Autonomous Terminal Data Communications System

This invention generally relates to the field of data communications, and, more particularly, to a system providing data communication among a plurality of terminals over a common data communication medium, with each terminal having autonomous access to the data communication medium independent of any other terminal or device.

In modern aircraft, it is desirable to integrate, as far as possible, the functions of previously-independent avionic systems to permit an attendant reduction in weight, space and power requirements of the avionic systems and a simplification in wiring between physically separated avionic systems or subsystems thereof. Such integration has been achieved by the use of a common data communication medium, or data bus, to which each avionic system, or subsystem thereof, has access through an associated terminal, each of which is capable of transmitting and receiving data on the data bus. Data transmitted on the data bus by one terminal associated with a particular system or subsystem can be received by the terminals associated with the remaining systems or subsystems, thus eliminating the requirement for separate wiring interconnections between the systems or subsystems. In addition, data generated in a particular system or subsystem can be used by any other system or subsystem without the necessity of having to independently generate that data.

In order for a data communication system using a data bus to properly and reliably operate, without degradation upon the performance of any of the systems or subsystems coupled thereto, it is desirable that the data communication system allow tranmission of data originating from any terminal to any or all of the remaining terminals using a minimum physical length of the data bus. Also, the data communication system must be capable of accommodating periodic tranmission of any or all data. For example, if a particular terminal is inhibited from transmitting in a periodic manner, then those terminals that depend upon such data may malfunction. Further, operation of any one of the systems, subsystems or terminals associated therewith, must not be affected by the operational status of any other system, subsystem or terminal, whether such other system, subsystem or terminal be operating normally or be malfunctioning. Finally, the data bus, and any devices for coupling the data bus to each terminal which conceivably could render the data bus unusable, must be of such high functional integrity so as to make the possibility of data bus failure extremely remote.

The data communication systems using a data bus that have been particularly proposed for use in aircraft have taken two forms. The first, known as a broadcast bus, includes a single data bus which is exclusively assigned to one data source terminal. Data transmitted on the data bus from the data source terminal can be received by any number of receiver terminals coupled to the data bus. Although the broadcast bus is capable of accommodating periodic data transmissions, it is not capable of bilateral data communications. Accordingly, a separate data bus must be used for each data source terminal. Also, any device requiring the use of data from more than one data source terminal must be separately coupled with each required data bus. Accordingly, the broadcast bus does not permit a significant reduction to be made in aircraft wiring interconnections.

The second, known as a multiplex bus using a central bus controller, includes a single data bus having coupled thereto a plurality of remote terminals and a central bus controller terminal. The bus controller terminal determines (under software control) the mode of operation of any remote terminal, and controls such mode of operation by the transmission of command words on the bus. The multiplex bus, although capable of significantly reducing aircraft wiring interconnections, is not inherently suitable for periodic data transmission by each remote terminal. More important, the operation of remote terminals is not autonomous due to their dependence on the bus controller terminal, so that failure of the bus controller terminal constitutes failure of all remote terminals associated with the data bus.

It is therefore an object of this invention to provide an improved data communication system using a data bus.

It is a further object of this invention to provide such a data communication system which permits each participating terminal to have autonomous access to the data bus.

It is yet a further object of this invention to provide such a data communication system in which normal operation of any participating terminal, or malfunction thereof, will not affect the operation of the data communication system or of any other participating terminal associated therewith.

It is another object of this invention to provide such a data communication system which permits each participating terminal to engage in periodic data tranmissions on the data bus.

It is yet another object of this invention to provide such a data communication system which affords multiplexed data communications among participating terminals without the requirement for a central bus controller terminal.

It is still another object of this invention to provide such a data communication system which permits a significant reduction in the wiring requirements between participating systems and a significant increase in the integration of such systems.

The foregoing objects, and other objects and advantages that will be apparent to those of ordinary skill in the art, are achieved by the use of a method which permits a plurality of terminals to autonomously and periodically transmit messages on a data communication medium. At each terminal, the absence of messages on the data communication medium is detected. Transmission of a message by each terminal is enabled, however, only upon the occurrence of an absence of messages on the data communication medium whose duration substantially equals that of an inter-message gap unique to that terminal, and, the elapse of a transmission interval from a previous transmission of a message by that terminal. The duration of the transmission intervals of all terminals is substantially the same and is preferably chosen to be greater than the sum of all terminal inter-message gaps and of the durations of all messages transmitted by the terminals since a previous transmission of a message by each terminal.

The invention can best be understood by reference to the following portion of the specification, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of the data communication system using a data bus and having a plurality of participating terminals coupled thereto;

FIG. 2 is a block diagram of a typical participating terminal;

FIG. 3 is a timing diagram illustrating the operation of the data communication system under ideal, steady-state conditions;

FIG. 4 is a block diagram of a portion of a terminal control unit in FIG. 2;

FIG. 5 is a block diagram of a simplified version of a participating terminal, and of an associated utilization device and interface unit;

FIG. 6 is a block diagram of a demodulator and a receiver in FIG. 5;

FIG. 7 is a block diagram of a transmitter and a modulator in FIG. 5;

FIG. 8 is a timing diagram illustrating the operation of the transmitter and modulator in FIG. 7;

FIG. 9 is a block diagram of a sync pattern detection circuit in FIG. 6; and,

FIGS. 10 and 11 are signal diagrams illustrating the operation of the sync pattern detection circuit in FIG. 9.

With reference now to FIG. 1, the data communication system illustrated therein is adapted to provide data communications among a plurality of utilization devices UD.sub.1 -UD.sub.5. In practical applications, the utilization devices UD.sub.1 -UD.sub.5 may comprise a plurality of avionic systems, or subsystems, or, a plurality of computer-related devices such as a central processing unit (CPU), input/output (I/O) devices, displays, memories, or the like. Each of the utilization devices UD.sub.1 -UD.sub.5 may include means for transmitting data, means for receiving data, or both. In FIG. 1, utilization devices UD.sub.1 -UD.sub.4 are capable of transmitting and receiving data, whereas utilization device UD.sub.5 is capable only of receiving data. The data communication system of the present invention includes a plurality of terminals TL.sub.1 -TL.sub.5 which are associated with respective ones of the utilization devices UD.sub.1 -UD.sub.5, and also includes a common data communication medium to which the plurality of terminals TL.sub.1 -TL.sub.5 are coupled. In the embodiment shown in FIG. 1 and discussed hereinafter, the data communication medium comprises a data bus DB, which may take many forms, such as one or more electrical conductors, a magnetic member, a waveguide, or a fiber-optic member. Preferably, the data bus DB includes a single twisted pair of wires extending to the physical locations of the utilization devices UD.sub.1 -UD.sub.5. The present invention does not require, however, that the data communication medium be limited to a physical apparatus such as data bus DB, so that the data communication medium may comprise any appropriate carrier, such as audio, radio or light frequency waves or pulses, which is capable of carrying information by way of modulation thereof.

In FIG. 1, each terminal includes a data communications link 10 for exchanging data with its associated utilization device, an output 12 upon which transmitted data appears and which is coupled to the data bus DB by a bus coupler 13, and an input 14 for receiving data present on the data bus DB, with the input 14 being coupled to the data bus DB by a bus coupler 15. In the embodiment shown in FIG. 1 and discussed hereinafter, the data to be transmitted and received using the data bus DB is in the form of messages comprising one or more, succesive serial digital words.

Referring now to FIG. 2, each terminal includes a demodulator 16 for demodulating the messages on the data bus DB received via coupler 15 and input 14 and for supplying the demodulated messages to a receiver 18. The demodulated messages are temporarily stored within the receiver 18, and the data therein is sent in an appropriate format to the associated utilization device under control of a terminal control unit 20 which exchanges "interface handshake" information with the utilization device and which provides the utilization device with an identification of the data that has been received. A transmitter 22 receives data from its associated utilization device, again under control of the terminal control unit 20 and outputs such data in the form of messages comprising one or more successive, several digital words at periodic transmission intervals determined by the terminal control unit 20. A modulator 24 modulates the messages in a desired manner and supplies the modulated messages to the data bus DB via the output 12 and bus coupler 13.

Normally, the receiver 18 in each terminal is enabled and the transmitter 22 is disabled. The terminal control units 20 in the plurality of terminals TL.sub.1 -TL.sub.4 utilize an identical and unique terminal control routine, or protocol, which insures that only one terminal is transmitting at a given time and that, under steady-state conditions, transmission by any terminal occurs at periodic transmission intervals.

In detail, the protocol requires that: (a) the transmission interval for each terminal be the nominal time interval between the initiation of periodic data transmissions by the terminal, and that the transmission intervals for all terminals be approximately the same in duration; (b) each transmitting terminal be capable of transmitting one message, including one or more data words, with appropriate labels, check words and parity words during each transmission interval; (c) each message can have any desired duration so long as such duration is fixed during times when periodic transmission by the terminal is required and so long as the sum of the durations of the messages of all terminals, and all inter-message gaps, plus a growth gap to be described hereinafter, does not exceed the duration of each transmission interval; (d) each transmitting terminal initiates data transmission only upon the concurrence of the expiration of an inter-message gap on the data bus which is unique to that transmitting terminal, and, the elapse of a period of time, from a previous transmission by the terminal, which is substantially equal to the transmission interval; and, (e) any inter-word gaps in any message have a duration which is less than that of the shortest intermessage gap.

Requirements (a) and (d) of the protocol must be met in every application of the invention and can be embodied in each terminal control unit 20 by the use of circuitry such as illustrated in FIG. 4. A transmission interval timer 30 is reset at the time of a previous message transmission by the terminal, as explained hereinafter, and provides an output signal at elapse of a predetermined period of time thereafter, the predetermined period being substantially equal to the transmission interval. A word gap detector 32 receives the demodulated messages from the receiver 18 and provides an output signal only when such demodulated messages are absent, signifying an inter-word or inter-message gap on the data bus DB. A gap timer 34 is adapted to time the duration of each output signal from word gap detector 32 and to provide an output signal only when the duration of the output signal from word gap detector 32 exceeds the unique inter-message gap assigned to the terminal. An AND gate 36 provides a transmitter enable signal to the transmitter 22 only upon concurrence of the output signals from transmission interval timer 30 and gap timer 34, so that transmitter 22 is enabled to transmit a message on the data bus DB. The resultant transmission of a message causes word gap detector 32 to terminate its output signal, thereby resetting gap timer 34, and the transmitter enable signal is used to reset transmission interval timer 30, so that the terminal control unit 20 is again enabled to detect the conditions that will permit a subsequent message transmission by the terminal.

To more fully understand the operation of the data communication system, reference should be made to FIG. 3 which comprises a timing chart illustrating a plurality of successive cycles of operation of the data communication system under ideal, steady-state conditions.

Terminals TL.sub.1 -TL.sub.4 are capable of transmitting messages, with the message being transmitted by each of the terminals TL.sub.1 -TL.sub.3 including two successive data words, each data word being of a predetermined length and being preceded by a label identifying the data therein, and with the data words in each message being separated by an inter-word gap. As illustrated, the messages M.sub.11, M.sub.21 and M.sub.31 transmitted by the terminals TL.sub.1 -TL.sub.3 during the first illustrated cycle of operation of the data communiction system are of equal duration. The message M.sub.41 of terminal TL.sub.4 includes three data words, each data word being of a predetermined length and being preceded by a label identifying the data therein, the data words again being separated by an inter-word gap and with message M.sub.41 therefore being longer in duration than messages M.sub.11, M.sub.21 and M.sub.31. During the second cycle of operation of the data communication system illustrated in FIG. 3, terminals TL.sub.1 -TL.sub.3 transmit messages M.sub.12, M.sub.22 and M.sub.32, each of identical duration to corresponding messages M.sub.11, M.sub.21 and M.sub.31, and terminal TL.sub.4 transmits message M.sub.42 of identical duration to message M.sub.41.

The terminal control unit 20 in each of the terminals TL.sub.1 -TL.sub.4 establishes (through its gap timer 34) a unique inter-message gap for the associated terminal (that is, gaps g.sub.1, g.sub.2, g.sub.3 and g.sub.4 for terminals TL.sub.1, TL.sub.2, TL.sub.3 and TL.sub.4, respectively, with g.sub.1 <g.sub.2 <g.sub.3 <g.sub.4). The terminal control unit 20 in each of the terminals TL.sub.1 -TL.sub.4 also establishes a transmission interval through its transmission interval timer 30 (that is, transmission intervals T.sub.1, T.sub.2, T.sub.3 and T.sub.4 for terminals TL.sub.1, TL.sub.2, TL.sub.3 and TL.sub.4, respectively, with T.sub.1 .perspectiveto.T.sub.2 .perspectiveto.T.sub.3 .perspectiveto.T.sub.4). As previously indicated, the transmitter 22 in each of the terminals TL.sub.1 -TL.sub.4 is not enabled until concurrence of expiration of the unique inter-message gap associated with the terminal and elapse of the transmission interval for that terminal.

With reference again to FIG. 3, let it be assumed that transmitter 22 in terminal TL.sub.1 is transmitting message M.sub.11, during its associated transmission interval T.sub.1, and that the transmission intervals T.sub.2 -T.sub.4 have not yet elapsed. At the termination of message M.sub.11, the absence of message on the data bus DB causes the gap timers 34 in each of the terminals TL.sub.1 -TL.sub.4 to begin timing. After the expiration of a time equal to gap g.sub.1, the gap timer 34 in terminal TL.sub.1 provides an output signal. However, since the transmission interval T.sub.1 for terminal TL.sub.1 has not yet again elapsed, no transmitter enable signal is provided by AND gate 36 in terminal TL.sub.1, and accordingly transmitter 22 therein is not enabled. Subsequently, the gap timer 34 in terminal TL.sub.2 provides an output signal upon expiration of the gap g.sub.2 associated therewith. Under the steady-state operation illustrated in FIG. 3, the transmission interval T.sub.2 for terminal TL.sub.2 will also have elapsed, so that a transmitter enable signal is provided by the AND gate 36 in terminal TL.sub.2 to enable the transmitter 22 therein, whereby the message M.sub.21 is transmitted.

After the transmission of the first data word in message M.sub.21, the gap timers 34 in the terminals TL.sub.1 and TL.sub.3 -TL.sub.4 are again enabled. However, since the inter-word gap is shorter than any of the inter-message gaps (the shortest of which is gap g.sub.1), the initiation of transmission of the second data word in message M.sub.21 occurs before any of the gap timers 34 provides an output signal, theeby inhibiting the terminals TL.sub.1, TL.sub.3 and TL.sub.4 from transmission during message M.sub.21. At the termination of message M.sub.21, the gap timers 34 in each of the terminals TL.sub.1 -TL.sub.4 are again enabled. Upon successive expiration of the gaps g.sub.1 and g.sub.2, the gap timers 34 in terminals TL.sub.1 and TL.sub.2 again provide an output signal. However, the transmitters 22 in terminals TL.sub.1 and TL.sub.2 are not enabled inasmuch as their respective transmission intervals T.sub.1, T.sub.2 have not again elapsed. Upon subsequent expiration of the gap g.sub.3, the gap timer 34 in terminal TL.sub.3 provdes an output signal. Since the transmission interval T.sub.3 associated with terminal TL.sub.3 has also elapsed, a transmitter enable signal is provided by AND gate 36 in terminal TL.sub.1 to enable transmitter 22 therein so that message M.sub.31 is transmitted. Similarly, upon termination of message M.sub.31, and after expiration of gap g.sub.4, transmitter 22 in terminal TL.sub.4 is enabled to transmit message M.sub.41.

After the termination of message M.sub.41, the gap timers 34 in each of the terminals TL.sub.1 -TL.sub.4 are again enabled. As previously noted, the length of each transmission interval T.sub.1 -T.sub.4 is chosen to be somewhat longer than the duration of all messages during each cycle of operation of the data communication system (e.g., M.sub.11 +M.sub.21 +M.sub.31 +M.sub.41) and the sum of all inter-message gaps (e.g., g.sub.1 +g.sub.2 +g.sub.3 +g.sub.4). Accordingly, at least the gap timer 34 associated with the terminal having the shortest inter-message gap (e.g., gap g.sub.1 associated with terminal TL.sub.1) provides an output signal before the first-in-time transmission interval (e.g., interval T.sub.1) again elapses. Upon the subsequent elapse of transmission interval T.sub.1, the transmitter 22 in terminal TL.sub.1 is next enabled to transmit the message M.sub.12, whereupon a second cycle of the data communication system is initiated. Operation of the data communication system then proceeds in a manner identical to that previously described for the first cycle of operation thereof, with the successive transmission of messages M.sub.22, M.sub.32 and M.sub.42 upon expiration of the gaps g.sub.2, g.sub.3 and g.sub.4 and the elapse of the transmission intervals T.sub.2, T.sub.3 and T.sub.4 respectively associated with terminals TL.sub.2, TL.sub.3 and TL.sub.4.

Since the receivers 18 in each of the terminals TL.sub.1 -TL.sub.5 associated with the data bus DB are continuously enabled it can be seen that the data words on the data bus DB can be detected by any of the participating terminals and transferred, upon proper identification, to their associated utilization device.

In order that only one terminal be in a transmitting mode at any one time, it is essential that the protocol previously discussed be strictly adhered to. Although it is not necessary that the transmission interval timers 30 be synchronized with each other, it is necessary that all transmission intervals be approximately equal in duration so that a successive transmission interval (e.g., T.sub.2) does not elapse at or before the elapse of a previous transmission interval (e.g., T.sub.1). Likewise, although it is not necessary that the gap timers 34 be synchronized, it is necessary that they have a stable time base so that the gap of any terminal does not approximate the gap of any other terminal. Finally, the time base establishing the inter-word gap in each of the terminal control units 20 must be stable so that the inter-word gap of any terminal does not equal or exceed the inter-message gap of any terminal.

In order to insure that the protocol is strictly adhered to, each terminal includes a terminal monitor 40 (FIG. 2) which receives signals from the receiver 18, the terminal control unit 20, and the transmitter 22 therein. Each terminal monitor 40 includes an independent time base which establishes the desired inter-word gap, inter-message gap, and transmission interval for its associated terminal. The terminal monitor 40 also includes circuitry for comparing the actual inter-word gap, inter-message gap, and transmission interval provided by its associated terminal with the desired inter-word gap, inter-message gap, and transmission interval established by the independent time base therein. If any of these comparisons fail to be made, the terminal monitor 40 supplies a signal to a modulator enable switch 42 which responsively removes power from the modulator 24 to inhibit further transmissions by the associated terminal.

The cycles of operation illustrated in FIG. 3 represent an ideal, steady-state situation which will not likely be encountered in actual practice. To give an example, let it be assumed that the data communication system has been inoperative and that power is subsequently applied to terminals TL.sub.1 -TL.sub.5. Since the power supplies in the terminals TL.sub.1 -T.sub.5 will not all come up to operating specifications at the same time, it can be seen that one of the terminals TL.sub.1 -TL.sub.5 will be capable of transmission before the other terminals. Let is be assumed that terminal TL.sub.4 is first capable of transmission, and accordingly initiates transmission of a message therefrom on the data bus DB and initiates its associated transmission interval T.sub.4. After termination of the message from terminal TL.sub.4, let it be assumed that only terminal TL.sub.2 is then capable of transmission. Accordingly, after termination of the inter-message gap g.sub.2, terminal TL.sub.2 initiates transmission of a message on the data bus DB and initiates its transmission interval T.sub.2. Subsequently, let it be assumed that terminals TL.sub.1 and TL.sub.3 successively engage in transmissions upon the data bus DB, and initiate their respective transmission intervals T.sub.1, T.sub.3. As a result, it can be seen that a cycle of operation of the data communication system will consist of successive transmissions from terminals TL.sub.4, TL.sub.2, TL.sub.1 and TL.sub.3, with simultaneous transmission by two or more terminals being inhibited by the protocol which requires concurrence of the expiration of the unique inter-message gap and the elapse of the transmission interval associated with each terminal.

It should also be noted that the "growth gap", or the time between the termination of the last message in the cycle of operation of the data communication system and the initiation of a successive cycle of operation thereof, always precedes the transmission interval of the terminal having the slowest time base, that is, the terminal whose transmission interval timer 30 has the longest actual period. To understand this point, reference should again be made to FIG. 3. Let it be assumed that the transmission interval timer 30 in terminal TL.sub.4 has the longest actual period. Accordingly, the time at which a message transmitted by terminal TL.sub.4 occurs, relative to the time ocurrence of the termination of the message transmissions from the remaining terminals TL.sub.1 -TL.sub.3, is successively delayed in successive cycles of operation of the data communication system, with the result that the growth gap illustrated in FIG. 3 becomes sucessively shorter, until the growth gap is reduced to the inter-message gap g.sub.1 associated with terminal TL.sub.1. At the same time, the gap between termination of each message transmission by terminal TL.sub.3 and the initiation of each message transmission by terminal TL.sub.4 successively increases until it reaches a duration equal to the duration of the growth gap illustrated in FIG. 3. As a result, the periodicity of each cycle of operation of the data communication system will be determined by that one of the terminals TL.sub.1 -TL.sub.4 having the slowest transmission interval timer 30.

To further illustrate the operation of the data communication system of the present invention, reference will now be made to the construction and operation of a simplified version of each transmitting terminal TL.sub.1 -TL.sub.4.

In FIG. 5, the data bus DB comprises a twisted pair of wires 100 extending to all terminals, with the terminations (not illustrated) of the twisted pair of wires 100 being short-circuited so that the data bus DB comprises a single, continuous current loop. Messages on the data bus DB are in the form of currents, and are coupled to and from each transmitting terminal by an associated bus coupler 102 which includes a core 104 whose legs are inserted into two adjacent loops formed by the twisted pair of wires 100 so that each wire thereof constitutes a single turn of a primary winding of the bus coupler 102. A secondary winding 106 is also wound around the core 104 and is interconnected with the transmitting terminal by a terminal stub 108 comprising a twisted pair of wires. Preferably, the data bus DB and the bus coupler 102 are those disclosed and claimed in my copending application entitled "Current Mode Data or Power Bus," filed concurrently herewith, and reference should be made to that application for further details thereof.

Within the terminal, the terminal stub 108 is connected in common to the input of a demodulator 110 and to the output of a modulator 112. The output from demodulator 110 comprises a signal DMR which represents the demodulated message and which is applied to the input of a receiver 114 whose construction and operation will be described hereinafter with reference to FIG. 6. The outputs from receiver 114 comprise a signal ADA, which is produced upon the absence of messages upon the data bus DB, and a plurality of parallel, binary output signals DR, which represent the data within any message on the data bus DB and received by the receiver 114. An interface unit 116 is provided for controlling the interchange of data between the terminal and its associated utilization device 118. Included in the interface unit 116 is a latch circuit 120 to which is applied the parallel, binary output signals DR from receiver 114 and which is operative to store the received data represented by such signals DR under control of signal LD from receiver 114 as described hereinafter. The received data in latch 120 is converted into an analog signal by a D/A converter 122, amplified by an amplifier 124, and supplied to the utilization device 118.

The utilization device 118 provides an analog output signal, representing the data to be transmitted, to interface unit 116. Within interface unit 116, the analog output signal from utilization device 118 is amplified by an amplifier 126, converted into a parallel, digital format by an A/D converter 128, and stored in a shift register 130. Under control of signals T, TS and XD from a transmitter 132 within the terminal, as described hereinafter, the transmitted data within shift register 130 is transferred as a signal DT, in serial, digital form, to transmitter 132. At periodic transmission intervals in accordance with the above-described protocol, and in response to the signal ADA from receiver 114, transmitter 132 provides an output signal DM, in serial digital form, representing the transmitted data, the label or address of the terminal, and synchronizing and parity information. The signals DM and T from the transmitter 132 are applied to modulator 112 whose output has three distinct states. During the time that the signal T is absent, the output of modulator 112 has a high impedance so as to decouple the modulator 112 from the data bus DB. During the time that the signal T is present, signifying the transmission of a message from the terminal, the output of modulator 112 has present thereon an output signal which alternates between first and second levels in response to the signal DM, with the first and second levels being respectively positive and negative. The output signal from modulator 112 is applied directly to the input of demodulator 110, and via terminal stub 108 and bus coupler 102 to the data bus DB.

With reference now to FIG. 8, only one message is transmitted by each terminal during its transmission interval. As a particular example, each message comprises a serial digital word having, in sequential order, a three-bit SYNC word, a three-bit LABEL word, an eight-bit DATA word, a PAD bit, and a PARITY bit. The format of each message transmitted is Manchester bi-phase level modulation, wherein successive positive and negative levels in the output signal from modulator 112 represent a "1" and successive negative and positive levels in the output signal from modulator 112 represent a "0".

With reference now to FIG. 6, the demodulator 110 includes an amplifier 140 which provides a POS output for the duration of each positive level of the signal on data bus DB, and which provides an NEG output for the duration of each negative level of the signal on the data bus DB. At all other times, sense amplifier 140 provides no output. The POS and NEG outputs of sense amplifier 140 are applied to respective inputs of an OR gate 142 whose output comprises the signal ADA. As illustrated in FIG. 8, when either the POS or NEG outputs are being provided by sense amplifier 140, the signal ADA is at a high logic level, signifying the presence of a message on the data bus DB. When neither the POS nor the NEG outputs from sense amplifier 140 are present, the signal ADA switches to a low logic level, signifying the absence of any message on the data bus DB.

Let it be assumed that the terminal under discussion is terminal TL.sub.1 (FIG. 1), with inter-message gap g.sub.1 and transmission interval T.sub.1. With reference to FIG. 7, the signal ADA is applied to a gap timer 150 within transmitter 132. Gap timer 150 may comprise a monostable multivibrator which is adapted to provide a momentary output signal TG at a predetermined time after each high level-low level transition in the signal ADA, with the predetermined time corresponding to the desired inter-message gap g.sub.1. Transmitter 132 also includes a transmission interval timer 152, which also may comprise a monostable multivibrator, and which is adapted to provide a momentary output signal T1 at a predetermined time after the application of a signal ST1 thereto. As will be described hereinafter, signal ST1 is provided at the end of a message transmission from the terminal, and the predetermined time of transmission interval timer 152 is chosen so that the signal T1 is provided at a time corresponding to the desired transmission interval T.sub.1 for the terminal. Signals TG and T1 are applied to respective inputs of an AND gate 154 whose output is connected to the input of a clock control circuit 156, which may comprise a resettable RS flip-flop. With reference to FIG. 8, let it be assumed that message transmission on the data bus DB has terminated. Accordingly, the signal ADA goes to a low logic level. At a time g.sub.1 thereafter, and at the elapse of the transmission interval T.sub.1, signified by the signals TG and T1, respectively, AND gate 154 provides an output signal which sets clock control circuit 156 into a first state thereof, wherein an output signal T thereof goes to a high logic level to initiate a message transmission from the terminal. The signal T is applied to a transmit clock 158, a program counter 160, a shift register 162, a line driver 164, and a parity generator 184 in transmitter 132, as well as to the shift register 130 in interface unit 116, to control the operation of those components as described hereinafter.

When the signal T goes to a high logic level, transmit clock 158 is enabled to produce clock signals TS and TCL (FIG. 8), with the frequency of the signal TCL being twice that of the signal TS. The clock signal TCL is applied to the program counter 160. The clock signal TS is applied to a modulation matrix circuit 168, an AND gate 166, and an AND gate 182 in transmitter 132, as well as to the shift register 130 in interface unit 116.

The program counter 160, which may comprise a fixed-length, binary counter, provides a plurality of binary output signals 160A to a transmit control ROM 170 which is appropriately programmed to respond to the binary output signals 160A to provide signals XSL, XD, CP, XP and XS2 which have a high logic level during a desired transmission of the SYNC word and LABEL word, a desired transmission of the DATA, a desired parity check, a desired transmission of the PARITY bit, and a desired modification of the SYNC word, respectively. More specifically (FIG. 8), XSL goes to a high logic level at the time signal T goes to a high logic level, and goes to a low logic level upon the initiation of the thirteenth pulse in the clock signal TCL. XD goes to a high logic level upon the initiation of the thirteenth pulse in clock signal TCL, and goes to a low logic level at the initiation of the thirty-first pulse in the clock signal TCL. CP goes to a high logic level at the initiation of the seventh pulse in the clock signal TCL, and goes to a low logic level at the initiation of the twenty-first pulse in the clock signal TCL. XP goes to a high logic level at the initiation of the thirty-first pulse in the clock signal TCL, and goes to a low logic level at the time the signal T goes to a low logic level. Finally, XS2 goes to a high logic level at the initiation of the fourth pulse in the clock signal TCL, and goes to a low logic level at the initiation of the seventh pulse in the clock signal TCL.

The three-bit SYNC word and the three-bit LABEL word are stored, respectively, in a SYNC pattern generator 172 and a LABEL generator 174 whose outputs are coupled to respective inputs of shift register 162. When the signal T goes to a high logic level at the beginning of the message transmission, the SYNC word and the LABEL word are transferred into shift register 162. Since the signal XSL is at a high logic level at this time, AND gate 166 applies the clock signal TS to the clock input of shift register 162 so that the SYNC word and LABEL word are strobed out of shift register 162 to form the signal SL. In the example under discussion, the SYNC word is represented by binary "110" and the LABEL word, for terminal TL1, is represented by binary "001". The signals SL and XSL are applied to respective inputs of an AND gate 176, whereby the signal SL is gated to a first input of an OR gate 178. The output signal from OR gate 178 comprises the signal DM which is applied to the modulator 112, and, more specifically, to the modulation matrix circuit 168 therein. Therefore, from the initiation of message transmission to the time that the signal XSL goes to a low logic level, the signal DM comprises the signal SL, as illustrated in FIG. 8.

Although the SYNC word will remain the same for all participating terminals in the data communication system, the LABEL word will of course vary from terminal to terminal. It will also be recognized that the LABEL can identify not only a specific terminal, but also a specific message from that terminal, in which case the LABEL may comprise part of the data to be transmitted by the terminal.

When the signal XSL goes to a low logic level, the signal XD goes to a high logic level. When signal XD goes to a high logic level, shift register 130 is enabled, and the contents thereof are strobed by the signal TS to form the output signal DT. In the example under consideration, the transmitted data is represented by binary "10100010" plus a binary "0" PAD bit which is provided for expansion of such transmitted data. The signals XD and DT are applied to respective inputs of an AND gate 180 whose output is coupled to a second input of OR gate 168. Accordingly, the signal DM comprises the signal DT, for the duration of the signal XD, as illustrated in FIG. 8.

To generate the PARITY bit, the signals DM and TS are applied to respective inputs of an AND gate 182 whose output is coupled to the input of parity generator 184. The output of parity generator 184 is a signal P, representing the PARITY bit, and is applied along with the signal XP to respective inputs of an AND gate 186 whose output is coupled to a third input of OR gate 178. Parity generator 184 is also provided with the signals T and CP, and may comprise a J-K flip-flop. When the signal T goes to a high logic level at the initiation of message transmission, parity generator 184 is set to a state whereupon the output signal P thereof is at a low logic level. During the time that the signal CP is at a high logic level, parity generator 184 is enabled and is responsive to low-high level transitions in the signal DM, as strobed through AND gate 182 by clock signal TS, to toggle its output signal P between low and high logic levels. In the example under consideration, the signal P goes to a high logic level upon the first low-high logic level transition in the signal DM, and then goes to a low logic level at the next low-high logic level transition in the signal DM. When the signal CP returns to a low logic level, the signal P remains at a low logic level and is gated through AND gate 186 at the time that the signal XP goes to a high logic level. Accordingly, the signal DM comprises the signal P, for the duration of the signal XP, as illustrated in FIG. 8.

The signal DM is applied to the modulation matrix circuit 168 which responds to the clock signal TS to develop a Manchester bi-phase level modulated output signal DB therefrom.

The operation of modulation matrix circuit 168 is largely conventional. However, the last bit of the SYNC word is "zero", which would normally result in a low-high logic level transition in the output signal DB.To insure the proper operation of the receivers in the participating terminals of the data communication system, the signal XS2 causes the modulation matrix circuit 168 to maintain the output signal DB at a low logic level during the time occurrence of the third bit in the SYNC word, as best seen in FIG. 8.

The output signal from modulation matrix circuit 168 is applied to the input of the line driver 164. During the time that the signal T from clock control circuit 156 is at a low logic level, the output from line driver 164 is at a high impedance. When the signal T switches to a high level, line driver 164 converts each high logic level of the output signal from modulation matrix circuit 168 into a corresponding positive level, and each low logic level of the output signal from modulation matrix circuit 168 into a corresponding negative level.

At the time that the count in program counter 160 has advanced to its maximum value, program counter 160 provides signal ST1 which resets clock control circuit 156 into a second state. As a result, the signal T goes to a low logic level to terminate the message transmission by resetting program counter 160, clearing shift register 162, clearing shift register 130 in interface unit 116, and returning line driver 164 to a high impedance state. Simultaneously, signal ST1 re-initiates the timing period of transmission interval timer 152. The transmission interval T1 from the terminal TL.sub.1 is accordingly determined by the duration of the message transmission for the terminal, which is of fixed duration, plus the time period established by transmission interval timer 152. Accordingly, even though the gap timer 150 will subsequently provide one or more signals TG, a subsequent message transmission from terminal TL.sub.1, cannot occur until the elapse of the transmission interval T.sub.1 as determined by the signal T1 from transmission interval timer 152.

With reference back to FIG. 6, the signal ADA in the receiver 114 is applied to the input of a clock control circuit 190 and to the input of a word gap detection circuit 192. The clock control circuit 190 may comprise an R/S flip-flop, and the word gap detection circuit 192 may comprise a monostable multivibrator. When the signal ADA goes to a low logic level, signifying the presence of a message on the data bus, the clock control circuit 190 is set to a first state in which an output signal R therefrom has a high logic level. The signal R is applied to a receiver clock 194 which is operative to provide a clock signal RCL whenever the signal R has a high logic level. A shift register 196 is provided for storing messages, as coupled thereto by the POS output of sense amplifier 140 and under control of the clock signal RCL. Therefore, when a message appears on the data bus DB, the signal R goes to a high level, the clock signal RCL is produced, and the message is sequentially transferred into the shift register 196.

The shift register 196 has a plurality of outputs (outputs "1"-"32" in the example under consideration) which are interconnected by means of a matrix circuit 198 with a sync pattern detection circuit 200, a parity checker circuit 202, and a label decoder 204. Specifically, outputs "1"-"6" of the shift register 196, representing the bit locations of the SYNC word, are coupled to respective inputs of sync pattern detection circuit 200. Outputs "7"-"20" of the shift register 196, representing the message bits used for generation of the PARITY bit, and outputs "31"-"32" of the shift register 196, representing the bit locations of the PARITY bit, are coupled to the respective inputs of parity checker circuit 202. Outputs "7"-"12" of the shift register 196, representing the bit locations of the LABEL word, are applied to respective inputs of the label decoder 204.

Sync pattern detection circuit 200 may comprise a decoder whose output signal SPD goes to a high logic level when the signals applied to its input represent the desired SYNC word. The signal SPD is applied to a second input of clock control circuit 190, and to an AND gate 206. The parity checker circuit 202 includes a decoder which is operative to provide a signal representing the binary state of the desired PARITY bit as developed from the message bits used to develop that PARITY bit, and a comparator which is operative to provide an output signal GP which has a high logic level when the value of the desired PARITY bit is equal to that of the actual PARITY bit. When the signals SPD and GP are at a high logic level, AND gate 206 provides an output signal OE which is applied to an ENABLE input of label decoder 204.

As the message on the data bus DB is transferred into the shift register 196, the presence of the proper SYNC word in the appropriate storage locations of shift register 196 causes the output signal SPD from sync pattern detection circuit 200 to go to a high logic level. When signal SPD goes to a high level, clock control circuit 190 is reset to a second state, whereupon signal R therefrom goes to a low level, thereby terminating the signal RCL from receiver clock 194 and accordingly terminating the transfer of the message into shift register 196.

With reference now to FIGS. 9, 10 and 11, the sync pattern detection circuit 200 includes an AND gate 218 whose output is coupled to the input of a monostable multivibrator 220 which is capable of providing the output signal SPD. The "1" output of shift register 196 (as routed through matrix circuit 198) is coupled directly to a first input of AND gate 218; the "2" output of shift register 196 is coupled through an inverter 210 to a second input of AND gate 218; the "3" output of shift register 196 is coupled directly to a third input of AND gate 218; and, the "4", "5" and "6" outputs of shift register 196 are coupled through respective inverters 212, 214 and 216 to a fourth input of AND gate 218. As will be recalled from a review of FIG. 8, the SYNC word precedes the remainder of the message and is represented in the message by a positive bit level, a negative bit level, a positive bit level, and three negative bit levels, in succession. As the message on the data bus DB is transferred into the shift register 196, the SYNC word therefore has the pattern illustrated in FIG. 10(a). When this pattern reaches the last six bit locations in shift register 196 (to which outputs "1"-"6" are connected), the bits of the DATA word are in the proper bit locations in shift register 196 (e.g., those to which the outputs "13"-"28" are connected) and the AND gate 218 accordingly provides an output signal to cause monostable multivibrator 220 to provide the output signal SPD to terminate the transfer of the message into the shift register 196. Only the odd-numbered outputs in the group of outputs "13"-"28" of the shift register 196 are connected to the latch circuit 120 in the interface unit 116, as illustrated by the numeral underlining immediately above FIGS. 11(a), 11(b) and 11(c). Also, the PAD bit is discarded. The parallel, binary output signals DR (FIG. 6) presented to the latch circuit 120, which are those present on the odd-numbered outputs in the group of outputs "13"-"28" of shift register 196 (FIG. 11(a)), therefore represent "10100010", or, the data that has been transmitted.

Let it be assumed that the polarity of the message has been inverted from that originally transmitted (such as might occur due to a polarity reversal in the bus coupler 102) so that the message being transferred into shift register 196 is the logical complement of that originally transmitted. In this situation, the SYNC word will have a pattern as illustrated in FIG. 10(b) which will not result in AND gate 218 providing an output signal. To provide decoding of the SYNC word during conditions of polarity reversal, the sync pattern detection circuit 200 includes a second AND gate 228 whose output is coupled to the input of monostable multivibrator 220. The "1" output of shift register 196 is coupled through series inverters 222, 224, and the "2" output of shift register 196 is coupled through an inverter 226, to a first input of AND gate 228. The "3", "4" and "5" outputs of shift register 196 are coupled directly to respective second, third and fourth inputs of AND gate 228. When the polarity of the message is the inverse of that originally transmitted, the presence of the SYNC word having a pattern as illustrated in FIG. 10(b) will not result in the AND gate 218 providing an output signal, as previously described, and the DATA word will appear as illustrated in FIG. 11(b). Accordingly, the receiver clock 194 (FIG. 6) remains enabled and the clock signal RCL therefrom continues to transfer the message into and through the shift register 196. When the message has been transferred one additional bit location, the pattern of the SYNC word, as illustrated in FIG. 10(c), causes AND gate 228 to provide an output signal to cause monostable multivibrator 220 to provide the output signal SPD to terminate the message transfer. At this time, the DATA word will appear as illustrated in FIG. 11(c), so that the signals DR present on the odd-numbered outputs in the group of outputs "13"-"28" represent "10100010", or, the data that has been transmitted.

Accordingly, the embodiment of the sync pattern detection circuit 200 in FIG. 9 allows proper decoding of the transmitted data, irrespective of the polarity with which the message has been provided to the receiver 114. It also should be noted that the pattern of the SYNC word in the message is an improper code for Manchester bi-phase level modulation, that is, the pattern of the SYNC word includes three successive logic levels of a given type, whereas a proper Manchester bi-phase level code permits only two successive logic levels of the same type. Accordingly, messages not having the pattern of the SYNC word do not cause the sync pattern detection circuit 200 to provide its output signal SPD, with the result that such messages are transferred through shift register 196 without decoding.

Referring again to FIG. 6, if the binary value of the desired PARITY bit compares with that of the actual PARITY bit, then the signal GP from parity checker circuit 202 will be at a high logic level, so that the signal OE from AND gate 206 is provided to enable label decoder 204. An output signal LD is provided by label decoder, however, only if the LABEL word in the message identifies data which the terminal is to receive. The signal LD is applied to an enable input of latch circuit 120 in interface unit 116 (FIG. 5). As previously described, the outputs "13"-"28" of shift register 196, on which appear the parallel, binary output signals DR representing the DATA word, are connected to respective signal inputs of latch circuit 120. If the LABEL word in the message is correct, the signal LD enables latch circuit 120 so as to transfer the signals DR thereto.

When the signal ADA again goes to a high logic level, signifying the absence of a message on the data bus DB, the word gap detection circuit 192 is enabled. At a predetermined time thereafter, word gap detection circuit 192 provides a signal WGD which clears shift register 196, thereby conditioning the terminal for the reception of a successive message.

It will therefore be appreciated that the present invention permits each participating terminal to have autonomous access to the data bus DB. For example, the receiver in any one terminal is capable of detecting and storing any messages transmitted on the data bus DB, and to transfer the data within any such message to its associated utilization device upon proper message identification, irrespective of the operation of any other terminal. Likewise, the transmitter in any one terminal can transmit a message on the data bus DB in response only to the absence of messages on the data bus DB, and thus irrespective of the transmission by any other terminal. By the use of the protocol previously described, the invention permits periodic transmission of messages by each terminal without the requirement for a central bus controller terminal, and yet avoids the simultaneous transmission of messages by two or more terminals. Further, failure of any terminal or terminals will not constitute failure of any other terminal. For example, the failure of a receiver in any one terminal will not affect the operation of the receiver or transmitter in any other terminal. Likewise, the failure of the transmitters in all terminals, except one, to transmit messages will not result in the transmitter in that one terminal failing to periodically transmit its message, inasmuch as message transmission from that one terminal is dependent only upon the expiration of the unique inter-message gap (which in this case will be measured from the termination of the message transmission of that one terminal) and the elapse of the transmission interval for that one terminal.

Finally, the failure of a demodulator, receiver, transmitter or modulator in any one terminal will not result in failure of the data bus DB, provided that a data bus, and associated couplers, such as disclosed and claimed in my previously referred to copending application are used. For example, a short circuit across the secondary winding 106 of the bus coupler 102, or a broken core 104, will reduce the inductance of the bus coupler 102 so as to have the same effect upon the operation of the data bus DB as would be caused by removal of the bus coupler 102. Likewise, an open secondary winding 106, or a severed terminal stub 108, will result in an increase in the inductance presented by the bus coupler 102 to the data bus DB, since the secondary winding 106 is no longer in parallel with the demodulator 110 and modulator 112. By considering this increased inductance as the maximum terminal design load, and accordingly designing the data bus DB therefor, it can be seen that such failure will have no effect upon the operation of the data bus DB.

While the invention has been described with reference to a preferred embodiment, it is to be clearly understood by those skilled in the art that the invention is not limited thereto. Rather, the scope of the invention is to be interpreted only in conjunction with the appended claims.