05B  96        JONZ,CAS                                                   
05C  54                                                                   
05D  54        CALL2,DT0                                                  
                       Call routine to set up                             
05E  6A                the absolute byte register                         
                       for sequential transmission                        
                       when only 1 input module is                        
                       present out of a pair of                           
                       input modules.                                     
               Delay Routine                                              
05F  B9        MR1I(0AH)                                                  
                       Increment the accumulator                          
060  0A                to 256 counts by                                   
061  27   CLA: CLRA    10 μ sec./sec. count @ 6 MHZ.                   
062  17   INA: INCA    then repeat 10 times.                              
063  00        NOOP                                                       
064  96        JONZ,INA                                                   
065  62                                                                   
066  E9        DJNZ1,CLA                                                  
                       Total dwell time equals                            
067  61                value in R1(10) times                              
                       2.56 msec., dwell complete.                        
068  BB        MR3I(0H)                                                   
                       Turn off rack fault indi-                          
069  00                cator during 0R operation                          
                       in PSI.                                            
               Continue Initialization                                    
06A  D5   REST:                                                           
               SLRB1   Change to register bank I.                         
06B  B8        MR0I(20H)                                                  
                       Load BIR0 W/20H.                                   
06C  20                                                                   
06D  B9        MR1I(04H)                                                  
                       Point B1R1 to B0R4.                                
06E  04                                                                   
06F  BA        MR2I(0FAH)                                                 
                       Initialize register B1R2.                          
070  FA                                                                   
071  99        NP1I(0BFH)                                                 
                       Disable DIP switch buffers.                        
072  BF                                                                   
073  F0   PST: MA0     Is slot in rack range?                             
074  F2        JB7,PS1 If yes, go to PS1.                                 
075  7A                                                                   
076  54        CALL2,D25                                                  
077  5C                                                                   
078  04        JMP0,MNP                                                   
                       If not, go to module not                           
079  84                present.                                           
07A  53   PS1: NAI(0FH)                                                   
                       Isolate slot enable address.                       
07B  0F                                                                   
07C  41        OA1     Generate slot enable.                              
07D  4B        OAR3    Set rack fault.                                    
07E  39        OUTP1                                                      
07F  0A        INP2    Check P2 for input module                          
                       present.                                           
080  43        OAI(82H)                                                   
                       Force receive data line                            
081  82                and active line true so                            
                       it will function as an                             
                       input.                                             
082  D2        JB6,MP1 If true, go to input module                        
083  8F                present MP1.                                       
084  FA   MNP: MAR2    Load desired port 2.                               
085  43        OAI(01H)                                                   
                       Input module not present,                          
086  01                set transmit data out line                         
                       to mark state and store                            
087  AA        MR2A    status in B1R2.-088 27  CLRA Load transmit data    
                       out                                                
089  AB        MR3A    register B1R3 with 0&#39;s, so                         
                       all ones are sent out when                         
                       no module is present.                              
08A  AD        MR5A    Store 0&#39;s in R5 also.                              
08B  00        NOOP                                                       
08C  00        NOOP                                                       
08D  04        JMP0,MP2                                                   
08E  98                                                                   
08F  53   MP1: NAI(0FEH)                                                  
                       Input module is present,                           
                       set transmit data out line                         
                       to space state and store                           
090  FE                status in B1R2.                                    
091  AA        MR2A                                                       
092  80        MXA0    Float the data bus.                                
093  08        INBS    Read input data.                                   
094  37        CLPA    Invert data (low true).                            
095  AB        MR3A    Store data in B1R3.                                
096  27        CLRA    Preset transmitted data                            
                       parity register.                                   
097  AD        MR5A    Store in R5.                                       
098  89   MP2: OP1I(20H)                                                  
                       Disable I/O data bus.                              
099  20                                                                   
09A  27        CLRA    Reset receive data                                 
09B  AC        MR4A    register B1R4 and load                             
09C  17        INCA    serial data out mask                               
09D  AE        MR6A    into R6.                                           
09E  FA        MAR2    Move port 2 register to                            
                       accumulator.                                       
09F  25        ENTI                                                       
0A0  8A        OP2I(01H)                                                  
                       Change transmit data out                           
                       line to mark state.                                
0A1  01                                                                   
0A2  56   SBD: JT1,SBD Dwell till receive start                           
                       bit.                                               
0A3  A2                                                                   
               Start Bit Received, Start                                  
               Transmission Here                                          
0A4  65   SBT: STPTC   Stop timer/counter.                                
0A5  85        CLRF0   Clear received data parity                         
                       flag.                                              
0A6  3A        OUTP2   Transmit start bit.                                
0A7  A5        CLRF1   Clear flag F1 for fault                            
                       detection test during                              
                       stop bit times 2 and 3.                            
0A8  0A        INP2                                                       
0A9  56        JT1,SNV Check for valid start bit:                         
0AA  CO                                                                   
                       T1=1, not valid, go to SNV;                        
                       T1=0, valid, continue.                             
0AB  32        JB1,MNA Rack adapter module active?                        
0AC  BE                                                                   
                       If B1=1, go to ERR MNA.                            
0AD  80        MXA0    Ready I/O data bus for input.                      
0AE  BE        MR6I(01H)                                                  
                       Load R6 with bit mask.                             
0AF  01                                                                   
0B0  FE        MAR6    Isolate first bit to be                            
0B1  5B        NAR3    sent, which is the least                           
                       significant bit in R3 (R1=01).                     
0B2  DA        XOAR2   Assemble data to be sent.                          
               Start Bit Time Complete                                    
0B3 3A         OUTP2   Transmit data bit 1.                               
0B4 DD         XOAR5   Generate and store                                 
                       transmitted data parity                            
0B5 AD         MR5A    in B1R5.                                           
0B6 FB         MAR3    Ready next bit for                                 
                       transmission.                                      
0B7 56         JT1,B1A Read input, if bit=1,                              
0B8 C2                 go to B1A.                                         
0B9 2C         XCAR4   Bit=0.                                             
0BA 27         CLRA    Initialize input byte                              
                       to 00H.                                            
0BB 2C         XCAR4   Store in B1R4.                                     
0BC 04         JMP0,B1B                                                   
                       Go to B1B.                                         
0BD C7                                                                    
0BE 44    MNA: JMP2,ER2                                                   
                       Rack adapter module                                
                       not active.                                        
0BF B0                                                                    
0C0 44    SNV: JMP2,ER3                                                   
                       Start bit invalid.                                 
0C1 B6                                                                    
0C2 95    B1A: CPLF0   Toggle parity flag.                                
0C3 2C         XCAR4   Get data bits.                                     
0C4 FE         MAR6    Initialize to 01H.                                 
0C5 77         RRA     Advance bit pointer.                               
0C6 2C         XCAR4   Store.                                             
0C7 77    B1B: RRA     Rotate next bit                                    
0C8 AB         MR3A    in R3 for transmission.                            
0C9 5E         NAR6    Ready port 2 register                              
0CA DA         XOAR2   for next transmission.                             
               1st Bit Time Complete                                      
0CB  3A        OUTP2   Transmit data bit 2.                               
                       Reference notes in bit 1.                          
0CC  DD        XOAR5                                                      
0CD  AD        MR5A                                                       
0CE  FB        MAR3                                                       
0CF  56        JT1,B2A Receive data bit 2.                                
0D0  D6                                                                   
0D1  2C        XCAR4                                                      
0D2  77        RRA                                                        
0D3  2C        XCAR4                                                      
0D4  04        JMP0,B2B                                                   
0D5  DB                                                                   
0D6  95   B2A: CPLF0                                                      
0D7  2C        XCAR4                                                      
0D8  17        INCA                                                       
0D9  77        RRA                                                        
0DA  2C        XCAR4                                                      
0DB  77   B2B: RRA     Ready data bit 3 for                               
                       transmission.                                      
0DC  AB        MR3A                                                       
0DD  5E        NAR6                                                       
0DE  DA        XOAR2                                                      
               2nd Bit Time Complete                                      
0DF  3A        OUTP2   Transmit data bit 3.                               
                       Reference notes in bit 1.                          
0E0  DD        XOAR5                                                      
0E1  AD        MR5A                                                       
0E2  FB        MAR3                                                       
0E3  56        JT1,B3A Receive data bit 3.                                
0E4  EA                                                                   
0E5  2C        XCAR4                                                      
0E6  77        RRA                                                        
0E7  2C        XCAR4                                                      
0E8  04        JMP0,B3B                                                   
0E9  EF                                                                   
0EA  95   B3A: CPLF0                                                      
0EB  2C        XCAR4                                                      
0EC  17        INCA                                                       
0ED  77        RRA                                                        
0EE  2C        XCAR4                                                      
0EF  77   B3B: RRA     Ready data bit 4 for                               
                       transmission.                                      
0F0  AB        MR3A                                                       
0F1  5E        NAR6                                                       
0F2  DA        XOAR2                                                      
               3rd Bit Time Complete                                      
0F3  3A        OUTP2   Transmit data bit 4.                               
                       Reference notes in bit 1.                          
0F4  DD        XOAR5                                                      
0F5  AD        MR5A                                                       
0F6  FB        MAR3                                                       
0F7  56        JT1,B4A Receive data bit 4.                                
0F8  FE                                                                   
0F9  2C        XCAR4                                                      
0FA  77        RRA                                                        
0FB  2C        XCAR4                                                      
0FC  24        JMP1,B4B                                                   
0FD  03                                                                   
0FE  95   B4A: CPLF0                                                      
0FF  2C        XCAR4                                                      
100  17        INCA                                                       
101  77        RRA                                                        
102  2C        XCAR4                                                      
103  77   B4B: RRA     Ready data bit 5 for                               
                       transmission.                                      
104  AB        MR3A                                                       
105  5E        NAR6                                                       
106  DA        XOAR2                                                      
               4th Bit Time Complete                                      
107  3A        OUTP2   Transmit data 5.                                   
                       Reference notes in bit 1.                          
108  DD        XOAR5                                                      
109  AD        MR5A                                                       
10A  FB        MAR3                                                       
10B  56        JT1,B5A Receive data bit 5.                                
10C  12                                                                   
10D  2C        XCAR4                                                      
10E  77        RRA                                                        
10F  2C        XCAR4                                                      
110  24        JMP1,B5B                                                   
111  17                                                                   
112  95   B5A: CPLF0                                                      
113  2C        XCAR4                                                      
114  17        INCA                                                       
115  77        RRA                                                        
116  2C        XCAR4                                                      
117  77   B5B: RRA     Ready data bit 6 for                               
                       transmission.                                      
118  AB        MR3A                                                       
119  5E        NAR6                                                       
11A  DA        XOAR2                                                      
               5th Bit Time Complete                                      
11B  3A        OUTP2   Transmit data bit 6.                               
                       Reference notes in bit 1.                          
11C  DD        XOAR5                                                      
11D  AD        MR5A                                                       
11E  FB        MAR3                                                       
11F  56        JT1,B6A Receive data bit 6.                                
120  26                                                                   
121  2C        XCAR4                                                      
122  77        RRA                                                        
123  2C        XCAR4                                                      
124  24        JMP1,B6B                                                   
125  2B                                                                   
126  95   B6A: CPLF0                                                      
127  2C        XCAR                                                       
128  17        INCA                                                       
129  77        RRA                                                        
12A  2C        XCAR4                                                      
12B  77   B6B: RRA     Ready data bit 7 for                               
                       transmission.                                      
12C  AB        MR3A                                                       
12D  5E        NAR6                                                       
12E  DA        XOAR2                                                      
               6th Bit Time Complete                                      
 12F 3A        OUTP2   Transmit data bit 7.                               
                       Reference notes in bit 1.                          
130  DD        XOAR5                                                      
131  AD        MR5A                                                       
132  FB        MAR3                                                       
133  56        JT1,B7A Receive data bit 7.                                
134  3A                                                                   
135  2C        XCAR4                                                      
136  77        RRA                                                        
137  2C        XCAR4                                                      
138  24        JMP1,B7B                                                   
139  3F                                                                   
13A  95   B7A: CPLF0                                                      
13B  2C        XCAR4                                                      
13C  17        INCA                                                       
13D  77        RRA                                                        
13E  2C        XCAR4                                                      
13F  77   B7B: RRA     Ready data bit 8 for                               
                       transmission.                                      
140  AB        MR3A                                                       
141  5E        NAR6                                                       
142  DA        XOAR2                                                      
               7th Bit Time Complete                                      
143  3A        OUTP2   Transmit data bit 8.                               
                       Reference notes in bit 1.                          
144  DD        XOAR5                                                      
145  5E        NAR6    Ready parity bit for                               
                       transmission.                                      
146  DA        XOAR2                                                      
147  56        JT1,B8A Receive data bit 8.                                
148  4E                                                                   
149  2C        XCAR4                                                      
14A  77        RRA                                                        
14B  2C        XCAR4                                                      
14C  24        JMP1,B8B                                                   
14D  53                                                                   
14E  95   B8A: CPLF0                                                      
14F  2C        XCAR4                                                      
150  17        INCA                                                       
151  77        RRA                                                        
152  2C        XCAR4                                                      
153  2D   B8B: XCAR5   Prepare to clear registers                         
                       B1R3,B1R5.                                         
154  27        CLRA                                                       
155  AB        MR3A    Clear register B1R3.                               
156  2D        XCAR5   Clear transmitted data                             
                       parity register B1R5,                              
                       parity bit available for                           
                       transmission.                                      
               8th Bit Time Complete                                      
157  3A        OUTP2   Transmit parity bit.                               
158  BF        MR7I(FEH)                                                  
                       Load R7 with FE(H).                                
159  FE                                                                   
15A  18        INCR0   Get next slot file.                                
15B  56        JT1,BPA Receive parity bit.                                
15C  5F                                                                   
15D  24        JMP1,BPB                                                   
15E  61                                                                   
15F  95   BPA: CPLF0   Process parity.                                    
160  00        NOOP                                                       
161  F0   BPB: MA0                                                        
162  F2        JB7,SOK If bit 7=1, go to SOK.                             
163  7A                                                                   
                       Slot in range of this rack.                        
164  B2        JB5,SSO If B5=0 select data out                            
165  6D                from another rack,                                 
                       slot not in rack range.                            
166  00        NOOP                                                       
167  00        NOOP                                                       
168  8A        OP2I(01H)                                                  
                       Transmit stop bit.                                 
169  01                                                                   
16A  00        NOOP                                                       
16B  24        JMP1,CP4                                                   
16C  74                                                                   
16D  23   SSO: MAI(EEH)                                                   
                       Prepare to select data out                         
16E  EE                from another rack;                                 
                       force B0, B4 to 0; B4                              
                       selects other transmit data                        
                       out line; B0 clears B0 to                          
                       insure mark state.                                 
16F  8A        OP2I,01H                                                   
                       Transmit stop bit.                                 
170  01                                                                   
171  5A        NAR2                                                       
172  AA        MR2A    Must output B1R2 later.                            
173  1A        INCR2   Force bit 0 to a &#34;1.&#34;                              
174  0A   CP4: INP2    Read and store.                                    
175  AF        MR7A                                                       
176  54        CALL2,D17                                                  
177  5F                                                                   
178  24        JMP1,CP1                                                   
179  A5                                                                   
17A  92   SOK: JB4,CP5                                                    
17B  8D                                                                   
17C  53        NAI(0FH)                                                   
                       Slot is within rack range.                         
17D  0F                                                                   
17E  8A        OP2I(01H)                                                  
                       Transmit stop bit.                                 
17F  01                                                                   
180  41        OA1     Isolate slot address.                              
181  39        OUTP1   Enable backplane signals.                          
182  0A        INP2                                                       
183  D2        JB6,MP3 Read P2 and check for                              
184  9E                input module present.                              
                       If present, go to MP3.                             
                       If not present, then set                           
185  2A        XCAR2   port 2 register to mark                            
186  4E        OAR6    state.                                             
187  2A        XCAR2                                                      
188  AF        MR7A    Store stop bit in B1R7.                            
189  00        NOOP                                                       
18A  00        NOOP                                                       
18B  24        JMP1,CP1                                                   
18C  A5                                                                   
18D  53   CP5: NAI(0FH)                                                   
                       Select this path when an                           
18E  0F                input transmission must                            
                       take place even with the                           
                       absence of an input                                
                       module.                                            
18F  8A        OP2I(01H)                                                  
                       Transmit stop bit.                                 
190  01                                                                   
191  41        OA1     Isolate slot address.                              
192  39        OUTP1   Enable backplane signals.                          
193  0A        INP2    Read and check for input                           
                       module present.                                    
194  D2        JB6,MP3 If B6=1, go to MP3.                                
195  9E                If B6=0, continue.                                 
196  2A        XCAR2   Set part 2 register to                             
197  5F        NAR7    space state.                                       
198  2A        XCAR2                                                      
199  AF        MR7A    Store stop bit in B1R7.                            
19A  BB        MR3I(00H)                                                  
                       Transmit zeroes.                                   
19B  00                                                                   
19C  24        JMP1,CP1                                                   
19D  A5                                                                   
19E  2A   MP3: XCAR2   Set port 2 register to                             
19F  5F        NAR7    space state.                                       
1A0  2A        XCAR2                                                      
1A1  AF        MR7A    Store stop bit in B1R7.                            
1A2  08        INBS    Read input slot data.                              
1A3  37        CPLA    Invert data (low true).                            
1A4  AB        MR3A    Store data in transmit                             
                       data out register B1R3.                            
               1st Stop Bit Time Complete                                 
               Plus 5 μsec.                                            
1A5  C8   CP1: DECR0   Select previous absolute                           
                       byte.                                              
1A6  F0        MA0     Isolate and store this                             
1A7  53        NAI(0FH)                                                   
                       slot address.                                      
1A8  0F                                                                   
1A9  2F        XCAR7   Save in B1R7 and get first                         
                       stop bit from B1R7.                                
1AA  26        JNT0,TDC                                                   
                       Check transmit data from                           
1AB  AE                other racks for two consecu-                       
1AC  24        JMP1,CP0                                                   
                       tive valid stop bits.                              
1AD  B0                                                                   
1AE  B5   TDC: CPLF1   If B0=0, complement F1;                            
1AF  00        NOOP    If B0=1, do not alter F1.                          
1B0  46   CP0: JNT1,ES1                                                   
                       Check received data for                            
                       stop bits.                                         
1B1  B6                If T1=1, check B7 of B1R7:                         
                       if B7=1, 2 good stop bits,                         
                       go to ES3; if B7=0, 1 stop                         
                       bit BAD, go to ES2, ES5.                           
                       If T1=0, go to ES1, check                          
                       B7 of B1R7; if B7=1, 1 stop                        
                       bit bad, go to ES2, ES5;                           
                       if B7=0, 2 bad stop bits,                          
                       go to ERF, ER7.                                    
1B2  F2        JB7,ES3                                                    
1B3  C6                                                                   
1B4  24   ES2: JMP1,ES5                                                   
1B5  BA                                                                   
1B6  E2   ES1: JB7,ES2                                                    
1B7  B4 -1B8                                                              
          44           JMP2,ER4 2 low stop bits from                      
                       central processor.                                 
1B9  BC                                                                   
1BA  F0   ES5: MA0     Check previous transmission                        
                       for error.                                         
1BB  D2        JB6,ES4 B6=0, error this scan only.                        
1BC  C4                Update fault tolerance bit,                        
                       go to CP2.                                         
                       B6=1, 2 consecutive errors                         
                       (major fault), go to ES4.                          
1BD  43        OAI(40H)                                                   
1BE  40                                                                   
1BF  A0        M0A                                                        
1C0  54        CALL2,D17                                                  
1C1  5F                                                                   
1C2  24        JMP1,CP2                                                   
1C3  D5                                                                   
1C4  44   ES4: JMP2,ER5                                                   
                       2 consecutive errors, go                           
                       to ER5 fault routine.                              
1C6  B6   ES3: JF0,ES5 Check parity: if F0=1,                             
                       parity error, go to ES5;                           
                       if F0=0, no parity error.                          
1C8  FC        MAR4    Position received data.                            
1C9  37        CPLA    Invert (low true).                                 
1CA  02        OUTBS   Output to backplane.                               
1CB  FF        MAR7    Enable and write                                   
1CC  39        OUTP1   lines active.                                      
1CD  F0        MA0     Load slot address and                              
                       error flag.                                        
1CE  53        NAI(BFH)                                                   
                       Reset and store.                                   
1CF  BF                                                                   
1D0  A0        M0A     2 consecutive error flag.                          
1D1  9A        NP2I(F7H)                                                  
                       Strobe on, 5 μsec.                              
1D2  F7                                                                   
1D3  8A        OP2I(08H)                                                  
                       Strobe off.                                        
1D4  08                                                                   
1D5  89   CP2: OP1I(A0H)                                                  
                       Disable port 1 lines.                              
1D6  A0                                                                   
               3rd Stop Bit Plus 7.5 μsec. Complete                    
1D7  23        MAI(D0H)                                                   
                       Increment byte counter.                            
1D8  D0                                                                   
1D9  26        JNT0,TDD                                                   
                       Check transmit data for 2                          
1DA  DD                                                                   
1DB  24        JMP1,CP3                                                   
                       consecutive stop bits.                             
1DC  DF                                                                   
1DD  76   TDD: JF1,ERF Jump if 2 low stop bits from                       
1DE  E6                next adapter down line.                            
1DF  18   CP3: INCR0   Mask to determine if 16 bytes                      
1E0  68        AAR0    transmissions have occurred.                       
1E1  C6        JOZ,GSW                                                    
1E2  FF                If data in accumulator = 0,                        
                       generate status word (GSW).                        
                       If data in accumulator does                        
                       not = 0, continue.                                 
1E3  FA        MAR2    Move port 2 register status                        
                       to the accumulator.                                
1E4  04                                                                   
1E5  A4                                                                   
1E6  44   ERF: JMP2,ER7                                                   
1E7  CE                                                                   
               Status Byte Transmission Time                              
1FF  FA   GSW: MAR2    Generate status byte                               
                       start bit.                                         
200  43        OAI,10H Select this module.                                
201  10                                                                   
202  53        NAI,0FEH                                                   
203  FE                                                                   
204  3A        OUTP2   Transmit status byte                               
                       start bit.                                         
205  BB        MR3I(04H)                                                  
206  04                                                                   
207  AA        MR2A    Save port 2 register                               
                       contents.                                          
208  0A        INP2    Check for rack adapter                             
209  32        JB1,ERN module active.                                     
20A  54                                                                   
20B  BE        MR6I(01H)                                                  
20C  01                                                                   
20D  BF        MR7I(07H)                                                  
20E  07                                                                   
20F  FE        MAR6                                                       
210  5B        NAR3    Prepare to send first bit.                         
211  DA        XOAR2                                                      
               Start Bit Time Complete                                    
212  3A   SWT: OUTP2   Transmit status byte                               
                       data bits 1-7.                                     
213  DD        XOAR5   Generate and store                                 
214  AD        MR5A    parity.                                            
215  FB        MAR3    Ready next                                         
216  77        RRA     bit for                                            
217  AB        MR3A    transmission.                                      
218  5E        NAR6    Ready accumulator to send                          
219  DA        XOAR2   next bit.                                          
21A  54        CALL2,D12                                                  
21B  61                                                                   
21C  EF        DJNZ7,SWT                                                  
21D  12                                                                   
21E  3A        OUTP2   Transmit data bit 8.                               
21F  DD        XOAR5   Prepare parity bit.                                
220  54        CALL2,D27                                                  
221  5B                                                                   
222  5E        NAR6                                                       
223  DA        XOAR2                                                      
224  3A        OUTP2   Transmit parity bit.                               
225  27        CLRA                                                       
226  AD        MR5A                                                       
227  AF        MR7A                                                       
228  54        CALL2,D27                                                  
229  5B                                                                   
22A  8A        OP2I,01H                                                   
                       Transmit first stop bit.                           
22B  01                                                                   
22C  54        CALL2,D30                                                  
22D  5A                                                                   
22E  54        CALL2,D40                                                  
                       2nd stop bit time.                                 
22F  56                                                                   
230  54        CALL2,D40                                                  
                       3rd stop bit time.                                 
231  56                                                                   
232  54        CALL2,D40                                                  
                       4th stop bit time.                                 
233  56                                                                   
               Start Dwell Transmission                                   
234  54        CALL2,D40                                                  
                       Equates to start bit time.                         
235  56                                                                   
236  54        CALL2,D40                                                  
                       Equates to fist data                               
237  56                bit time.                                          
238  54        CALL2,D40                                                  
                       Equates to second data                             
239  56                bit time.                                          
23A  54        CALL2,D40                                                  
                       Equates to third data                              
23B  56                bit time.                                          
23C  54        CALL2,D40                                                  
                       Equates to fourth data                             
23D  56                bit time.                                          
23E  54        CALL2,D40                                                  
                       Equates to fifth data                              
23F  56                bit time.                                          
240  54        CALL2,D40                                                  
                       Equates to sixth data                              
241  56                bit time.                                          
242  54        CALL2,D40                                                  
                       Equates to seventh data                            
243  56                bit time.                                          
244  54        CALL2,D40                                                  
                       Equates to eighth data                             
245  56                bit time.                                          
246  54        CALL2,D40                                                  
                       Equates to ninth data                              
247  56                bit time.                                          
248  8A        OP2I,10H                                                   
                       Initialize select data                             
                       out to select data from                            
                       this I/O interface rack.                           
249  10                                                                   
24A  23        MAI,0E7H                                                   
24B  E7                                                                   
24C  62        MTA     Load transmission loss                             
                       timer.                                             
24D  B8        MR0I(20H)                                                  
                       Reset absolute byte                                
                       slot center.                                       
24E  20                                                                   
24F  BB        MR3I(0H)                                                   
                       Turn off rack fault                                
                       indicator.                                         
250  00                                                                   
251  55        STRTT   Start timer.                                       
252  04        JMP0,PST                                                   
                       Go to prepare slot for                             
253  73                data transmission.                                 
254  44   ERN: JMP2,ER6                                                   
255  C8                                                                   
               Delay Routine                                              
256  00   D40: NOOP    40 μsec.                                        
257  00   D37: NOOP    37.5μsec.                                       
258  00   D35: NOOP    35 μ sec.                                       
259  00   D32: NOOP    32.5 μsec.                                      
25A  00   D30: NOOP    30 μsec.                                        
25B  00   D27: NOOP    27.5 μsec.                                      
25C  00   D25: NOOP    25 μsec.                                        
25D  00   D22: NOOP    22.5 μsec.                                      
25E  00   D20: NOOP    20 μsec.                                        
25F  00   D17: NOOP    17.5 μsec.                                      
260  00   D15: NOOP    15 μsec.                                        
261  00   D12: NOOP    12.5 μsec.                                      
262  83   D10: RETU    10 μsec.                                        
               Interrupt Routine                                          
263  65   RT:  STPTC   Stop timer.                                        
264  35        DISTI   Disable timer interrupt.                           
265  93        RETR    Reenable interrupts.                               
               Time Out Routine                                           
266  54   TOUT:                                                           
               CALL2,RT                                                   
267  63                                                                   
268  44        JMP2,ER1                                                   
                       Time out routine.                                  
269  AA                                                                   
               Routine To Correct For Multiple                            
               Transmissions When Only 1 of 2                             
               Input Modules Are Present                                  
26A  B8   DT0: MR0I(20H)                                                  
                       Reset absolute byte counter.                       
26B  20                                                                   
26C  B9        MR1I(10H)                                                  
26D  10                                                                   
26E  F0   DT1: MA0     Slot in rack range?                                
26F  F2        JB7,DT2 If B7=1, go to DT2.                                
270  7C                                                                   
271  18        INCR0   If B7=0, dwell for 2                               
272  54        CALL2,D40                                                  
                       absolute byte register                             
273  56                check times and                                    
274  54        CALL2,D40                                                  
                       advance B1R0.                                      
275  56                                                                   
276  54        CALL2,D40                                                  
277  56                                                                   
278  54        CALL2,D35                                                  
279  58                                                                   
27A  44        JMP2,DT5                                                   
27B  A3                                                                   
27C  53   DT2: NAI(0FH)                                                   
                       Isolate slot address and                           
27D  0F                check for input module                             
27E  4C        OAR4    present.                                           
27F  39        OUTP1                                                      
280  0A        INP2                                                       
281  D2        JB6,DT3 If B6=1, input module is                           
282  90                present, go to DT3.                                
283  18        INCR0   If B6=0, not present, check                        
284  F0        MA0     for input module present in                        
285  53        NAI,0FH next absolute byte register.                       
286  0F                                                                   
287  4C        OAR4                                                       
288  39        OUTP1                                                      
289  0A        INP2                                                       
28A  D2        JB6,DT4 If B6= 1, input module present,                    
28B  9B                go to DT4;                                         
28C  54        CALL2,D15                                                  
                       If 6=0, not present, dwell                         
28D  60                to end of this register time                       
28E  44        JMP2,DT5                                                   
                       and go to DT5.                                     
28F  A3                                                                   
290  F0   DT3: MA0     Set bit 4 for this absolute                        
291  49        OAR1    byte register and for next                         
292  A0        M0A     absolute byte register.                            
293  18        INCR0                                                      
294  F0        MA0                                                        
295  49        OAR1                                                       
296  A0        M0A                                                        
297  54        CALL2,D25                                                  
298  5C                                                                   
299  44        JMP2,DT5                                                   
29A  A3                                                                   
29B  F0   DT4: MA0     Input module present so set                        
29C  49        OAR1    B4 in this and previous abso-                      
29D  A0        M0A     lute byte register.                                
29E  C8        DECR0                                                      
29F  F0        MA0                                                        
2A0  49        OAR1                                                       
2A1  A0        M0A                                                        
2A2  18        INCR0                                                      
2A3  18   DT5: INCR0   Advance absolute register                          
2A4  F8        MAR0    counter. Check for register                        
2A5  D3        XOAI(30H)                                                  
                       counter overflow. If R0=30H,                       
2A6  30                routine complete.                                  
2A7  96        JONZ,DT1                                                   
                       If not, go to DT1.                                 
2A8  6E                                                                   
2A9  83        RETU                                                       
               Error Routine with Diagnostics                             
2AA  99   ER1: NP1I(0F0H)                                                 
2AB  F0                                                                   
2AC  89        OP1I(01H)                                                  
                       ER1, least significant nibble                      
2AD  01                in port 1=1.                                       
2AE  44        JMP2,ERR                                                   
                       ER1 transmission loss time                         
2AF  D4                out.                                               
2B0  99   ER2: NP1I(F0H)                                                  
2B1  F0                                                                   
2B2  89        OP1I(02H)                                                  
                       ER2, least significant nibble                      
2B3  02                in port 1=2.                                       
                       ER2 rack adapter module not                        
                       active in start bit time.                          
2B4  44        JMP2,ERR                                                   
2B5  D4                                                                   
2B6  99   ER3: NP1I(F0H)                                                  
2B7  F0                                                                   
2B8  89        OP1I(03H)                                                  
                       ER3, least significant nibble                      
2B9  03                in port 1=3.                                       
2BA  44        JMP2,ERR                                                   
                       ER3 start bit not valid.                           
2BB  D4                                                                   
2BC  99   ER4: NP1I(F0H)                                                  
2BD  F0                                                                   
2BE  89        OP1I(04H)                                                  
                       ER4, least significant nibble                      
2BF  04                in port 1=4.                                       
2C0  44        JMP2,ERR                                                   
                       ER4, low stop from central                         
2Cl  D4                processor.                                         
2C2  99   ER5: NP1I(F0H)                                                  
2C3  F0                                                                   
2C4  89        OP1I(05H)                                                  
                       ER5, least significant nibble                      
2C5  05                in port 1=5.                                       
2C6  44        JMP2,ERR                                                   
                       ER5 2 consecutive errors                           
2C7  D4                (major fault) in 2 I/O scans.                      
2C8  99   ER6: NP1I(F0H)                                                  
2C9  F0                                                                   
2CA  89        OP1I(06H)                                                  
                       ER6, least significant nibble                      
2CB  06                in port 1=6.                                       
2CC  44        JMP2,ERR                                                   
                       ER6, rack adapter module not                       
2CD  D4                active in status byte trans-                       
                       mission time.                                      
2CE  99   ER7: NP1I(F0H)                                                  
2CF  F0                                                                   
2D0  89        OP1I(07H)                                                  
                       ER7, least significant nibble                      
2D1  07                in port 1=7.                                       
2D2  44        JMP2,ERR                                                   
                       ER7, two low stop bits from                        
2D3  D4                next rack adapter.                                 
2D4  65   ERR: STPTC   Stop timer.                                        
2D5  35        DISTI   Disable timer interrupts.                          
2D6  D5        SLRB1   Select register bank 1.                            
2D7  05        ENI     Enable reset button.                               
2D8  89        OP1I(70H)                                                  
                       Select DIP switches. -2D9 70   Disable I/O data    
                       bus.                                               
                       Turn on rack fault LED.                            
2DA  8A        OP2I(01H)                                                  
                       Force transmit data out                            
2DB  01                line to mark state.                                
2DC  80        MXA0    Float I/O data bus for                             
                       input.                                             
2DD  08        INBS    Read DIP switches.                                 
2DE  92        JB4,NRST                                                   
                       DSW4 on, no reset.                                 
2DF  E4                                                                   
2E0  8A        OP2I(04H)                                                  
                       If off, reset for                                  
2E1  04                                                                   
2E2  9A        NP2I(FBH)                                                  
                       5 μsec.                                         
2E3  FB                                                                   
2E4  BB   NRST:                                                           
               MR3I(FFH)                                                  
                       Delay 10 msec. to                                  
2E5  FF                                                                   
2E6  54   DLAY:                                                           
               CALL2,D40                                                  
                       cause scanner error.                               
2E7  56                                                                   
2E8  EB        DJNZ3,DLAY                                                 
2E9  E6                                                                   
2EA  08   LOOP:                                                           
               INBS    Look at DSW5.                                      
2EB  37        CPLA                                                       
2EC  B2        JB5,LOOP                                                   
                       Loop if processor is to be                         
2ED  EA                locked out.                                        
2EE  BB        MR3I(10H)                                                  
2EF  10                                                                   
2F0  04        JMP0,REST                                                  
2F1  6A                                                                   
END                                                                       
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                                           APPENDIX B                              
__________________________________________________________________________
Component Listing                                                         
                 Reference                                                
Component        Numbers                                                  
                       Description                                        
__________________________________________________________________________
Rack interface processor                                                  
                 130   Intel 8048 microcomputer                           
PROM             145   manufactured by Intel                              
RAM              146   Corporation                                        
D-type flip-flop 140   SN74LS74 flip-flop manu-                           
                       factured by Texas Instru-                          
                       ments, Inc.                                        
Clock line inverter                                                       
                 139   SN7406 hex inverter/buffer/                        
                       driver manufactured by                             
                       Texas Instruments, Inc.                            
Receiving line NAND gate                                                  
                 66    SN74LS38 quad NAND                                 
Select data circuit NAND gates                                            
                 206,208                                                  
                       buffers with open collec-                          
Reset NAND gate  153   tors manufactured by                               
                       Texas Instruments, Inc.                            
Read/write inverters                                                      
                 184-185                                                  
                       SN74LS04 hex inverters                             
                 and   manufactured by Texas                              
                 190-191                                                  
                       Instruments, Inc.                                  
Read and write NAND gates                                                 
                 193 and 196                                              
                       SN74LS38 quad NAND                                 
Rack fault NAND gate                                                      
                 153   with open collectors                               
                       manufactured by Texas                              
                       Instruments, Inc.                                  
Data shaper NAND gates                                                    
                 114, 116,                                                
                       SN 74LS00 quad NAND                                
                 122 and 125                                              
                       gates manufactured by                              
                       Texas Instruments, Inc.                            
Data shaper inverters                                                     
                 112, 118                                                 
                       SN74LS04 hex inverters                             
                       manufactured by Texas                              
                       Instruments, Inc.                                  
Termination sensing NAND gates                                            
                 236, 238                                                 
                       SN74LS00 quad NAND gates                           
                 and 241                                                  
                       manufactured by Texas                              
                       Instruments, Inc.                                  
Termination sensing ex OR gate                                            
                 240   SN74LS86 quad ex OR                                
                       gates manufactured by                              
                       Texas Instruments, Inc.                            
Reset circuit transistor                                                  
                 172   MPS-3643-S PNP transistor                          
Select data out circuit inverters                                         
                 209 and 211                                              
                       manufactured by Fairchild                          
                       Semiconductor                                      
Reset circuit transistor                                                  
                 172   MPS-3643-S PNP transistor                          
                       manufactured by                                    
Input circuit transistors                                                 
                 107 and 108                                              
                       2N3904 PNP transistor                              
                       and 2N3906 NPN transistor                          
                       manufactured by Texas                              
                       Instruments, Inc.                                  
Optical coupling circuits                                                 
                 42,55,81                                                 
                       Four HP5082-4370 optical                           
                 and 82                                                   
                       coupling isolation cir-                            
                       cuits manufactured by                              
                       Hewlett-Packard Co.                                
First set of buffers                                                      
                 220   8304 octal bidirectional                           
                       buffer-line drivers                                
                       manufactured by National                           
                       Semiconductor, Inc.                                
Second set of buffers                                                     
                 223   SN74LS240 octal buffer/                            
                       line drivers manufactured                          
                       by Texas Instruments, Inc.                         
Decoder          149   SN74154 4-line-to-16-line                          
                       decoder manufactured by                            
                       Texas Instruments, Inc.                            
DIP switch circuit                                                        
                 224   AMP Inc. P/N 435626-5                              
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