Industrial Control System

The present invention relates to control systems of the type having a plurality of remotely located process control units connected together through a communications link and, more particularly, to a control system in which each of the remote units sequentially assumes supervisory communication control of the communication link and in which high reliability information transfer is achieved between remotes.

Many system type industrial installations, for example, those related to industrial process-type manufacturing and electrical power generation, employ a large number of physically distributed controlled-devices and associated sensors for effecting coordinated operation of the overall system. In the past, coordinated control of the various devices has been achieved by manual operation and various types of semi-automatic and automatic control systems including electromagnetic relay systems, hardwired solid-state logic systems, and various types of computer control systems. The computer systems have included central systems in which the various sensors and controlled devices are connected to a central computer; distributed control systems in which a remotely located computer is connected to each of the controlled devices and to one another; and hybrid combinations of the central and distributed systems. The successful functioning of the control system is vital to any industrial process and, accordingly, distributed systems have generally been preferred over central systems because the failure of one of the remotely located control computers generally does not cause a system wide failure as in the case of the failure of the central computer in the central system. However, in many distributed computer systems, one of the remotes or a specially designed control unit generally handles supervisory communication control of the communication buss and, for these systems, failure of the communication buss supervisor can lead to a system-wide failure.

In many industrial control systems, the various communication busses that extend between the remotely located computer process control units are exposed to high electrical noise environments. Accordingly, the information transferred over the communication buss can be subjected to error-inducing interference because of the harsh electrical environment. In view of this, a control system must have a means for detecting errors within the transmitted information in order to provide high reliability data transmission between remotes.

In view of the above, it is a broad overall object of the present invention, among others, to provide an industrial control system for controlling an industrial process or the like having a high overall system operating reliability.

It is also an object of the present invention to provide an industrial control system which may take the form of a distributed control system, a central control system, or a combination thereof to provide high overall operating efficiency and reliability.

It is another object of the present invention to provide an industrial control system defined by a plurality of remotely located process control units (remotes) interconnected through a communication buss which each of the remotely located units adapted assume supervisory control of the communication buss in accordance with a predetermined sequence.

It is still another object of the present invention to provide an industrial control system having a plurality of remotely located process control units interconnected through a buss in which high reliability information transfer is achieved between remotes.

In accordance with these objects, and others, the present invention provides a control system for controlling an industrial process including a plurality of remote process control units R.sub.n (remotes) connected to various controlled devices and sensors and communicating with one another through a communications link having at least two independent communication channels. Each remote is assigned a unique succession number or position in a predetermined succession order with each remote unit assuming supervisory communication control of the communications link on a revolving or master for the moment basis in accordance with the remote's relative position in the succession order. Information transfer including process data and command control information is accomplished between a source remote R.sub.s and a destination remote R.sub.d by successively transmitting two identical information blocks over each communication channel with the destination remote R.sub.d testing the validity of the blocks on one of the channels and, if valid, responding with an acknowledgement signal (ACK), and, if invalid, then testing the validity of the two blocks received on the other, alternate channel. An acknowledgement (ACK) or a non-acknowledgement signal (NAK) is sent by the destination remote R.sub.d if the information on the alternate channel is found, respectively, valid or invalid. The source remote R.sub.s will retransmit the information blocks in response to a non-acknowledgement signal from a destination remote with the retransmission from the source remote R.sub.s limited to a predetermined, finite number.

A control system in accordance with the present invention advantageously provides a means for controlling an industrial process in which high overall system operating reliability is achieved. The system is equally suitable for use with central (master/slave), distributed, and hybrid system configurations.

The above description, as well as the objects, features, and advantages of the present invention will be more fully appreciated by reference to the following detailed description of a presently preferred but nonetheless illustrative embodiment in accordance with the present invention when taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic diagram of an exemplary process control system including a plurality of remote process control units (remotes) connected to a common, dual-channel communications link;

FIG. 2 is a schematic block diagram of an exemplary remote process control unit of the type shown in FIG. 1;

FIG. 3 is a schematic block diagram of an exemplary modulator/demodulator for the remote process control unit shown in FIG. 2;

FIG. 4 is a schematic block diagram of an exemplary communication protocol controller for the remote process unit shown in FIG. 2;

FIG. 5 illustrates the format of an exemplary or illustrative information block for transferring information between remotes;

FIG. 5A illustrates the format of a header frame of the information block shown in FIG. 5;

FIG. 5B illustrates the format for a data/information frame of the information block shown in FIG. 5;

FIG. 5C illustrates the format for an acknowledgement block (ACK) for acknowledging successful receipt of an information block;

FIG. 5D illustrates the format for a non-acknowledgement block (NAK) for indicating the unsuccessful transmission of an information block between remotes;

FIG. 6 illustrates, in pictorial form, two identical data blocks having the format shown in FIG. 5 successively transmitted on each communication channel of the communication link illustrated in FIG. 1;

FIG. 7 is a flow diagram summary of the manner in which a source and a destination remote effect communications with one another;

FIG. 8A is a partial flow diagram illustrating in detail the manner in which a source and a destination remote communicate and validate information transferred between one another.

FIG. 8B is a partial flow diagram which completes the flow diagram of FIG. 8A and illustrates in detail the manner in which a source and a destination remote communicate and validate information transferred between one another;

FIG. 9 is a legend illustrating the manner in the flow diagrams of FIG. 8A and FIG. 8B are to be read; and

FIGS. 10A through 10F are exemplary tables illustrating the manner in which supervisory control of the communication link is transferred from remote to remote.

An industrial control system in accordance with the present invention is shown in schematic form in FIG. 1 and includes a communications link CL having a plurality of remote process control units (remotes) R.sub.1, R.sub.2, . . . R.sub.n-1, R.sub.n connected thereto. The communications link CL is shown as an open line, double channel configuration formed from dual coax, dual twisted pair, or the like with the individual communication links identified, respectively, by the reference characters CL.phi. and CL1. While the system configuration shown in FIG. 1 is a distributed open loop or shared global bus type, the invention is equally suitable for application to central systems or central/distributed hybrid configurations. The system of FIG. 1 is adapted for use in controlling an industrial process, e.g., the operation of a power generating plant, with each remote unit R.sub.n connected to one or more associated or corresponding input/output devices I/O.sub.1, I/O.sub.2, . . . I/O.sub.n-1, I/O.sub.n such as, but not limited to, various types of sensors (temperature, pressure, position, and motion sensors, etc.) and various types of controlled devices (motors, pumps, compressors, valves, solenoids, and relays, etc.).

The architecture of an exemplary remote R.sub.n is shown in FIG. 2. While the architecture of the remote R.sub.n can vary depending upon the control process requirements, the remote shown in FIG. 2 includes a modem 10; a communication protocol controller 12; an input/output management device 14; a central processing unit (CPU) 16; a memory 18; a peripheral device 20 that can include e.g., a CRT display, a printer, or a keyboard; and a common bus 22 which provides addressing, control, and information transfer between the various devices which constitute the remote. The devices shown in dotted line illustration in FIG. 2 (that is, the central processing unit 16, the memory 18, and the peripheral device 20) are provided depending upon the process control requirements for the remote R.sub.n. For example, in those remotes R.sub.n which function as an elemental wire replacer, only the modem 10, the communication protocol controller 12, and the input/output management 14 are provided. In more complex process control requirements, an appropriately programmed central processing unit 16 and appropriate memory 18 are provided to effect active control according to a resident firmware program. In still other remotes requiring a human interface, the appropriate peripheral devices(s) 20 may be connected to the common buss 22.

As shown in more detail in FIG. 3, the modem 10 provides two independent communication channels CH.phi. and CH1 connected respectively to the communication links CL.phi. and CL1. Each of the communication channels CH.phi. and CH1 is provided with substantially identical communication devices, and a description of the communication devices of the first communication channel CH.phi. is sufficient to provide an understanding of the second communication channel CH1. The communication channel CH.phi. includes an encoder/decoder 24.sub..phi. for providing appropriate modulation and demodulation of the digital data transmitted to and received from the communication link CL.phi.. In the preferred form, the encoder/decoder 24.sub..phi. converts digital information in non-return-to-zero binary (NRZ) format to base-band modulation (BBM) signal format for transmission and effects the converse for reception. Amplifiers 26.sub..phi. and 28.sub..phi. are provided, respectively, to drive a passive coupling transformer T.phi. with digital information provided from the encoder/decoder 24.sub..phi. and provide amplified signals to the encoder/decoder 24.sub..phi. from the coupling transformer T.phi.. A set of selectively operably relay contacts 30.sub..phi. are provided between the coupling transformer T.phi. and the corresponding communication link CL.phi. to effect selective interruption thereof to isolate the remote R.sub.n from the communications link CL, and another set of relay contacts 32.sub..phi. are provided to selectively connect the signal output of the coupling transformer T.phi. with a termination impedance Z.phi.. The termination impedance Z.phi. is used when the particular remote R.sub.n is at the end of the communication link CL to provide proper line termination impedance for the link. A selectively operable loop back circuit 34 is provided to permit looping back or re-circulation of test data during diagonistic checking of the remote R.sub.n. The isolation relays 30.sub..phi. and 31.sub.1, the impedance termination relays 32.sub..phi. and 32.sub.1, and the loop back circuit 34 are connected to and selectively controlled by a C-link control device 38 which receives its communication and control signals from the communication protocol controller 12 described more fully below. A watch-dog timer 40 is provided to cause the C-link control device 38 to operate the isolation relays 30.sub..phi. and 30.sub.1, to disconnect the remote R.sub.n from the communication link CL in the event the timer 40 times-out. The timer 40 is normally prevented from timing out by periodic reset signals provided from the communication protocol controller 12. In this way, a remote R.sub.n is automatically disconnected from the communication link CL in the event of a failure of the communication protocol controller 12.

As shown in more detail in FIG. 4, each communication protocol controller 12 includes input/output ports 42, 44, and 46 which interface with the above described modem 10 for the communication channels CH.phi. and CH1 and the modem C-link control device 38 (FIG. 3). A first-in first-out (FIFO) serializer 48 and another first-in first-out serializer 50 are connected between the input/output ports 42 and 44 and a CPU signal processor 52. The first-in first-out serializers 48 and 50 function as temporary stores for storing information blocks provided to and from the modems 10 as described more fully below. The CPU 52, in turn, interfaces with the bus 22 through bus control latches 54. A read only memory (ROM) 56 containing a resident firmware program for the CPU and a random access memory (RAM) 58 are provided to permit the CPU to effect its communication protocol function as described more fully below. A register 60 (for example, a manually operable DIP switch register or a hardwired jumper-type register) that includes registers 60a and 60b and timers 62 are also provided to assist the CPU in performing its communication protocol operation. An excess transmission detector 64, connected to input/output ports 42 and 44 (corresponding to communication channels CH.phi. and CH1) determines when the transmission period is in excess of a predetermined limit to cause the C-link control device 38 (FIG. 3) to disconnect the transmitting remote from the communications link CL and thereby prevent a remote that is trapped in a transmission mode from monopolizing the communications link CL.

The input/output management device 14, a detailed description of which is not necessary to an understanding of the present invention, is preferably a firmware controlled microprocessor-based device which is adapted to scan the various input/output hardware points, effect data collection and distribution to and from the input/output points, format the collected data in preferred formats, and assemble the data in selected sequences as well as place data in memory for subsequent transmission.

As mentioned above, one or more remotes R.sub.n may be provided with active processing capability by interfacing a central processing unit 16 and corresponding memory 18 with the remote bus 22. For example, in those remotes R.sub.n that monitor and control a specific operating characteristic of a process step, such as a temperature or pressure variable, a microprocessor 16 with an appropriate firmware program(s) can be provided to measure the actual operating characteristic, effect a comparison between the actual operating characteristic and the preferred set point, and effect one or more input/ouput actions in accordance with predetermined process control algorithms.

In the preferred embodiment, the signal processor for the CPU signal processor 52 is a 8.times.300 micro-controller manufactured by the Signetics Company of Sunnyvale, Calif., and the central processing unit 16, when used, is an 86/12 signal board 16-bit micro-computer manufactured by the Intel Company and adapted to and configured for the Intel MULTIBUS.

Each remote R.sub.n is adapted to communicate with the other by transmitting digital data organized in predetermined block formats. A suitable and illustrative block format 66 is shown in FIG. 5 and includes a multi-word header frame 66A, a multi-word data frame 66B, and a block termination frame or word 66C. Selected of the information block configurations are adapted to transfer process control information to and from selected remote units R.sub.n and other of the block configurations are adapted to transfer supervisory control of the communications link CL from one remote to the other remote as explained in greater detail below.

An exemplary format for the header and data frames of an information block 66 is shown, respectively, in FIGS. 5A and 5B. The header frame 66A preferably includes a `start of header` word(s) that indicates to all remotes that information is being transmitted; a `source` identification word(s) that indicates the identity of the source remote R.sub.s that is transferring the information; a `destination` word(s) that indicates the identity of the receiving or destination remote R.sub.d ; a `header-type` word(s) that indicates whether the data block is transmitting data, a parametered command block, or a parameterless command block; `block-type` word indicating the type of block (that is, a command block or a data block); a `block number` word that indicates the number of blocks being sent; a `block size` word indicating the length of the data frame; a `security code` word(s) that permits alternation of the resident software programming in a remote; and, finally, a two-byte `cyclic redundancy code` (CRC) validity word. The data frame for each data block, as shown in FIG. 5B, can include a plurality of data carrying bytes or words B.sub.1, B.sub.2, . . . B.sub.n of variable length terminated with a two-byte cyclic redundancy code word. As described more fully below, each of the remotes is adapted to acknowledge (ACK) successful receipt of data and command blocks and non-acknowledge (NAK) the receipt of data in which a transmission error is detected. When transmitting an acknowledgement block or a non-acknowledgement block, the header format used is shown in FIGS. 5C and 5D in which an acknowledgement (ACK) or non-acknowledgement (NAK) word occupies the `block type` word position. The block formats disclosed above are intended to be illustrative only and not limiting.

The various remote units R.sub.1, R.sub.2, R.sub.3, . . . R.sub.n communicate with one another by having each remote successively take control of the communications link CL and then passing digital information between the controlling source remote R.sub.s and a destination remote R.sub.d using a double transmission alternate line technique that provides for high reliability data transfer between remotes even when one of the two communication links CL.phi. or CL1 is inoperative, for example, when one of the two communication cables is severed as occassionally occurs in harsh industrial environments.

When a remote unit assumes control of the communication link CL (as explained more fully below) and, as a source remote R.sub.s, desires to send data blocks to another, destination remote R.sub.d, the data block is assembled at the source remote R.sub.s in accordance with the block formats discussed above in connection with FIGS. 5-5D and transmitted through the information channels CL.phi. and CL1 of the source remote R.sub.s to the communication links CL.phi. and CL1 with the header frame containing both the source remote R.sub.s and the destination remote R.sub.d identification information.

In accordance with the data transmission technique, the communication protocol controller 12 of the source remote R.sub.s transmits the information blocks twice on each communication link CL.phi. and CL1 as schematically illustrated in FIG. 6 to provide a first data block DB.sub.A and then a second, following data block DB.sub.B on each communication link CL.phi. and CL1. The transmitted information block headers include the identity of the destination remote, R.sub.d, which causes the destination remote R.sub.d to receive and act upon the information blocks. At the destination remote R.sub.d, the two data blocks DB.sub.A.phi. and DB.sub.B.phi. on the communication link CL.phi. are passed through the communication channel CH.phi. and the two data blocks DB.sub.A1 and DB.sub.B1 on the communication link CL1 are passed through the communication channel CH1 to, respectively, the first-in first-out serializers 48 and to (FIG. 4).

As shown in the summary flow diagram of FIG. 7, the destination remote R.sub.d checks the validity of the received data by selecting one of the two communication links (e.g. CL.phi. in FIG. 7) and then checks the first data block on the selected line (that is, DB.sub.A.phi.) by performing a cyclic redundancy check of the header frame and, if valid, performing a cyclic redundancy check of the data frame. If the data frame is valid, the communication protocol controller 12 of the destination remote R.sub.d then performs a bit-for-bit comparison between the CRC-valid first data block DB.sub.A.phi. and the second data block DB.sub.A.phi.. If the bit-for-bit comparison is good, an acknowledgement (ACK) signal is sent from the destination remote R.sub.d to the source remote R.sub.s to indicate the receipt of valid information and complete that data block information transaction. On the other hand, if the CRC validity checks of the header or the data frame or the bit-for-bit comparison check indicate invalid data, the protocol controller 12 of the destination remote R.sub.d then selects the other, alternate line (in this case, CL1) and performs the aforementioned cyclic redundancy checks of the header and data frame and the bit-for-bit comparison between the first and second data blocks DB.sub.A1 and DB.sub.B1 on the alternate line CL1. If these checks indicate valid data on the alternate line, the destination remote responds with an acknowledgement signal to conclude the data block transmission transaction. On the other hand, if these checks indicate invalid data on the alternate line (which means that the data blocks on both the first-selected line and the alternate line are invalid) the destination remote R.sub.d responds with a non-acknowledgement signal (NAK) to cause retransmission of the data blocks from the source remote R.sub.s. The non-acknowledgement block (NAK) includes a byte or bytes indicating the identity of the data block or blocks which should be retransmitted. A counter (not shown) is provided that counts the number of retransmissions from the source remote R.sub.s and, after a finite number of retransmissions (e.g. four), halts further retransmission to assure that a source remote R.sub.s and a destination remote R.sub.d do not become lost in a repetitive transmit/NAK/retransmit/NAK . . . sequence in the event of a hardware or software failure of the destination remote R.sub.d error checking mechanism.

The double message alternate line checking sequence summarized in FIG. 7 may be more fully appreciated by referring to the detail flow diagram shown in FIGS. 8A and 8B (as read in accordance with the flow diagram map of FIG. 9). At the start of the information validity checking procedure, the `line .phi.-first` flag register is checked; if a flag is present the `first-attempt fail` flag register is checked, and, if there is no flag in this register, the two data blocks DB.sub.A1 and DB.sub.B1 on channel CH1 are stored while the two data blocks DB.sub.A.phi. and DB.sub.B.phi. on channel CH.phi. are used for the first attempt information check. Thereafter, the header frame of the first data block DB.sub.A.phi. on channel CH.phi. undergoes a CRC check and, if acceptable, the data frame of this data block DB.sub.A.phi. undergoes a CRC check. If the header and data frames CRC checks indicate valid data a `good message` register is incremented. If the number of good messages is less than two, the error checking procedure returns to the initial part of the flow diagram and, after determining there is no channel CH.phi. first flag or first-attempt flag present, checks the second, following data block DB.sub.B.phi. by repeating the header and data CRC cylcic redundancy checks. If the header and data frames pass the CRC checks, the `good message` register is incremented again to indicate that a total of two messages in succession (that is, DB.sub.A.phi. and DB.sub..phi.) have passed the cyclic redundancy check for the header and data frames. Thereafter the two data blocks DB.sub.A.phi. and DB.sub.B.phi. received on line .phi. are checked by performing a bit-by-bit comparison between the two. If the data blocks DB.sub.A.phi. and DB.sub.B.phi. pass the bit-by-bit comparison test, the communications protocol controller 12 of the destination remote R.sub.d sends an acknowledgement (ACK) message to the source remote R.sub.s to conclude the information block transfer and resets the various registers. If, on the other hand, either the data block DB.sub.A.phi. or DB.sub.B.phi. on line .phi. fail the header and data frame CRC checks or these two data blocks fail the bit-by-bit comparison check, the communication protocol controller 12 sets the `first-attempt fail` flag and returns to the start of the procedure to determine that the `line .phi.-first` flag and the `first-attempt` fail flag are present. The communication protocol controller 12 then uses the stored data blocks DB.sub.A1 and DB.sub.B1 from line 1 (which data blocks were previously stored in FIFO 50). The header block and data block of the data blocks DB.sub.A1 and DB.sub.B1 from line CL1 undergo the CRC check and, if successful, cause the incrementing of the `good message received` register to cause the communication protocol controller 12 to then check the validity of the second data block DB.sub.B1. If the data blocks DB.sub.A1 and DB.sub.B1 pass the CRC checks, they are compared with one another in a bit-by-bit comparison test and if this comparison check is successful, an acknowledgement (ACK) is sent. If, on the other hand, either data block DB.sub.A1 or DB.sub.B1 does not pass the CRC check or the data blocks do not pass the bit-for-bit comparison test, a non-acknowledgement (NAK) signal is sent to the source remote R.sub.s including information requesting the retransmission of the data blocks which failed the validity test at the destination remote R.sub.d. The source remote R.sub.s then retransmits the improperly received information blocks as described above with retransmission limited to a finite number. A register is provided for each of the communication links for recording, in a cumulative manner, the number of times a invalid message is received for each communication link. In this manner, it can be determined, on a statistical basis, whether one of the two communication links has suffered a deterioration in signal transmission capability and, of course, whether one of the communication links is severed.

As can be appreciated, the dual transmission of the identical messages on plural communication links vastly enhances the ability of the destination remote R.sub.d to detect errors and determine whether the information being transmitted is valid or not. In addition, the destination remote R.sub.d is able to operate and successfully receive messages even if one of the communication links is severed since the communication protocol controller 12 at the destination remote R.sub.d will examine the received signals on each line and will find invalid data on the severed line but will always examine the data blocks on the other line and, if necessary, request retransmission of the information blocks.

In selecting one of the two channels CH.phi. or CH1 for the first validity check, it is preferred that one of the two channels (e.g. CH.phi.) be selected for the first check on every other information transaction and that the other of the two channels (e.g. CH1) be selected for the first check for the other information transactions. While the system has been disclosed as having dual communication links CL.phi. and CL1, the invention is not so limited and can encompass more than two communication links with the remotes adapted to sequentially examine signals received on the various channels.

As mentioned above, each remote R.sub.n of the control system is adapted to accept and then relinquish supervisory control of the communication link CL on a master for the moment or revolving master arrangement. The communication protocol controller 12 of each remote R.sub.n includes a register which contains the remote succession number, another register which contains the total number of remotes in the system, and another register which contains the relative position of the remote from the present system master. The first two registers are schematically illustrated by the reference character 62 in FIG. 4. In addition, each remote R.sub.n includes a variable transfer-monitor timer the time-out interval of which is set in accordance with a predetermined control-transfer time constant (50 micro-seconds in the preferred embodiment) and the position of the particular remote relative to the present system master to permit, as explained in more detail below, the master for the moment transfer to continue even in the event of a disabled remote (that is, a remote that is unable to accept supervisory control because of a malfunction). Another timer is provided to force transfer of supervisory control of the communications link CL in the event a remote, because of a malfunction, is unable to transfer supervisory control to its most successive remote. The operation of the master for the moment transfer technique can be appreciated by consideration of the following example of an illustrative system that includes five remotes arranged in the open loop configuration of FIG. 1 and transferring supervisory control of the communications link CL in accordance with the tables of FIGS. 10A-10F. The upper row of each table indicates the succession sequence or order of the five remotes R.sub.0, R.sub.1, R.sub.2, R.sub.3, and R.sub.4 that comprise the system; the intermediate row identifies the remote that is the present master for the moment and also identifies the relative successive position of the other remotes from the present master, that is, the first (or next) successive remote from the present master, the second successive remote from the present master, the third remote from the present master, etc.; and the third row of each table lists the setting of the variable transfer-monitor timer for the particular remote.

The system is provided with initialization software so that the first remote in the successive R.sub.0 assumes supervisory control of the communication link CL after system start-up and becomes the initial master of the system (FIG. 10A). When the initial master R.sub.0 is in control of the communications link CL, it can send data to any of the other remotes, request status or other data from another remote, and send control blocks and the like over the communications link. When the master R.sub.0 determines that it no longer desires possession of the communications link CL, it passes supervisory control of the communication link CL, to the next or first successive remote in accordance with the succession order. Thus, when the present master R.sub.0 concludes its information transfer transactions, it transfers supervisory control of the communications link CL to its next or first successive remote R.sub.1 by transmitting a control block to the remote R.sub.1 with all the remaining remotes (that is, R.sub.2, R.sub.3, R.sub.4) being cognizant of the transfer of supervisory control from the present master R.sub.0 to its first or next successive remote R.sub.1. Since, in the present system, the transfer of supervisory control of the communications link CL is expected to take place within 50 micro-seconds, the second successive remote R.sub.2, as shown in the third row of the table of FIG. 10B, sets its variable transfer-monitor timer to 50 micro-seconds, the third successive remote R.sub.3 sets its variable transfer-monitor timer to 100 micro-seconds, and the fourth successive remote R.sub.4 sets its transfer-monitor timer to 150 micro-seconds. When the first successive remote R.sub.1 receives the control block from the present master R.sub.0, it accepts supervisory control of the communications link CL by responding with an acknowledgement message (ACK). If the control block is mis-received, the first successive remote R.sub.1 can respond with an non-acknowledgement (NAK) to request retransmission of the control block transferring supervisory control of the communications link CL. During the time interval that the present master remote R.sub.0 is attempting to transfer supervisory control of the communication link CL to its next successive remote R.sub.1, the transfer-monitor timers of the remaining remotes are counting down. If, for any reason, the next or first successive remote R.sub.1 fails to take control (e.g. a malfunction of the remote), the transfer-monitor timer of the second successive remote R.sub.2 will time-out at 50 micro-seconds and cause the second successive remote R.sub.2 to then accept supervisory control of the communication link CL from the present master R.sub.0 and thus bypass an apparently malfunctioning first successive remote R.sub.1.

Assuming that the initial system master R.sub.0 successively transfers supervisory control of the communications link CL to its first successive remote R.sub.1, that successive remote R.sub.1 then becomes the present master with the remaining remotes changing their position relative to the present master and setting their transfer-monitor timers in accordance with the second and third rows of the table of FIG. 10B. When the present master R.sub.1 concludes its information transfer transactions, if any, it attempts to transfer supervisory control to its first or next successive remote R.sub.2 by sending an appropriate control block to remote R.sub.2 which responds with an acknowledgement signal (ACK) or, in the event of a mistransmission of the control block, a non-acknowledgement signal (NAK) which causes retransmission of the control block. When the control block requesting transfer of supervisory control of the communication link CL is sent from the present master R.sub.1 to its next successive remote R.sub.2, all the remaining remotes reset their transfer-monitor timers in accordance with their position relative to the present remote as shown in the third row of table of FIG. 10C. Should the next successive remote R.sub.2 be unable to accept supervisory control of the communication link CL from the present master R.sub.1, the transfer-monitor timer of the second successive remote R.sub.3 will time-out in 50 micro-seconds and cause the second successive remote R.sub.3 to assume supervisory control of the communications link CL to thereby bypass an apparently malfunctioning first successive remote R.sub.2. As can be appreciated from a review of the transfer-monitor time-out settings of the various remotes, supervisory control of the communications link CL will transfer even if one or more successive remotes are malfunctioning, when the transfer-monitor timer of the next operable remote times out. This transfer sequence continues in succession as shown in the remaining tables of FIGS. 10D to 10F with supervisory control of the communication link CL being passed from remote to remote in succession with the last remote R.sub.4 returning supervisory control to the first remote R.sub.0.

By employing a master for the moment transfer technique in which the receiving remote acknowledges control from the transferring remote and in which retransmission of a mis-received control block is provided for in response to a non-acknowledgement signal from the receiving remote, it is possible to positively transfer supervisory control of the communication link. This technique advantageously transfers control using the data and information carrying communication link rather than as, in other systems, by providing separate communication lines or channels dedicated solely to supervisory control transfer functions. Also the provision of a variable transfer-monitor timer at each remote that is set in accordance with the remote's relative position to the present master and a transfer time-constant automatically transfers supervisory control of the communications link even if one or more of the successive remotes are malfunctioning.

As will be apparent to those skilled in the art, various changes and modifications may be made to the industrial control system of the present invention without departing from the spirit and scope of the invention as recited in the appended claims and their legal equivalent.