DATA PROCESSING SYSTEM INCLUDING A DATA TRANSMISSION BUS

Abstract: A data processing system, such as a point-of-sale (POS) system, has a central subsystem (12) connected by a bus (16) having a pair of conductors (20, 22) connected to a plurality of remote subsystems (14). The remote subsystems (14) are each electrically isolated from the bus by a receiver including a light-emitting diode (34) connected across the conductor pair (20, 22) and a transmitter including a phototransistor (40) connected across the conductor pair (20, 22). Current flows in one direction along the bus (16) when the central subsystem (12) is transmitting a message, and in an opposite direction when a remote subsystem (14) is transmitting a message. In an alternate embodiment, at least one (14B) of the remote subsystems (14) is not isolated from the bus, and is connected to the bus by a differential receiver (48) and a dual output transmitter (49).

Description: DATA PROCESSING SYSTEM INCLUDING A DATA TRANSMISSION BUS Technical Field This invention relates to data processing systems of the kind including a central subsystem and a plurality of remote subsystems, said remote subsystems being adapted to receive signals from said central sub¬ system and to transmit signals to said central subsys¬ tem. Background Art In data processing systems, such as point-of- sale (POS) systems, it is normally desired to intercon¬ nect a central subsystem (such as a POS terminal) to a plurality of peripheral or remote subsystems (such as receipt printers, coin dispensers, display devices, and the like) . Problems often arise, however, in intercon¬ necting such subsystems. For example, if two of the subsystems each have their own power supply, a ground loop condition can arise in which noise and other spur¬ ious signals pass between the subsystems and cause error in their operation. While various solutions have been proposed to overcome ground loop conditions, each solu¬ tion has given rise to other problems or disadvantages . From U.S. Patent No. 4,241,330 there is known a digital communication system for communicating among two central consoles and a plurality of local control¬ lers. The devices are interconnected by two independent paths each including a bidirectional duplex two wire communication link. Each path extends from one of the consoles to all the local controllers in opposite direc- tions, thereby forming a noncontinuous loop configura¬ tion. In the known arrangement faulty local controllers can be isolated, yet communication maintained among the other controllers and both consoles. Disclosure of the Invention According to the present invention, there is -2- provided a data processing system of the kind specified, characterized by: a bus connected to said central sub¬ system and said remote subsystems, said bus including a first conductor and a second conductor; first unidirec- tional signal means connected to said bus at least at one of said remote subsystems and adapted to receive signals transmitted by said central subsystem, said first unidirectional signal means only passing signals in a direction from said first conductor to said second conductor; and second unidirectional signal means con¬ nected to said bus at said one of said remote subsystems and adapted to transmit signals to said central subsys¬ tem, said second unidirectional signal means passing signals only in a direction from said second conductor to said first conductor. It will be appreciated that in a data proces¬ sing system according to the invention the signals transmitted by one of the remote subsystems and passed by the associated second unidirectional signal means will not pass through any of the first unidirectional signal means, whereby the other remote subsystems do not receive the message. It will also be appreciated that a system according to the invention has the advantage that only a single, twoconductor bus is utilized. Another advantage is the avoidance of the problems which arise with loop- connected subsystems. It should be understood that in a loop—connected system each remote subsystem receives and amplifies the message from the previous subsystem and - passes the message to the next subsystem in the loop. It will thus be appreciated that problems arise when one of the subsystems in the loop fails. In the disclosed embodiments the first uni¬ directional signal means comprises an LED (light emit- ting diode) with the LED passing current only in a direction from a first one of the conductors to a second one of the conductors. The second unidirectional signal eans comprises a phototransistor in the disclosed em¬ bodiments, with the phototransistor passing current in a direction from the second conductor to the first con¬ ductor. As a result, when a signal is transmitted at the phototransistor in the transmitting interface, it will not pass through the LED at the receiver interface, and thus will not be diminished by having to drive the receiving interface. The use of LED's and phototransis¬ tors has a cost advantage as compared with transformers for connecting subsystems to a bus. In one described embodiment, all remote sub¬ systems are connected to the bus by an opto-isolator. A transceiver at the central subsystem passes current in one direction on the two conductors when the central subsystem is generating a message to be received by LED's at each remote subsystem, and generates a constant volt¬ age for passing current in an opposite direction to be modulated by a phototransistor at one of the remote sub¬ systems when the central subsystem is to receive a message from that remote subsystem. In a second embodiment, at least one of the remote subsystems is not electrically isolated but, rather, is operatively connected through electrical paths to the bus. That remote subsystem receives electrical signals from the bus by way of a receiver with a high resistance input, and passes signals to the bus by way of a transmitter with a high resistance output. Brief Description of the Drawings Two embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a simplified block diagram of a data processing system having plural subsystems inter- connected by a two-wire bus in accordance with the present invention. - - Fig. 2 is a simplified circuit diagram of the transceiver at the central subsystem shown in Fig. 1. Figs. 3A and 3B are signal waveforms illus¬ trating the operation of the transceiver of Fig. 2. Fig. 4 is a simplified block diagram of a data processing system illustrating an alternate embodiment of the present invention. Fig. 5 is a detailed circuit diagram of the transceiver at the central subsystem shown in Fig. 4. 0 Figs. 6A and 6B are signal waveforms illus¬ trating the operation of the transceiver of Fig. 5. Fig. 7 is a detailed circuit diagram of the receiver for connecting each electrically isolated remote subsystem to the bus in the system of either 5 Fig. 1 or Fig. 4. Fig. 8 is a detailed circuit diagram of the transmitter for connecting each electrically isolated remote subsystem to the bus in the system of either Fig. 1 or Fig. 4. 0 Best Mode for Carrying Out the Invention Referring now to Fig. 1, there is shown a data processing system 10 in accordance with the present invention. As illustrated, the system 10 includes a central subsystem 12 and a plurality of remote sub- 5 systems 14. While the system 10 generally represents any data processing system having a central subsystem linked to a plurality of remote subsystems, in the preferred embodiment the system 10 is a retail point-of- sale system. In particular, the central subsystem 12 is 0 a point-of-sale terminal at which sales information is entered (e.g., at a keyboard that is not shown) and processed. Each of the remote subsystems 14 represent peripheral units that are connected to the central subsystem 12, such as receipt printers, coin dispensers, 5 display devices, credit card readers, and the like. While only two remote subsystems are shown in the draw¬ ing, it should be appreciated that there could be any number of remote subsystems 14 in actual practice In the system 10, data and control signals are transferred by the central subsystem 12 to one or more of the remote subsystems 14 in order to cause the remote subsystems 14 to perform their intended function. In addition, the various control and data signals may be transmitted from any one of the remote subsystems 14 back to the central subsystem 12. It should be noted that the system 10 is of the type wherein each remote subsystem either receives a signal from the central subsystem 12 or transmits a message to the central subsystem 12. None of the remote subsystems 14 transmit data or other signals directly to each other. In the system 10 of Fig. 1, the remote sub¬ systems 14 are connected to the central subsystem 12 by a bus 16 that includes two conductors or wires 20 and 22. The central subsystem 12 transmits messages to or receives messages from the bus 16 by way of a trans¬ ceiver 24. Each of the remote subsystems 14 receives messages from the bus by way of a receiving interface or receiver 28 and.transmits messages to the bus 16 by way of a transmitting interface or transmitter 30. Each of the receivers 28 and transmitters 30 that connects the remote subsystems 14 to the bus 16 is an opto-isolator, so that the remote subsystems 14 are each electrically isolated, at their data and control paths, from the bus 16. Each receiver 28 includes a light-emitting diode (LED) 34 and a phototransistor (PT) 36, and each transmitter 30 includes an LED 38 and a phototransistor 40. The direction of current flow through the LED's 34 and 38 and the phototransistors 36 and 40 is illustrated by arrows on the lines leading to and from each receiver 28 and transmitter 30. This j st-mentioned direction of current flow is an important aspect of the present invention, since it permits the bus 16 to consist of only two wires. As will be described in greater detail later, the current flow along the conductors 20 and 22 during the trans¬ mission of a signal from the central subsystem 12 to the -6- remote subsystems 14 is opposite to the current flow when one of the remote subsystems 14 is transmitting a signal to the central subsystem 12. The LED 34 in each receiver 28 is connected between the conductors 20 and 22 of the bus 16 such that the receiver 28 draws current from the bus 16 only when the central subsystem 12 is transmitting a signal. When one of the remote subsys¬ tems 14 is transmitting a signal on the bus 16, the LED 34 is back-biased and does not draw current from the bus 16. In particular, when a message or signal is transmitted by the transceiver 24 from central subsystem 12, current flows from the transceiver 24 along con¬ ductor 20 of bus 16 to the LED 34 in each of the re- ceivers 28. The resulting optical signal generated by the LED 34 is passed to phototransistor 36, and the resulting voltage across phototransistor 36 causes current flow, representing the transmitted signal, to its associated remote subsystem 14. Each subsystem 14, of course, receives the same signal and, depending upon the particular one of many conventional protocols that may be used, the message will normally include a sub¬ system address for causing only the addressed one of the subsystems to act on the message. As is well known to those skilled in the art, a phototransistor has some characteristics of a diode in that it passes current in only one direction (except for slight leakage current) . Since the phototransisto s 40 in each of the transmitters 30 are back-biased during the time that current is flowing from transceiver 24 along conductor 20, no current is being passed through the phototransistors 40 as the LED 34 in each receiver 28 is receiving a signal from the central subsystem 12. When one of the remote subsystems 14 is to transmit a signal to the central subsystem 12, the transceiver 24 generates a voltage of opposite polarity (conductor 22 at a higher potential than conductor 20) OMPI ° so that current may flow from the transceiver 24 along the conductor 22. The control of the bus 16 is super¬ vised by the central subsystem 12, of course, so that normally only one of the remote subsystems 14 will try 5 to transmit a message at any point in time on the bus 16. The remote subsystem 14 that is to transmit a message does so by controlling the LED 38 in its asso¬ ciated transmitter 30 to generate optical signals. The 10 phototransistor 40 in that same transmitter receives those optical signals and is thereby controlled to pass current (in a modulated fashion) from conductor 22 to conductor 20. It should be noted that the current from the 15 phototransistor 40 at the transmitting remote subsystem 14 that is passed to the conductor 20 of bus 16 is carried by conductor 20 only to the transceiver 24 and to none of the receivers 28 in the remote subsystems (including the receiver 28 in the transmitting remote - ' -- ••• 20 • subsystem).-- This is' dύe to the arrangement of the LED's " ' 34. Each LED 34 is back-biased when conductor 22 is at a higher potential than conductor 20 and, consequently, none of the current is drawn off by any one of the LED's 34. 25 Of course, at the same time, each of the phototransistors 40 acts as an open circuit except for the phototransistor 40 at the transmitting subsystem. This feature just described is an important aspect of the present invention in that it permits the 30 small current that is passed by the phototransistor 40 at any transmitting subsystem 14 to be received sub¬ stantially undiminished at the transceiver 24. Fig. 2 shows, in simplified form, circuitry that could be used within the transceiver 24. As can be 35 seen, transceiver 24 includes two drivers 41 and 42. Driver 41 receives the signal to be transmitted (TDATA) directly at its positive (+) input and by way of an OMPI < ?N θ -8- open collector inverter 43 at its negative (-) input. Driver 42 receives TDATA directly at its negative input and by way of the inverter 43 at its negative input. Driver 42 is connected to ground at its output by a resistor 44, with the voltage across resistor 44 repre¬ senting a signal (RDATA) received from one of the remote subsystems. Fig. 3A shows the values of TDATA, Vg (the voltage across conductors 20 and 22) , and RDATA, all 10 when the transceiver 24 is transmitting a signal from the central subsystem 12 to one of the remote subsystems 14. Fig. 3B also shows the values of TDATA, V_, and RDATA, but when the transceiver 24 is receiving a signal from one of the remote subsystems 14. 15 Referring now to Fig. 3A (in conjunction with Figs. 1 and 2), when the central subsystem 12 is trans¬ mitting a signal, the value of TDATA changes between "0" and "1", representing the value of the binary data being sent to a remote subsystem 14. The outputs of drivers 20-.- • 1 a d 42. are- lways. of opposite polarity and the volt- • a ζf e v c across conductors 20 and 22 swings between posi- tive and negative values (+V and -V " τ ) , corres¬ ponding to the changes in the binary values of TDATA. However, the current flow is only in one direction (as 25 illustrated by arrow 45), since all of the LED's 34 will pass current only in one direction and none of the phototransistors 40 are conducting. Referring now to Fig. 3B (in conjunction with Figs. 1 and 2), when one of the remote subsystems 14 is 30 to transmit a signal (under the control of the central subsystem 12), the signal TDATA goes to a constant positive voltage value (represented by the binary "1" value in the drawings) . The current now flows in the opposite direction (illustrated by arrow 46) and V_- 35 remains at a reverse or negative polarity (with fluc¬ tuation between a minimum value -V R-,M..I-.N. x and a maximum value -V-,,..,-- due to the modulation of the current by the RMAX "B J E4 OMPI 'fa transmitting remote subsystem 14) . The modulated current returns to the transceiver 24 along conductor 20 and to ground by way of resistor 44. The resulting signal RDATA will change between "0" and "1", representing the binary values of the data received from the remote subsystem. Referring now to Fig. 4, there is shown a data processing system 10A that illustrates an alternate embodiment of the present invention. Like the system 10 of Fig. 1, the system 10A includes a central subsystem 12A and a plurality of remote subsystems, only two of which are illustrated and designated 14A and 14B. The remote subsystems 14A and 14B are connected to a bus 16A that includes two conductors 20A and 22A. The bus 16A is connected to the central subsystem 12A by way of a transceiver 24A. In the system 10A, the remote subsystem 14A represents a subsystem that is electrically isolated from the bus 16A, and the remote subsystem 14B repre- sents' a- " subsystem that'is not electrically 'isolated from the bus 16A. The system 10A, having both electrically isolated and non-electriσally isolated subsystems, represents a point-of-sale system that is a preferred embodiment, since it is more likely to be found in actual practice. The presence of both isolated and non-isolated subsystems is often required in a point-of-sale system where some peripheral units connected to the central point-of-sale subsystem or terminal may contain their own power supplies and, as illustrated by remote sub¬ system 14A, need to be electrically isolated from the bus 16A, and where other peripheral units, such as the illustrated remote subsystem 14B, may be powered by the same power supply as the point-of-sale terminal and do not need to be electrically isolated from the bus 16A. An example of such an isolated peripheral unit having its own power supply would be a remote printer or other unit that is at such long distance from the central subsystem 12A that the use of a power line stretching from the central subsystem to the remote subsystem would result in a significant loss in power. Since remote subsystem 14A is electrically isolated, its connection to bus 16A is similar to the connection of the remote subsystems 14 to bus 16 in Fig. 1. That is, the remote subsystem 14A has a receiver 28A with an LED 34A and a phototransistor (PT) 36A, and a transmitter 30A with an LED 38A and a phototransistor 40A. Since the remote subsystem 14B is not elec¬ trically isolated from the bus 16A, it has electrical connections both at its input and its output to bus 16A. At its input, remote subsystem 14B is connected to the bus 16A by way of a differential receiver 48 which detects signals from the central subsystem 12A on con¬ ductors 20A and 22A of bus 16A and passes those signals to the remote subsystem 14B. At its output, subsystem -14B- is- connected to-the- us 16A"by a dual" output- (double- - ended) transmitter 49, which receives signals from remote subsystem 14B, and passes those signals to the conductors 20A and 22A of bus 16A. For reasons which will become apparent later, it is important in the system 10A that the receiver 48 have a high resistance across its inputs and that transmitter 49 have a high resistance across its output. Although the specific operation of transceiver 2 A and the nature of the signals on conductors 20A and 22A in the system 10A of Fig. 4 will be described later in conjunction with Figs. 5, 6A and 6B, it should be apparent that the general operation of system 10A is similar to that of system 10 in Fig. 1. When a signal is to be transmitted from the central subsystem 12A to one of the remote subsystems 14A and 14B, the voltage V results in current being passed in a direction from the transceiver 24A along the conductor 20A. The LED ( OMPI 34A in the isolated subsystem 14A receives the current and generates an optical signal that is received by the phototransistor 36A. The voltage Vg across the conduc¬ tors 20A and 22A is also provided to the positive (+) 5 and negative (-) input terminals of receiver 48 at the non-isolated subsystem 14B, resulting in the same signal being received by remote subsystem 14B as was received by remote subsystem 14A. When a signal is to be transmitted by one of 10 the remote subsystems 14A or 14B, the polarity of the voltage V_ across, and the direction of the current on, the conductors 20A and 22A is reversed, so that current is now passed from transceiver 24A along conductor 22A. If the isolated subsystem 14A is providing a signal to 15 the central subsystem 12A, then the phototransistor 40A in the transmitter 30A modulates the current to repre¬ sent the transmitted signal. If the non-isolated sub¬ system 14B is providing a signal to the central sub¬ system 12A, then the transmitter 49 modulates the cur- ' •" 2 ' O ' " rent " arid passes' the ' current from conductor' 22A to con¬ ductor 20A in response to a controlling signal at the output of the subsystem 14B. When either the LED 38A in transmitter 30A of subsystem 14A or the transmitter 49 of the subsystem 25 14B is transmitting a signal to the central subsystem 12A, current is not drained off by the receivers 28A or 48. Like the system 10 in Fig. 1, the system 10A is so arranged that the LED 34A in receiver 28A of subsystem 14A is reversed or back-biased. The receiver 48 has a 30 sufficiently high input resistance to minimize the current drain from the bus. The transceiver 24A can have the same circuit¬ ry as transceiver 24 that was described earlier and shown in simplified form in Fig. 2. Such circuitry is 35 illustrated in greater detail in the detailed circuit diagram of Fig. 5. The transceiver 24A in Fig. 5, like the transceiver 24 in Figs. 1 and 2, passes current only in one direction (to the conductor 22A in the direction of arrow 51 in Fig. 5) when it is receiving a signal and passes current only in the opposite direction (to con¬ ductor 20A in the direction of arrow 53 in Fig. 5) when transmitting a signal. The transceiver 24A is shown in Fig. 5 as receiving a signal TDATA that represents data (in inver¬ ted form) that is to be transmitted on the bus 16A, and providing a signal RDATA that represents data (in inver- ted form) that is received from the bus 16A. The signal TDATA is provided to a set of inverters 50A, 50B, 50C, 50D and 50E, with inverters 50C and 50D receiving the signal TDATA by way of an open collector inverter 52. Four transistors 56, 58, 60 and 62 control the current provided to bus 16A, so that signals can be either transmitted or received by the transceiver 24A. The base of transistor 56 receives the tied outputs of inverters 50A and 50B, the base of transistor 58 re¬ ceives the output of inverter 50C, the base of trans- istor ' 60 rec ' ei ' ve ' s the output of inverter 50D ' , an the- ' base of transistor 62 receives the output of inverter 50E. Transistors 56 and 60 each have their collector connected to a voltage source of V and have their emitters connected to the collectors of transistors 58 and 62, respectively. The emitter of transistor 58 is connected both to ground by a resistor 64 and to the base of a transistor 66. The transistor 66 provides the inverted received data signal RDATA. The emitter of transistor 62 is connected to ground. The bases of transistors 56, 58, 60 and 62 are each connected to the voltage source V by way of resistors 70, 72, 74 and 76, respectively. The col¬ lector of transistor 66 is also connected to the voltage source V " cc by way of a resistor 78. '* . • As illustrated in Fig. 5 by the portions of the circuitry enclosed in broken lines, the inverters 50A-50C, the transistors 56 and 58, and the resistors 70 and 72 perform the function-of a driver 42A (analogous OMPI to driver 42 in Fig. 2) and the inverters 50D and 50E, transistors 60 and 62, and resistors 74 and 76 perform the function of a driver 41A (analogous to driver 41 in Fig. 2). 5 The operation of transceiver 24A will now be described, referring to Fig. 5 in conjunction with Figs. 6A and 6B. Fig. 6A illustrates the signals RDATA, TDATA, and V , when the central subsystem and its transceiver 24A are transmitting a signal to the remote 10 subsystems 14A and 14B, and Fig. 6B illustrates the signals RDATA, TDATA, and V when the central subsystem and its transceiver 24A are receiving a signal from one of the remote subsystems 14A and 14B. When the transceiver 24A is transmitting a 15 signal to one of the remote subsystems 14A and 14B, the signal TDATA, as illustrated in Fig. 6A, represents data having either a logic level "0" or a logic level "1". When TDATA is at a "0", the output of the inverters 50A, 50B, and 50E are at a "1", and the transistors 56 and 62 • .v20- * - are""enabredv." The- conductor 20A of bus-16A is thereby connected to the source V " cc by-* transistor 56 and con- ductor 22A is connected to ground by transistor 62. The voltage V q across conductors 20A and 22A thus goes to a positive transmitting maximum voltage level {+V ) , 25 and current flows from the voltage source V through transistor 56, out of the transceiver 24A and along conductor 20A (in the direction of arrow 53), and then returns along conductor 22A, through transistor 62 to ground. During this period, the outputs of inverters 30 50C and 50D are at a "0" and the transistors 58 and 60 are disabled. When the signal TDATA goes to a "1" during the transmission of a signal, the outputs of inverters 50C and 50D go to a "1", and transistors 58 and 60 are 35 enabled. As a result, conductor 22A is connected to the source Vcc by J transistor 60 and conductor 20A is con- nected to ground by transistor 58 and resistor 64, with OMPI s* ΪPO _ the voltage V reversed to a negative transmitting maximum voltage level (- v TMAχ ) • Current, however, will not flow from transceiver 24A to conductor 22A because of the characteristics of LED 34A and receiver 48. LED 34A is, of course, back-biased when conductor 22A is at a higher potential, and the high input resistance of receiver 48 prevents any substantial current flow to receiver 48. During the period that TDATA is at "1 / the outputs of inverters 50A, 5OB, and 50E are at a "0 r and transistors 56 and 62 are disabled. As illustrated in Fig. 6A, while the trans¬ ceiver 24A is transmitting a signal, the inverted re¬ ceived data signal RDATA is in a "don't care" condition. When the transceiver 24A is receiving a signal from one of the remote subsystems 14A and 14B, the nature of the signals RDATA, TDATA, and V g is as illus¬ trated in Fig. 6B. During the entire time that a signal is being received, the signal TDATA is held at a "1" by the central subsystem 12A. As a result, the outputs of - - • the " buf ers- 5OC and 5OD 'in Fig-. 5 * will- be at a "I", causing transistors 58 and 60 to be enabled. At the same time, the outputs of inverters 50A, 5OB and 50E will be at a "0", causing transistors 56 and 62 to be disabled. The voltage V across the conductors 20A and 22A is reversed or negative, between a minimum (-V ) and a maximum (-VR MΛV ) voltage level. When the transceiver is in the receiving mode, the phototransistor 40A (Fig. 4) in the subsystem 14A or the transmitter 49 in the subsystem 14B is controlled by its subsystem to transmit a signal, so that current now flows from the voltage source V through transistor 60 and along conductor 22A (in the direction of arrow 51), and then returns along conductor 20A. The current then passes through transistor 58 and resistor 64 to ground. The resulting voltage across the resistor 64 drives the transistor 66, which in turn causes the signal being transmitted by one of the subsystems 14A and 14B to appear (in inverted form) as the signal RDATA. - JHEAl OMPI As shown in Fig. 6B, RDATA will have the value (either "0" or "1") of the signal from the transmitting subsystem, and the signal TDATA, as mentioned earlier, is controlled by the central subsystem 12A and remains at a logic level "1" during this entire period of time. It should be apparent from the foregoing description of transceiver 24A and its operation that system 10A has the same characteristics of system 10 in order to permit the use of only a two wire bus; namely, that current flows in one direction when the central system is transmitting a signal and flows in the oppo¬ site direction when one of the remote subsystems is transmitting a signal. Turning now to Fig. 7, there is shown in greater detail circuitry 128 that could be used in either the receiver 28 in system 10 of Fig. 1, or the receiver 28A in the system 10A of Fig. 4. As can be seen, the circuitry 128 includes an LED 134 that per¬ forms the function of the previously described LED 34 in Fig. ϊ - or/ the ' LED' 34A I Fig." 4. In addition, the circuitry 128 includes a phototransistor 136 that per¬ forms the functions of the previously described photo¬ transistor 36 in Fig. 1 or the phototransistor 36A in Fig. 4. The receiver circuitry 128 is connected between an isolated remote subsystem (not shown in Fig. 7) and a bus 116 that has two wires or conductors 120 and 122. The LED 134 is connected to conductor 120 by way of a diode 82 and a current-limiting resistor 84. The diode 82 protects the LED 134 from back-bias break- down. The phototransistor 136 is connected at its collector to the voltag 3 e source Vcc and is connected at its emitter to both the base of a transistor 86 and to ground by way of biasing resistor 88. In order to shorten the turn-off time of phototransistor 136, its base is connected to ground by way of a resistor 90. The collector of transistor 86 is connected to the voltage source V by a current-limiting resistor 92 and OMPI the emitter of transistor 86 is connected to ground. The transistor 86 supplies a signal (PRDATA) that represents the signal received from the bus 116. When a signal is transmitted on the bus 116 by a central subsystem, current from conductor 120 is passed through LED 134, causing the phototransistor 136 to drive transistor 86. The resulting voltage across the transistor 86 represents the received signal PRDATA. In Fig. 8, there is shown in greater detail transmitter circuitry 130 that could be used to perform the function of the transmitter 30 in Fig. 1 or the transmitter 30A in Fig. 3. The transmitter circuitry 130 connects an isolated remote subsystem (not shown in Fig. 8) to the conductors 120 and 122 of the bus 116. As seen in Fig. 8, the transmitter circuitry 130 in¬ cludes an LED 138 that performs the function of the LED 38 in Fig. 1 or the LED 38A in Fig. 4, and a photo¬ transistor 140 that performs the function of the photo¬ transistor 40 of Fig. 1 or the phototransistor 40A of Figv-;4V -The transmitter- circuitry " 130 further includes an open collector inverter 96 for receiving a signal PTDATA that is to be transmitted on the bus 116 to the central subsystem. The output of the inverter 96 is connected to the LED 138, which in turn is connected to the voltage source V by way of a current-limiting resistor 98. The phototransistor 140 has its collector connected to the conductor 122, and has both its base and its emitter connected to the conductor 120. The base of phototransistor 140 is connected to conductor 120 by way of a resistor 100 that acts to shorten the turn-off time of the phototransistor 140. In the operation of the transmitter circuitry 130, when the signal PTDATA goes to a "1", the output of the inverter 96 is grounded, and current flows from the source V through the resistor 98 and LED 138 to ground at the inverter 96. The resulting optical signal at LED 138 enables phototransistor 140 and causes current flow from conductor 122, through the phototransistor 140, and back to conductor 120. When PTDATA goes to a "0", neither LED 138 nor phototransistor 140 are conducting and no current passes from conductor 122 to conductor 120. The following Table indicates part numbers or values for various circuit components seen in Figs. 4, 5, 7 and 8: 10 PART NUMBER COMPONENT OR VALUE RECEIVER 48 SN 75176 TRANSMITTER 49 SN 75176 INVERTERS 50 SN 7406/7407 15 INVERTER 52 SN 74LS04 TRANSISTOR 56 2N 4013 TRANSISTOR 58 2N 4013 TRANSISTOR 60 2N 4013 TRANSISTOR 62 2N 4013 -20" -RESXSTOR--64- ■' ■ '■ - ■■ • * ■ "- '100 OHMS- - - TRANSISTOR 66 2N 3904 RESISTOR 70 150 OHMS RESISTOR 72 2.7K OHMS RESISTOR 74 470 OHMS 25 RESISTOR 76 270 OHMS RESISTOR 78 560 OHMS DIODE 82 IN 4454 RESISTOR 84 47 OHMS TRANSISTOR 86 2N 3904 30 RESISTOR 88 300 OHMS RESISTOR 90 100K OHMS RESISTOR 92 1.8K OHMS INVERTER 96 SN 7406 RESISTOR 98 160 OHMS 35 RESISTOR 100 100 OHMS LED 134 H11A1 PHOTOTRANSISTOR 136 H11A1 LED 138 H11A1 PHOTOTRANSISTOR 140 H11A1 OMPI ... ^N A ? Ύ \

Claims: CLAIMS : 1. A data processing system including a central subsystem (12) and a plurality of remote sub¬ systems (14), said remote subsystems (14) being adapted to receive signals from said central subsystem (12) and to transmit signals to said central subsystem (14), characterized by: a bus (16) connected to said central subsystem (12) and said remote subsystems (14), said bus (16) including a first conductor (20) and a second conductor (22); first unidirectional signal means (34) 0 connected to said bus (16) at least at one of said remote subsystems (14) and adapted to receive signals transmitted by said central subsystem (12), said first unidirectional signal means (34) only passing signals in a direction from said first conductor (20) to said 5 second conductor (22); and second unidirectional signal means (40) connected to said bus (16) at said one of said remote subsystems (14) and adapted to transmit - - - " -signals -to-'said- central " subsystem' (12) ' ,.' said second unidirectional signal means (40) passing signals only in 0 a direction from said second conductor (22) to said first conductor (20). 2. A data processing system according to claim 1, characterized in that said first unidirectional signal means includes a light-emitting diode (34) con¬ nected between said first and second conductors (20, 22) and in that said second unidirectional signal means includes a phototransistor (40) connected between said first and second conductors (20, 22). 3. A data processing system according to claim 2, characterized in that central subsystem (12) includes a transceiver (24) having a first driver (42) adapted to pass current to said first conductor (20) 5 when said central subsystem (12) is transmitting a message and a second driver (14) adapted to pass current to said second conductor (22) when said central sub¬ system (12) is to receive a message from one of said remote subsystems (14), the current from_said second 10 driver (41) being modulated by the phototransistor (40) at the one of said remote subsystems (14) transmitting the message so that modulated current is passed through that phototransistor (40) and returned along said first conductor (20) to said transceiver (24). 4. A data processing system according to claim 3, characterized in that first driver includes a first transistor (56) connected to a voltage source and connecting said voltage source to said first conductor 5 (20) when said central subsystem (21) is to transmit a signal, said first transistor (56) being controlled in accordance with the message to be transmitted to said central subsystem (12), and in that said second driver includes a second transistor (60) connecting said i0 ' - . voltage" source- to- said- second- conductor (22) when one- of. said remote subsystems (14) is to transmit a message to said central subsystem (12) so that current passes along said second conductor (22) and is modulated at the one of said remote subsystems (14) that is to transmit a 15 message. 5. A data processing system according to claim 2, characterized by a further remote subsystem (14B) coupled to said bus (16) and including a high input resistance differential receiver (48) having first 5 and second inputs connected respectively to said first and second conductors (20, 22), and a high output re¬ sistance transmitter (49) having first and second outputs connected respectively to said first and second conductors (20, 22). - L\TΕ ι OMPI ^*NATlθ5

Inventors: ADAMOWSKY MICHAEL NICHOLAS, CHRISTIE JOHN BRADLEY, CULL DUNCAN ERSKINE, ADAMOWSKY, MICHAEL, NICHOLAS, CHRISTIE, JOHN, BRADLEY, CULL, DUNCAN, ERSKINE
Applicants: NCR CO, NCR CORPORATION
Application year: 1984
Main classification: G06F13
IPC codes: H04B10, H04L12