CA1182568A - Industrial control system - Google Patents

Abstract

INDUSTRIAL CONTROL SYSTEM

ABSTRACT OF THE DISCLOSURE

A control system for controlling an industrial process includes a plurality of remotely located process control units (remotes) each coupled to an associated input/output device(s) and adapted to communicate with one another through a dual channel communications link. Each remote has a unique success-ion number within a predetermined succession order with super-visory communication-control of the communication link sequential-ly transferred to each remote according to its succession number to provide a revolving or master for the moment control of the system. Digital information in the form of data and control information blocks is transmitted between the remotes with the blocks transmitted twice on each channel of the communications link. The destination remote tests the block validity on one of the two dual channels and, if validated, responds with an acknowledgement signal (ACK) and, if invalid, tests the blocks on the other, alternate channel and then responds with an acknowledgement or non acknowledgement signal (NAK) depending upon whether the data blocks tests on the alternate channel are found valid or invalid. A non-acknowledgement from the destination remote re-triggers the transmission of the blocks from the source remote. The system provides high overall operating efficiency since the remotes will maintain a system-like integrity on each side of a severed communication link and the redundant block transmission with alternate line checking provides very high information transfer reliability.

Description

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INDUSTRIAL CONTROT, SYSTEM
BACKGROUND OF THE INVENTION
___ _ This application is a division of Canadian Serial No. 368,795, filed January 19, 1981.
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 manu-facturing 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 solidwstate 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 cornbinations 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 preferr~a over central systems because the failure of one of the remotely located control computers generally does not cause a system jlide 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 sys.ems, failure of the communication buss supervisory can lead to a system-wlde 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 con-trol system must have a means for detecting errors within the transmitted information in order to provide high reliability data transmission between remotes.

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S[JMMARY OE' THE INVENTION
The present invention seeks to provide an industrial control system for controlling an industrial process or the like having a high overall system operating reliability.
The invention in one aspect to which this divisional is directed pertains to an information transfer system for transmitting digital information between active devices and testing the validity of the transmitted information. The system includes at least one active device for transmitting information in digital form, at least one other active decice for receiving information in digital form, and at least a first and a second independent communication channel connected to and e~tending be-tween the first-mentioned and the second mentioned active devices Eor conveying information there~etween. A transmitter means is associated with the first-mentioned active device for trans-mitting digital information arranged in blocks of predetermined format, the transmitter means transmitting, for each information transfer transaction, an identical block on each of the first and second communication channels. Receiving means is associated with the second-mentioned active device for receiving digital information transmitted by the first-mentioned active device and for selecting a one of the first and second communication channels and testing the validity of the received block and, when the received block from the first selected col~munication channel is found invalid, for selecting the other of the first and second communication channels and testing the validity of the received block on the other communication channel. Means associated with the receiver means is provided for first-selecting the first of the communication channels on alternate information transfer transactions and for first-selecting the second of the communication channels on the remaining information transfer transactions.

The invention herein also contempla~es a method for transferring digital information formatted in predetermined blocks between an .information transmitting device and an interconnected information receiving device, the method comprising the steps of transmltting, for each information transfer transaction, identical information blocks from a transmitter over plural independent communication channels to a receiver, receiving and storing the received information blocks at the receiver, selecting the information block received on one of the plural communication channels and testing the validity thereof, selecting the information block received on other of the plural COmmUniCatiOn channels and testing the validity thereof in the event the first-selected information block fails its validity check, and requesting retransmission of the information blocks in the event both the first selected and the second-selected information blocks fail their validi.ty test, the one communication channel first-selected on alternate information transfer transactions and the other communication channel first-selected on the remaining information transfer transaction.
More particularly, the disclosed invention provides a control system for controlling an industrial process including a plurality of remote process control units Rn (remotes) connected to various controlled devices and sensors and communicating with one another through a communicatio~ link having a-t least two independ~nt communication ~hannels. ~ach 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 Rs and a destination remote Rd by successively transmitting two identical in~ormati.on blocks over each communication channel with the destination remote Rd 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 (NAIC) is sent by the destination remote Rd if the information on the alternate channel is found, respectively, valid or invalid. The source remote Rs will retransmit the information blocks in response to a non-acknowledgement signal from a destination remote with the retransmission ~rom the source remote Rs 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 (master1slave), distributed, and hybrid system configurations.

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BRIEF DESCRIPTION OF THE DRAWIMGS
The above description as well as the as~ects features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of a presently preferred but none-theless 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), including both primary con-trol remotes and redundant 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 i~ FIG. l;
FIG. 3 is a schematic block diagram of an exemplary modulator/demodulator (MODEM) for the remote process control unit shown in FIG. 2;

2 n FIG. 4 is a schematic block diagram of an exemplary communicatlon protocol controller ror the remote process unit shown in FIG. 2;
~IG. 4A is a schemat c block diagram of an exemplary input/output management device for the remote ?rocess control unit shown in FIG. 2;
FIG. 4B is a flow diagram illustrating the .anner in which the change-in-status evenis of the controlled devices of FIC-. 1 are detected by the input/
output management device of FIG. 4A;
FIG. 5 illustrates the format of an exemplarv 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 lllustrates the format for an acknowledgement block (ACK) for acknowledging successful receipt of an information block;
FIG. 5~ illustrates the fo1~at for a non-acknowledgement block ~NAK) for indicating the unsuccessful transmission of an information bloc~ Detween remotes;
.FIG. 6 illustrates, in pictorial form, two identical data blocks having the format shown in FIG. 5 successivel~ transmitted on each communication channel of the co~unication link illustrated in FIG. 1;
FIG. 7 is a flow diagxam summa-y of the manner in which a source and a destination remote effect communi cations with one another;
FIG. 8A is 2 partial flow diagram illust.a'ing 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 w~ich com-pletes the flow diagram of FIG. 8A and illustrates in detail the manner in which a source and a destination remote communicate and validate infor~ation transferred between one another;

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FIG. 9 is a legend illustrating the manner in the flow diagrams of FIG. 8A and FIG. 8B are to be read;
FIGS. lOA through lOF are exemplary tables illustrating the manner in which supervisory control or the communication link is transferred from remote to remote;
FIG. 11 is a schematic block diagram of an exemplary redundant remote that is adapted to assume control from a failed or othexwise inoperative primary remote;
FIGS. llA and llB are flow diagrams of the manner in which the central processiny unit of the redundant remote R4 monitoxs the operating condition of its assisned primary remotes Rl, R2, and R3 and takes over operation when one of the primary remotes fails;
FIG. 12 is a flow diagram summary of the manner by which an interrogating remote Rx tests the integrity o~ the communication link between it and the remotes Rx 1 and RX~l i~nediately adjacent thereto in the succession order;
FIG. 12A is a partial flow diagram illustrating in detail the manner b~ whlch an interrogating remote R~
tests the cor~nunications i~tegrity of the comm~ications link between it and the next lower number remote Rx 1 in the succession order;
FIG. 12B i.s a partial flow diagram illustrating in detail the manner in which an interrogating remote Rx tests the communiations integrity of the communications link between it and the next higher number remote RX~1 in the succession order;
FIG. 12C is a partial flow diagram illustrating in detail the manner by which a line termination impedance is applied to the cor~nunications lin}c in the event of a co~nunications link degradation or interruption;

~2~i8 FIG 13 is a legend illustrating the manner in which the flow diagrams of FIGS. 12A, 12B, and 12C are to be read; and FIG. 14 is an exemplary table illustrating the status of various counters when an interro~ating remote is evaluating the integrity of the communications link in accordance with the flow diagram shown in FIG. 12A.
DESCRIPTION OF THE PREFERRED EMBODIM~NT
An industrial control system in accordance with the present invention is shown in schematic form in FIG. 1 and includes a communications link CL (C-link) having a plurality of remotely located process control units (remotes) Rl, R2,...R7, R8 connected thereto with the eight remotes (Rl-R8) shown being exemplary; it bein~ understood that the system is designed to be used with a much larger number of remotes. of the eight remotes illustrated, the remotes Rl-R3 and R5-R7 are 'primary' remotes and the remotes R4 and R8 are 'redundant' remotes. The communications link CL is shcwn as an open line, double channel configuration formed from dual coax, dual twisted pair, or the like with the individual co~munication links identified, respectively, by the reference characters CL0 and CLl. While the system configuration shown in FIG~ 1 is a distri~uted 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 primary remote unit Rl-R3 and R5~R7 connected to one or more associated or corresponding input/output devices I/Ol-I/03 and I/05-I/07, respectively. Each input/output device is, in turn, connected to an associated controlled device CDl-CD3 and CD5-CD7 (of which only CD6 and CD7 axe illustrated in FIG. 1) such as, bu-t not limited to, various types of sensors (temperature, pressure, position, and motion sensors, etc.) and various types of actuators (motors, pumps, compressors, valves, solenoids, and reiays, etc.).
Each primary remote may control a large number of output devices and respond to a ]arge number of inpllt devices, and the blocks labeled I/O in FIG. 1 can each represent many input and output devices.
The redundant remote R4 monitors the operation of primary remotes Rl, R2, and R3; and the redundant remote R8 monitors the operation of primary remotes R5, R6, and R7. Should any one of the remotes Rl R2, and R3 fail, the failure will be detected by the remote R4 in a manner to be described and the remote R4 will take over control of the input and output devices of the failed remote by receiving the data from the failed remote over the communications link CL and sending commands to the failed remote over the communications link CL in formated information blocks. Similarly, if one of the remotes R~, R6, or R7 fails, the redundant remote R8 will take o~Jer control of the operation of the input/output devices for the failed remote as described above withrespect to redundant remote R4. ~lthough only eight remotes have been shown in Fiyure 1, any number of remotes Rl, R2, R3, .-.. Rn_l, Rn could be utilized in a particular system.
The architecture of an exemplary remote Rn is shown in FIG. 2. While the architecture of the remote Rn can vary dependiny upon the control process require-men-ts, 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;

-- ~.0 --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 20J
are provided depending upon the process control require-ments for the remote Rn. For example, in those primary remotes Rn which function as an elemental wire replacer, only the modem lO, the communication protocol controller 12, and the input/output management device 14 are pro-vided. In more complex process control requirements, an appropriately programmed central processing unit 16 and associated memory 18 are provided to ef~ect active con-trol according to a resident firmware program. In still other remotes requiring a human interface, the appropriate peripheral device(s) 20 may be connected to the common buss 22.
As shown in more detail in PIG. 3, the modem lO
provides two independent communication channels CH0 and CHl connected, respectively, to the communication links CL0 and CLl. Each of the col~unication channels C~0 and CHl is provided with substantially identical communi cation devices, and a description of the communicatlon devices of the first communication channel CH0 is sufficient to provide an understanding of the second communic~tion channel C~l. The communication channel CH~ includes an encoder/decoder 24~ for providing appropriate modulation and demodulation of the digital data trans-mitted to and received from the communication link CL0.

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In the preferred form, the encoder/decoder 24~ converts digital information in non-return-to-zero binary (NRZ) format to base-band modulation (BB.~) signal format for transmission and effects the converse for reception.
.~mplifiers 26~ and 280 are provided, respectively, to drive a passive coupling transformer T0 with digital information provided from the encoder/decoder 24~ from the coupling transformer T~. A set of selectively operable relay contacts 30~ are provided between the coupling transformer ~0 and th~ corresponding communication link CL~ to effect selective interruption thereof to isolate the remote Rn from the communications link CL, and another set of relay contacts 32~ are provided to selectively connect the signal output of the coupling transformer T~ with a termination impedance Z~. The termination impedance Z~ is used when the particular remote Rn is at the end of the communication link CL to provlde proper line termination impedance for the llnk, or, as described in more detail below, to assist in termi.nating an open or degra-led portion of the communi--cations link CL.
A selectively operable loop-back circuit 34 is provided to permit looping back or recirculation of test data during diagnostic checking of the remote Rn. While not specifically shown in FIG. 3 7 the loop-back circuit 34 can take the form of a double pole, single throw relay that effects connection be~ween the channels CH~ and CHl in response to a loop-back command signal 'L~'O During the diagnostic checking of a remote, which checking takes place when a particular remote is a master-for~the-moment as explained below, the relay contacts of -the loop-back ~ 12 -circuit 34 are closed and a predetermined test word is sent from the channel CH~ to the channel CHl and from the channel CHl to the channel CH0 with the received word in each case being chec]ced against the original test word to verify the transmit/receive integrity of the particular remote.
The isolation relays 300 and 311, the impedance termination relays 320 and 321, and the loop-back circuit 34 are connected to and selectively controlled by a communications link control device 38 whi.ch receives its communication and control signals from the communications protrocol 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~ and 301 to disconnect the remote Rn 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 Rn is automatically disconnected rom the co~nunication link CL in the event of a failure of its communication protocol controller 12.
As shown in more detail in FIG. 4, each communi-cation protocol controller 12 includes input/output ports 42, 44~ and 46 which interface with the above described modem lO for the communication channels CH0 and CHl and the modem C-link control device 38 (FIG. 3). A first-in first-out (FIFO) serializer 43 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 buss 22 through buss control latches 54. A read only memory (ROM) 56 containing a resident firmware program for the CPU 52 and a random access memory (R~M) 58 are provided to permit the CPU 52 to effect its communication protocol function as described more fully below. Timers 62 and a register 60 (for example, a manually operable DIP switch register or a hardwired jumper-type register~
that includes registers 60a and 60b are also provided to assist the CPU 52 in performing its communication proto-col operation. An excess transmission detectox 64, connected to input/output ports 42 and 44 (corresponding to communication channels CH0 and CHl) ~etermines when the transmission period is in excess of a predetermiIIed 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 ls trapped in a transmission mode from monopolizing the communications link CL.
The input/output management device 14, the architecture of whlch is shown in FIG. 4A, is preferably a firmware controlled microprocessor-based device which is adapted to scan the various input/output hardware points of the controlled device, effect a point-by-point status comparison with a prior scan, and record the change-in-status events along with the direction of the change and the time the event occurred (ti~e-tagging), effect data collection and distribu~ion to and from the input/output points, format the collected data in preferred patterns, and assemble the patterned data in selected sequences.
As shown in FIG. 4A, the input/output management device 14 includes a processor 14A connected to the remote buss 22 through a processor buss 14B; read-only-memories 14C and 14D connected to the processor 14A
through appropriate connectlons with these memories in-cluding the firmware necessary to effect the above~
described functions of the input/output management device 14 including the change-in-status event monitoring (described in more detail below); a read~write memory 14E (RAM) for temporarily storing information incident to the operation of the processor 14A including the change-in-status event information; a time base 14F for providing time information for time tagging the change-in-status events; and an input/output interface 14G for connection, either directly or indirectly, to the controlled devices.
In the preferred embodiment, the input/output interface 14G is defined by one or more printed circuit control cards generally arranged in rack formation with each card having hardware points arranged in predetermined sets of eight points with each hardware polnt carrying a blnary indication for controlling or sensing the operation of the controlled device. The control and operational status of the controlled device can generally be xepresented by one or more eight-bit words (e.g., 00010001) with each bit position representing a control or operational characteristic of the controlled device.
As described in further detail below in connection with FIG. 4B, the input/output management device 14 effects the aforedescribed change-in-status monitoring and associated time-tagging by periodically scanning the input/output hard-ware points in eight-bit groups and effecting a comparison between the so-obtained eight-bit group and the eight-bit group obtained during the previous scan. If a change is detected in one or more of the bit positions, the latest eight-bit group, along with the time-of-day inEormation obtained from the time base 14~, and other in~ormation, if desired, representing the direction of change, is placed in a first-in first-discard memory (FIPO) of predetermined size. Thus, each change-o~-status evenk along with its time tag and other information such as direction of change, etc. is placed in a memory of selected size as the changes occur. When all the memory locations are filled, the first entered event (which now represents the oldest chronological event) is discarded as the latest event enters the memory. The memory loading is inhibited by the occurrence of any one of a selected number of inhibit signals. In the system, various con-ditions including alarm conditions which represent partial or full system failures can be assigned a priority with those conditions or cor~inations thereof designated as "high" priority signals being permitted to disable or inhibit further accessing of the memory. In the event one of these high priority conditions occurs, the memory is inhibited from storing additional change-in-status information and the change-in-status events occurring prior to the high priority condition are preserved for subsequent analysis. Alarm conditions which are not designated as high priority, of course, do not inhibit the memory. This techni~ue advantageously differs from those prior techniques in which the controlled device status was only placed in memory at the moment of a high priority signal (in which case a historical pre~failure record-of-events was not available) or those techniques in which the change-in-status events were logged in a memory ~hich was periodically cleared, refilled, and cleared in which case the probability of obtaininy a complete histor~ of even~s prior to a predetermined high priority condition diminished in those instances in which the logging memory was cleared just prior to the occurrence of the high priority condition.
The manner b~ which the input/output management device 14 effects the change-of-status event logging is shown in FIG. 4B. During initialization, the processor 14B (referred to also as the RTZ in FIG. 4B) moves an image of the various input/output points, that is, the current status of the various input/output hardware points, to preassigned locations in the memory 14E (local) of the input/output management device 14 and the memory 18 (system) of the remote Rn (FIG. 2).
Thereafter, the address~s) of the first input/output card is obtained and the input/output hardware points for that card are scanned to obtain an input/output image which takes the form of an eight-bit word (e.g., 00000000) with each bit position representing the control or operational status of the controlled device. The input/output points so obtained are then compared with the previously obtained image of the points (e.g., 00100000), for example, by effecting a bit-by-bit exclusive OR (XOR) comparison. If the comparison indicates no change in status, (that is, the words are identical) the input/output points in the remaining cards are likewise scanned with the process repeated on a cyclic or looped basis. Howevex, if a change is detected in the exclusive OR comparison, that new input/output scan, along with the time tag information and the direction o~
change is placed in the memory 18 of the remote Rn, and, in addition, the latest scan is moved to the memroy 14E
of the input/output management device. This process continues with each new change-in-status event loaded into the memory 18 of the remote on a first-in first-discarded basis. The first-in first discard memory may be configured by assigning a preselected number o~ memory locations in the memory 18 of the remote Rn (e.g., fifty locations) for the logging information and providing an address pointer that points to each successive location in a serial manner with the pointer returning to the first location arter pointing at the last available pre-assigned location in the memory.
In the preferred embodiment, the processor 14A o~ the input/output management device 14 (FIG. 4A) and the processor 52 (FIG. 4) of the communication protocol controllex 12 i5 8X300 micro-controller manufactured by the Signetics Company of Sunnyvale, 5~

~alifornia, and the central proce33ing unit 16 (FI5. 2) is an 86/12 single board 16-bit micro-com~uter manu-factured by the Intel Company of Santa Clara, California and adapted to and configured for the ~ntel MULTIB~5~M
Each remote Rn is adapted to communicate with the other by transi~ting digital d~ta organized in pre-determined block formats. A sultable and illustrative block format 66 is shown in FIG. S and include~ a multi-word header frame 66A, a multi-word data frame 66a, and a look termination frame or word 66C. Selscted of the information block configuration~ aro adapt~d to transfer process control information to ~d from s~lected r~mote ~mits Rn and other of th~ block configuration~ ar~ adapted to transfer supervisory control of the c~mmunications llnk CL from one remote to the other remote AS explained in greater detail below.
An exemplary format for the header and data frames o~ an informlltion block 66 is shown, rèsp~ctively, in FIGS. 5A and SB~ The head~ frame 66A preferably 2~ include~ a 'start of header~ word(~) that indica~es to all remotes that infor~ation is b~ing transmitted; a 'source' identification word(s) that indlcate~ th~ identity of the sou~ce remote Rs that i~ transferring the infor~ation; a 'destination' word(s) that indicat~s the identi~y of the receiving or destination r~mote Rd; ~ Ih~ad~r-type' word(s) th~l~ indica~es whethex the data block i~ tran~ferring data, a parametared command block, or a paramet~rle~s ccmmand block;
'block-type' word indicating the type o~ block (that is, a command block or a data b~ock)s a 'block number' word that 5~

indicates the number sf blocks beiny sent; a 'block size' word indicating the length of tha data frame; a 'security code' word(s) that permits altexation of the resident soft-ware 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 in-clude a plurality of data carrying bytes or words Bl, B2~...Bn 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 o data and ~ommand blocks and non-acknowledge ~NAR) the receipt of data in which a trans-mission error i5 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.
~rhe vaxious remote uni~s Rl, R2~ R3,.. Rn communi-cate with one another by having each remote successively take control of the communications link CL and the controlling remote Rs then sending digital inormation between itself and a destination remote Rd 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~ or CLl is inoperative, for example, when one of the two co~munication cables i5 severed or otherwise degraded as occassionally occurs in harsh industrial environments.
~ 20 -~ en a remote unit assumes control of the communi-cation link GL (as explained more fully below) and, as a source remote Rs~ desires to send data blocks to another, destination remote Rd, the data block is assembled at the source remote Rs.in accordance with the block formats discussed above in connection with FIGS. 5-5D and trans-mitted through the information channels CL~ and CLl of the source remote R5 to the communication links CL~ and CLl with the header frame containing both the source remote Rs and t~.e destination remote Rd identification information.
In accordance with the data transmission technique, the communication protocol controller 12 of the source remote Rs transmits the in~ormation blocks twice on each communication link CL~ and CLl as schematically illustrated in FIG. 6 to provide a first data block DB~ and then a second, following data block DBB on each communication link CL~ and CLl.
The transmitted info~nation block headers include the id~ntity o~ the destination remote, Rd, which causes the des~ination remote Rd to receive and act upon the information blocks. At the destlnation remote Rd, the two data blocks DBA0 and DBB0 on the communication link CL0 are passed through the communication cha~nel CH~
and the two data blocks DBAl and DBBl on the communication link CLl are passed through the communication channel CHl to, respectively, the first-in first-out serializers 48 and 50 (FIG. 4).
As shown in the summary flow diagram o FIG. 7, the destingation remote Rd checks the validity o the received data by selecting one of thP two communication links (e.g., CL0 in F~G. 7) and then checks the first data block on the selected line (that is, DBA~3 by performing a cyclic redundancy check of the header frame and, if valld, performing a cyclic redundancy check of the data frame. If the data frame is valid, the commur.i-cation protocol controller 12 of the destination remote Rd then performs a bit-for-bit comparision between the CRC-valid first data block DBA~ and the second following data block DBB~. If the bit-for-bit comparision is good, an acknowledgement (ACR) signal is sent from the destination remote Rd to the source remote RS to indicate the receipt of valid information and complete tha~ 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 Rd then selects the other, alternate line (in this case, CLl) and perorms the aforementioned cyclic redundancy checks of the header and data frame and the bit-for-bit comparison between the fixst and second data blocks DBA1 and DB
on the alternate line CLl. If these checks indic2te valid data on thP alternate line, the destination remote Rd responds with an acknowledgement signal (ACK) to conclude the data block transmission transaction. On the other hand, if these checks indicate invalid data on the al~Prnate line (which means that the data blocks on both the first-selected line and the alternate line are invalid) the destination remote Rd responds with a non-acknowledgement signal (NAK) to cause retransmission of the data blocks from the source remote Rs. The non-

3~ acknowledgement block (NAK) includes a byte or bytes - 2~ -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 Rs and, after a finite number of re-transmissions (e.g., four~, halts further retransmission to assure that a source remote RS and a destination remote Rd do not become lost in a repetitive transmit/NAX/xe-transmit/NAK... sequence in the event of a hardware or software failure of the destination remote Rd error checXing 10 mechanism.
The double message alternate line checking sequence summarized in FIG. 7 may be more fully appreciated by referring to the detailed 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 ~-first' flag register is checked; if a flag is present, the 'irst-attempt fail' flag register is checked, and, if there is no flag in th~s register, the two data blocks DBAl and DBBl on channel OEIl are stored while the two data blocks DBA~ and D~B~ on channel CH~ are used for the first attempt information check.
Thereafter, the header ~xame of the first data block DBA0 on channel CH~ undergoes a CRC check, and, if acceptable, the data frame of this data block DBA~ undergoes a CRC check.
If the header and data frames CXC chec~s indicate valid data a 'good message' register is incremer~ted. 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 ~L~8~

detenmining there is no channel CH~ first flag or first-2ttempt flag present, checks the second following data block DB3~ by repeating the header and data CRC cyclic 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, D~A0 and DBB~) have passed the cyclic redundancy check for the header and data rames. Thereafter, the two data blocks DBA~ and DBB0 received on line CH~ are checked by performing a bit-by-bit comparision between the two. I the data blocks DBA~ and DBB~ pass the bit-by-bit comparision test, the co~muni-cations protocol controller 12 of the destination remote Rd sends an acknowledgement (ACK) message to the source remote Rs to conclude the information block transfer and xesets the various registers. If, on the other hand, eithex the data ~lock DBA~ or DBB0 on line CL~ ~ail the headex and data fxame CRC checks or these two data blocks fail the bit~by bit comparison check, the communication protrocol controller 12 sets the 'first-attemp~ fail' flag and xeturns ~o the start of the procedure ~o determine that the 'line ~-first' flag and the 'first-attempt' fail flag are present. The communi-cation protocol controller 12 then uses the s~ored data blocks DBAl and DBBl from line CLl (which data blocks were previously stored in FIFO SO). The header block and data block of the data blocks DBAl and DBBl from line CLl undergo the CRC
check and, if successful, cause the incrementing of the 'good 2~ -message' register to cause the communication protocol controller 12 to then check the validity of the second data block DBBl. If the data blocks DB~l and DBBl 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~l or DBBl does no~ pass the CRC check or the data blocks do not pass the bit-by-bit comparison test, a non-acknowledgement (NAK) is sent to the source remote Rs including information requesting the retransmission of the data blocks which ailed the validity test at the destination remote Rd. The source remote Rs 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 ~or recording, in a cumulative manner, the number of times an invalid message is received ~ox each communication link. In this manner, i~ can be dete~mined, on a statistical ba~is, whether one o~ 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 o~ plural communication links vastly enhances the ability o~ the des~ination remote Rd to detect errors and determine whether the in~ormation being.transmitted is valid or not. In addition, the destination remote Rd is able to operate and successfully receive messages even if one of the communicatiQn links C~ or CLl is severed since the communication protocol controller 12 at the dP.stination Rd - - -will examine the received signals on each line and will find invalid data on the severed line, but will always e~amine the data blocks on the other line and, if necessary, request retransmission of the information blocks.
In selecting one o the two channels CH~ or C~1 for the first validity check, it is preferred that one of the two channels ~e.g., CH0) be selected for the first check on every other information transaction and that the other of the two channels (e.g., C~1) be selected or the first check for the other intenmediate information transactions. While the system has been disclosed as having dual communication links CL~ and CLl, 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 Rn of the control system is adapted to accept and then relin~uish supervisory control of the communication link CL on a master-for-the-moment or revolving master arrangement. The communicakion protocol controller 12 o~ each remote Rn includes a register ~Jhich contains the remote succession number, another register which contains the total number of remotes in the system, and another xegister which contains th~ relative position of the remote from the present system master. The first two registers are schematically illustrated by the reference character 60 in FIG. 4. In addition, each remote Rn includes a variable transfer-monikor timer having a time-out interval that is set in accordance with a predetermined con~rol-tran~fer time constant (50 micro-seconds in the preferred embodiment) and the position of the particular remote relative to the present system m~ster to permit, as explained in more detail below, the master-for-the-moment transfer to continue even in the event of ~ disabled remote (that i5, 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 next successive remote. The operation of the master-for-the-moment transfer techni~ue can be appreciated by consideration of the followiny example of an illustrative system that includes five remotes arranged in the open loop configuration of FIG. 1 and transferxing supervisory control o the communications link CL in accord~nce with the tables of FIGS. lOA-lOF. The upper row of each ~able indicates the succession sequence or order of the five ~emotes Ro~ Rl, R2, R3 and R4 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 prese~t 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 succession, R
assumes supervisory contxol of the communication link CL ater system start-up and becomes the initial master of the syst~m (FIG. lOA). When the initial ma~ter Ro is in control of the communications link CL, it can send data to any o the other remotes, request s~at~ts or other data from another remote, and send control blocks and the like over the communications link CL. ~en the master Ro determines that it no longer desires possession of the communications link CL, it passes supexvisory control of the communications link CL to the next or first successive remote in accordance with the succession order. Thus, when the present mastex Ro concludes iks information transfer transactions, it transfers supervisory control of the communications link C~ to its next ox first successive remote Rl by transmitting a control block to the remote R
with all the remaining remotes (that is, R~, R3, R4) being cognizant of the transfer of supervisory control from the present mastex Ro to its fixst or next ~uccessive remote Rl. Since, in the present system, the txansfer o super~isory control of the communications link CL is expected to take place within 50 micxo-seconds, the second successive remote R2, as shown in the thlrd row of the table of FIG. lOB, sets its variable transfer-monitor tlmer to 50 micro-seconds, the third successive remote R3 sets its vaxiable transf~r~monitor timer to 100 micro-seconds, and the fourth successive remote R4 sets it trans~er-monitor timer to 150 micro-seconds. When the first successive re~ote R1 receives the control block from the present master Ro~ i~ accepts supervisory control of the communications link CL by responding with an acknowledgement message (ACR). If the control block is misreceived, the ~irst successive remote Rl can respond with a non-acknowledgement (NAK) to request retransmission of ~he control block transferring supervisory control of the communications link CL. During the time interval that the present master remote Ro is attempting to transfer supervisory control of the communi-cation link CL to its next successive remote Rl, the transfer-monitor timers of the remaining remotes are counting down. If, for any reason, the next or first successive remote Rl fails to take control (e.g., a malfunction of the remote), the transfer-monitor timer of the second successive remote R2 will time-out at 50 micro-seconds and cause the second successive remote R2 to then accept supervisory control of the cornmunication link CL
from the present master Ro and thus bypass the apparently malfunc~loning irst successive remote Rl.
Aassuming that the initial system master Ro successively ~ransexs supervisory con~rol of the communi-catins link CL to its first successive remote Rl, that successive remote Rl then becomes the present master with the remaining remotes changing their position relative ~o the present master and setting their transfex-monltor timers in accordance with the second and third rows of the table of FIG. lOB. When the present master Rl concludes its in~ormation transfer transactions, if any, it attempts ~o transfer supervisory control to its first or next successive remote R2 by sending an appropriate Gontrol block to remote ~2 which responds with an acknowledgement signal (ACK) or, in the event of a mistransmission of the control block, a non-acknowledgement signal (N~K) which causes re-transmission of the control block. When the control block requesting transfer of supexvisory control of the communi-cation link CL is sent from the present master Rl to its next succassive remote ~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 o the table of FIG. lOC. Should the next successive remote R2 be unable to accept supervisory control of the communication link CL from the present master Rl, the transfer-monitor timer o~ the second successive remote R3 will time-out in 50 micro-seconds and cause -the second successive remote R3 to assume supervisory control of the communiations link CL to thereby bypass an apparently malfunctioning ~irst successlve remote R2. 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 succe~sive remotes are malfunctioning, when the transer-monitor timer of the next operable remote times outO This transfer sequence continues in succession as shown in the remaining tables of FIGS. lOD to lOF with supervisory control of the communication link CL being passed from remote to remote in succession with the last remote R~
returning supervisory control to the first remotP Ro~

5~i~

By employing a master~for-the-moment transfer technique in which the receiving remote acknowledges control from the transferrlng remote and in which re-transmission of a mis-received control bloc~ i5 provided for in response to a non-acknowledqement signal from the receiving remotP, it is poss~ble to positively transer supervisory control of the communication link~ Thi~
technique ad~antageously transfers co~trol u~ing the data and inormation carry~ng communication lin~ rather than, as in other systems, by proviaing ~eparate communi-cation lines or channels ded~cated solely to supervisory control transfer function. Al~o, the provls~on of a variable trans~er-monitor timer at each remo e that i~ set in accordance with the remote's relative position to the present master and a transfex time-constant automa~ically txansfers supervisory control o~ the con~unicatio~s li.nk even i~ one or more o~ the succe~iv~ remote~ are mal~
func~ioning.
The architecture of a radundant remote (R4 and ~8 in FIG. 1), as sho~n in ~IG. 11, i~ essentially the same as that of a primary remote except that it has no input~
output devices asslgned to it. Each redundant remot2 functions to take over control responsibility of a controlled device from a primary xemote in the event the primary remote malfunctionY.

In each primary remote, preassigned memory locations are designated ~o act as a 'mailbox' register for that remote. Each time the central processing unit 16 of the primary remote cycles through its applications program, in which it responds to and controls the input/
output devices of the remote via the input/output management device 14, it stores a predetermined n~nber in its mailbox.
Each time the processor 14A of the input/outpu-t management device 14 cycles through its program, it decrements the number stored in the mailbox. The time for the CPU 16 to cycle through it~ program and for the input/output management device 14 to cycle through its prosram is approximately 1:1 so that the number stored in the mailbox will be maintained at or near the predetermined value set by the applications program of the CPU 16 unless the CPU 16 ceases to cycle through its applications program.
Should this happen, the number ~tored in the mailbox memory 18 will be decremented by the input/output management device 14 until it reaches a zero value.
Each time a redundant remote which i5 serving as a back-up for its associated primary remotes takes its turn in the master-for-the-moment sequence described above, the redundant remote will request and obtain the value of the number in the mailbox of its assigned primaxy remotesO
Xf the number in the mailbox i5 not zero, the redundant remote will know that the central processing unit 16 in the so~
queried primary remote is carrying out its applications program and has not gone into an emergency mode of operation or otherwise ceased to operate. If the redundant remote detects that the number in the mailbox for one of its assigned primary remotes is zero, then the redundant remote will determine that the central processing unit 16 of the zero-mailbox remote is not carrying out the applications program and, in response to this determination, the redundant xemote will first attempt to restart the applications program in the central processing unit 16 of the primary remote. If it fails to successfully restart the applications program, the redundant remote will carry out the applications program for the failed remote. In carrying out the applications program, the redundant remote will respond to the input devices and control the output devices assigned to the failed primary remote by sending commands and receiving data from the failed remote over the communications link CL.
The redundant remote, in addition to checking the status of its assigned primary remotes for which the redundant remote sexves as a back-up, also must maintain an up-to-date record of the status of the applications program in each of these assigned primary remotes. The redundant remote checks the status of the mailbox and gets the current applications program status fxom each of the primary remotes by sending requests for information over the communications link CL when the redundant remote takes its turn in the master~for-the~moment sequence as described above.

~ 33 -2S~8 The operation of the redundant remote in carrying out its function as a back-up for the primary remotes will be more fully understood ~ith reference to FIGS. llA and llB
which illustrate a flow chart of the program in the redundant remote R4 (FIG. 1), which serves as a back-up for its assigned primary remotes Rl, R2~ and R3. The other redundant remote R8 will have the same program except that it will be applied to its assigned remotes R5, R6, and R7.
As shown in FIGS. llA, after the program in the redundant remote R4 is started, it enters into a decision instruction sequence 101 to check the status of remote Rl. As explained above, it does this by sending a request for information over the communications link CL to remote Rl as~ing for the current number in the mailbox of remote Rl. It then determines whether this number ls greater than zero. If the number is greater than zero, the status of remote Rl is determined to be operating and the program of the redundant remote R4 advances to instruction step lG3 in which it resets a fail fl~g for Rl to 'off' and then enters subroutine lOS, in which the current applications program status in remote R1 is obtained. This means that the redundant remote R4 requests and obtains the current status of the input and output devices in remote Rl and the current status of the timers and the counters and the flags being used in the applications program of remote Rl. In other ~ 34 -words, in subroutine 105, all of ~he information that would be needed for the redundant remote R4 to take over the applications program is obtained from remote Rl.
This information is obtained by sending requests for data and receiving data back over the communications link CL.
Following the obtaining of the current appli-cations program status of remote Rl, th~ xedundant remote R4 progxam proceeds to decision instruction sequence 107, in which the status of remote R2 is checked in the same manner that was done with respect to Rl. If the status of remote R2 is operating, the program advances to instruction step 109, in which the program sets a fail flag for re~ote R2 and then pro~eeds into subxoutine 111, in which the status of the applications program for xemote R2 is obtained in the same manner as for Rl in sub-routine 105. The program then proceeds into a decision instruction sequence 113 to check the status of remote R3. If the status of remote R3 is operating, then the program resets the fail flag for remote R3 in instruction step 115 and proceeds into subroutine 117 to obtain the applications program st~tus for remote R3 in the same manner as ~or Rl in subroutine 105. Following subroutine 111, the program return~ again to decision instruction sequence 101 to check the status of remote Rl and the process cyclically repeats.
If in decision instruction se~uence 101, the program determines that the status Rl is not operating as indicated by the number in the mailbox of the remote Rl, bein~ zero, the program then advances to decision instruction sequence 119, in which the program determines if the fail flag for Rl is 'on' or 'off'. If the fail flag is 'off', the program proceeds into instruction sequence 121, in ~hich the program attempts to restaxt the applications program for remote Rl. It does this by sending a co~mand over the communications link CL to remote Rl to direct the communica-tions protocol controller 12 (FI5. 2) to attempt a hardware restart of the applications program.
This is carried out by the communications protocol controller 12 pulling a restart wire to ground in the common buss 22. When this restart wire is pulled to ground, it starts the applications program back through its initiali~ation program and sets all of the flags, timers, and counters just as if power had been turned on. Such a restart is called a hardware res~art. Alterna~ively, the redundant remote R~ could effect a software restart in the failed remote. ~ software restart would mexely start the applications program through its initialization proyram with the timexs, eo~mters and flags left in their present status.
Ater completing instruction sequence 121, the redundant remote R4 progxam then sets the fail flag for remote Rl to 'on' in instruction step 123 and then proceeds into decision instruction sequence 125 to again check the status of remote Rl by checlcing the number in the mailbox of remote Rl in the same manner as in decision instruction sequence 101. If the applicatio~s program in remote Rl was successully started in instruction sequence 121, the number in the mail~ox will not be zero and the pxogram will determine that the statlls of remote Rl is operating, whereupon the program will jump to decision inst~uction sequence 107 to check the status of remote R2 as already described.

- 3~ -If the program determines that the status of remote Rl is not operating in decision instruction sequence 125, then this means that the attempt to restart the applications program in remote Rl in instruction sequence 121 failed and the xedundan~ remote R4 program then proceeds into instruction sequence 127 to initialize the input/output management device 14 (also identified in FIG. llB as 'RTX') in remote Rl to recei.ve instructions and data from the redundant remote R4 instead of from the central processing unit 16 in the remote Rl and to send data on the status of the input and output devices to the redundant remote R4.
If the program of the redundant remote R4 determines that the fail flag was 'on' instead of 'off 7 ln decis.on instruction sequence 119, the redundant remote program would proceed directly into the instruction sequence 127 to initialize the input/output management device 14 of remote Rl to respond to the redundant remote R4.
The purpose of the fail flag which is set to 'on' in instruction step 123 and is reset to 'off' in instruction step 103 is to prevent the redundan~ remote program from getting hung-up in a csndition in which it success~ully restar~s the remote Rl only to have the remote Rl fail again by khe time the program of the redundant remote recycles around to checking the mailbox o~ the remote Rl again in decision instruction sequence 101. If this should happen, the fail flag for remote Rl will have been set to 'on' in instruction step 123 after the successful restarting of the applications program. Then, the next time that the redundant remote program cycles back to decision instruction sequence 101, and determines that the status of remote Rl is not operating, the fail flag for remote Rl will be 'on'. Accordingly, the program will jump from decision instruction sequence 119 into the instruction sequence 127 to initialize the remote Rl to respond ~o redundant remote R4. If the next time ~he redundant remote program recycles back to decision instruction sequence 101 to check the status o Rl, it determines that the status of Rl is operating, the program will then reset the fail flag to 'off' in instruction step 103 so that in subsequent cycles, should the program determine that the remote Rl has again failed, the program will again go into the restart instruction sequence 121 instead of immediately jumping to the initialization instruction sequence 127.
After the rPdundant remote program has comple-ted the initialization instruction sequence 127, it then proceeds to subroutine 129. In this subroutine, the status of the applications program oE remote Rl last received by the redundant remote R4, which status is stored in the memory o~ the redundant remote R4, i5 loaded into predetermined registers o the memory of the redundant remote R~ in order to carry out the applications program of remote Rl in the redundant remote R4. After this subroutine is completed, the program proceeds into instruction sequence 130 and then into the subroutine 131 in which it start~ and carries out the applications program. The redundant remote R4 carries out the R~ applications program by receiving data from remote Rl as to the sta~us of the input and output devices - 3a -of the remote Rl and sending instructions to remote R1 to direct operation of the input/output management device 14 of the remote Rl. The program in the redundant remote R4 will then continue to cycle through the applications program for the remote R1 until it receives a command from the operator to reset it back into its main cyc]e of checking the status of the remotes Rl, R2, and R3.
Should the redundant remote R4 determine that the status of remote R~ or remote R3 is not operating, it then performs the same program with respect to these remotes as described with respect to remote Rl as is illustrated in FIGS. llA and llB.
The redundant remote R8 will take over the applications program should any of the primary remotes R5-R7 become nonoperative in the same manner as described above with respect to R4 serving as a back-up for the primary remotes Rl-R3.
It will be appreciated that the provision of the redundant remotes decreases malfunctioning of the control system due to one of the primary remotes becoming inoperative as a result of failure of the central processing unit 16 of the primary remote. Because each redundant remote serves as a back-up for several primary remotes, the cost of providing the redundancy is significantly reduced. Because the redundant remotes are themselves each a remote control uni-t which takes its turn in the master-for-a-moment sequence communicating with the other remotes over the dual channel communications link, the redundant remotes can ~e provided in the system very inexpensively.

Each remote Rn, as described above, is provided with termination impedances Z~ and Zl for the first and second communication channels CH0 and CHl (FIG. 3) and a line termination relay 32~ and 321 under the control of the communications link control device 38. The termination impedances are connected across each channel of the communi-cations link when the particular remote is the first or the last remote in the system ~e.g., R1 and R8 in FIG. 1) to establish proper line termination impedance to prevent signal level degradation and the presence of reflected signals, both conditions which can adversely affect the performance of the system. The termination impedances Z0 and Zl are also applied across the appropriate communi-cations channels when a remote determines, as described below, that the communications link CL between it and its immediately adjacent higher or lower number remote i5 severed or sufficiently degraded that reliable data transmission cannot be maintained therebetween. The detenmination as to communications link degradation can be made by providing each remote wi~h a regi~ter fox each communications channel that records, in a cumulative manner, the numbex of invalid messages received from the immediately ~a~jacent remote(s) and terminate one or both of the communications link CL0 and CL1 in the direction of the remote from which the number of invalid messag~s received exceeds a threshhold value. More preferably, however, each remote is provied with an active testing diagnostic routine to enable it to test the communication integxity of t~e communications link betw~en it and its immediately adjacent remote(s) in accordance with the flow diagrams illustrated in FIGS. 12, 12A, 13B and 12C as xead in accordance with FIG. 13 and the table of FIG~ 14~

~2~

The flow diagram illustrated in FIG. 12 is a summary of the manner by which each remote is capable o testing the communication integrity of the communications link CL between it and its immediate adjacent remote or remotes and termina~ing one or bo~h of ~he communications links, CL~ and CLl, when a degraded or interrupted line condition is detected. As shown in FIG. 12, the remote Rx is initialized a~d then, in sequence, tests the communi-cations integrity of the communications link CL~ in the downstream directio~ between it and its immediately adjacent lower number remote (that is, R~ 1) and then tests the communication integrity of the communications link CLl in the downstream direction with the same remote. If either the communications link CL~ or CLl in the downstxeam direction is faulty, an appropriate ~lag is set in a register in the remote Rx reserved for this purpose. In a sLmilax manner, the remote Rx then tests the communications integrity of the communications link CL~ and CLl in the up-stream direction with its immediately adjacent higher number remote (that is, remote RX~l) and sets the appropriate ~lag, as and if required. Aft~r ~his initial diagnostic checking takes place, the remote Rx will terminate the failed communi-cations lire CL~ and~or CLl by actuating the appropriate relay contacts.320 and/or.321 as requixed. The line checking test utili~ed in FIGo 1~ preferably takes place when the remote Rx is master-for-the-moment (that is, Rm).
A more de~ailed explanation of the communications line integrity chec~ a~d automatic line termination may be had by referriny to FIGS, 12A, 12B and 12C (as xead in accordance ~ 41 -with the flow chart legend of FIG. 13) in which FIG. 12A
represents the downstream integrity check with the next lower number remote, FIG. 12B represents the upstream integrity check with the next higher number remote, and FIG. 12C represents the line termination function in response ~o the results of the integrity test performed in FIGS. 12A and 12B.
In FIG. 12A, the line checking diagnostic is started by first loading three registers or counters, namely, a 'retry counter', a 'CL0 retry counter', and a 'CLl retry counter' with an arbitrarily selected number, for example, five. The 'retry counter' is then decremented by one and a message sent from the remote Rx to the remote Rx 1 requesting an acknowledgement ACX signal. If the communications link CL~ and CLl between the interrogating remote and the responding remote is fully ~unctional, a valid ACK signal will be received by the interrogating remote Rx on both CL~ and CL1. The diagno~tic checking will then route to the part o~ the program (FIG. 12B) for checking the communications integrity of the communications link CL0 and CLl hetween the interrogating remote Rx and the n~xt higher nu~ber remote in the system, that i5, RX~l. On the other hand, if a valid ACK signal is not received on one or both of the communications links CL~ or CLl by the requesting remote Rx from the immediately adjacent lower number responding remote Rx l~ the appropriate retry counter (that is, 'CL~ retry co~nter' or ICLl retry co~nter ? ) will be decremented by one and the procedure repeated until the 'retry counter' is ~ero at ~hich time the appropriate C~

- 4~ -and/or CLl terminate flag register will be set; thereafter, the program will route to the upstream communications integrity check shown in FIG. 12B.
The flow diagram of FIG. 12B is basically the same as that oE ~IG. 12A except that the communications integrity chec~ occurs for that portion of the co~nunications link CL between the interrogating remote Rx and the next higher number responding remote R~l. More specifically, the three registers or counters, that is, the 'retry counter', the 'CL~ retry co~nter', and the 'CLl retry counter' are loaded with the arbitrarily selected value of five. The 'retry counter' is then decremented by one and a message sent from the interrogating remote R~ to the remote RX~l requesting an acknowledgement signal. If the communications link CL0 and CLl ~etween the interrogating remote Rx and the respondiny remote RX~l is i~tegral, a valid acknowledgement signal will be received by the interrogating remote Rx and the program will route to the tennlnation impedance portion of the procedure shown in FIG. 12Co On the other hand, if a valid acknowledgement signal is not received on one or both of the communications lines CL~
or CLl by the interrogating remote Rx from the higher order responding remote RX~1, the appropriate retry counter, that is, the 'CL0 or CLl retry counter' will be decremented by one and the procedure repeated until the 'retry counter' is zero at which point the appropriate CL~ and~or CLl termination ~lag register will be set; thereafter, the program diag~ostic will route to the line i.mpedance termination portion shown in FlG. 12C.

~256~

In the flow diagram of FIG. 12C, the various termination registers are examined for set flags and appropriate commands issued to the C-link control device 38 (FIG. 3) to terminate the line by appropriate actuation of the relay contacts 32~ and/or 321. As is also shown in FIG. 12C, a line termination relay can also be released (that is, reset) to remove a previously applied line termination impedance. Accordingly, the system provides each remote with the ability to remove a line texmination as well as apply a line termination. This particular feature is desirable when a communication link is temperarily degraded by the presence of non-recurring electrical noise to permit the system to automatically re-configure its line impedances.
The following specific example illustrates the operation of the line termination procedure in whi~h it is assumed that the communicatio~s link CL~ in FIG~ 1 is severed at point A as shown therein and that the remot~
R4 is the presen~ master.(Rm) of the system and testing the ~ommunications integrity of the comm~nications link between itself as the interroyating remote (Rx) and its next lower order number remote R3 (that is, Rx 1) In accordance with the ~low diagram of FIG. 12A, the 'retry counter', and the 'CL0 retry counter'/ and the 'CLl retry counter', as shown in the tabulation table of FIG. 14, are set to the pre determined value o~ five~ The 'retry counter' is decremented b~ one and the requesting interrogating remote R4 (Rx) requests an acknowledgement from the responding remote R3 (tha~ is~ ~x-l) The requested acknowledgement will be provided on line CLl but not line CL~ because of the - ~4 aforementioned interruption at point A (FIG. 1).
The interrogating remote R4, not receiving the requested acknowledgement signal on communications link CL~, will decrement the 'CL~ retry counter' by one. Thereafter, the retest procedure will be sequentially continued with the 'C~0 retry counter' being decremented with each additional unsuccessful attempt to obtain an acknowledgement from remote R3 through the communications link CL0. When the 'retry counter' decrements to zero, the 'CL0 retry counter' will also be decremented to zero at which time the CL0 lower order termination ~lag will be set. The remote R4 will thereafter continue the diagnostic checking procedure to test the communications integrity of that portion of the co~unications link between the remote R~ (Rx) and the next adjacent higher remote R5 (that is, RX~l) in accordanc0 with the 10w diagrc~m of FIG. 12B~ At the conclusion of the test o the communications link between th~ inter-rogating remote R~ and the immediately adjacent lower number and higher n~mber remotes R3 and R5, the termination relay contacts 32~ ~FIG. 3) will be set to texminate the communi-cations link CL~ at the remote R4. In a similar manner, the remote R3, when it b~comes master-for-the-moment, will also apply a termination impedance ~cross the communications link CL~.
As can be appreciated from the foregoing, the remotes Ro~Rn have the ability~ even when one or both of the communication links CL~ and C~l are severed to still ~25~

function on a master-for-the-moment basis and also to effect appropriate line termination to minimize the adverse effect on digital data signal strength and the generation of reflected signals from mismatched line impedance caused by deteriorated or severed communication lines. In addition, the system is self-healing, that is, when reliable communications is restored over the severed or degraded portion of the communications link the remotes Rn will then again function to remove the line impedances to resume full system operation.
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 calims and their legal e~uivalent.

Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An information transfer system for transmitting digital information between active devices and testing the validity of the transmitted information, said system comprising:
at least one active device for transmitting information in digital form;
at least one other active device for receiving informat-ion in digital form;
at least a first and a second independent communication channel connected to and extending between said first-mentioned and said second-mentioned active devices for conveying information therebetween;
a transmitter means associated with said first-mentioned active device for transmitting digital information arranged in blocks of predetermined format, said transmission means transmitting, for each information transfer transaction, an identical block on each of said first and second communication channels;
receiving means associated with said second-mentioned active device for receiving digital information transmitted by said first-mentioned active device and for selecting a one of said first and second communication channels and testing the validity of the received block and, when said received block from said first selected communication channel is found invalid, for selecting the other of said first and second cornmunication channels and testing the validity of the received block on said other communication channel; and means associated with said receiver means for first-selecting the first of said communication channels on alternate information transfer transactions and for first-selecting the second of said communication channels on the remaining information transfer transactions.

2. An information transfer system as recited in Claim 1, wherein said receiving means operates to send an acknowledge-ment signal to said transmitter means when either said received block from said first communication channel or from said second communication channel is found valid and for sending a non-acknowledgement signal to said transmitting means when both the received block from said first communication channel and the received block from said second communication channel are found invalid.

3. An information transfer system as recited in Claim 2, wherein said transmitter means operates to retransmit at least a portion of the digital information transmitted in a block to said receiving means in response to receiving a non-acknowledgement signal from said receiving means.

4. A method for transferring digital information formatted in predetermined blocks between an information transmitting device and an interconnected information receiving device, said method comprising the steps of:
transmitting, for each information transfer transaction, identical information blocks from a transmitter over plural independent communication channels to a receiver;
receiving and storing the received information blocks at the receiver;
selecting the information block received on one of said plural communication channels and testing the validity thereof;
selecting the information block received on other of said plural communication channels and testing the validity thereof in the event the first-selected information block fails its validity check; and requesting retransmission of said information blocks

Claim 4 - cont'd in the event both the first-selected and said second-selected information blocks fail their validity test, said one communication channel first-selected on alternate information transfer transactions and said other communication channel first-selected on the remaining information transfer trans-action.