Fault-tolerant multi-computer system4356546Abstract A Fault-Tolerant Multi-Computer System for control applications is disclosed. The system has a plurality of Computers (10a-10n), each having an assigned set of tasks which it is capable of executing. No one Computer in the system acts as a master and no one Computer executes all of the tasks. Communication between the Computers is by individual communication links (16, 18, 20) over which each Computer sends information directly to all other Computers in the system. Each Computer comprises an Applications Computer (100) and an Operations Controller (200). The Operations Controller receives messages over the communication links and selects, from the assigned tasks, the tasks to be performed by the associated Applications Computer. Each Operations Controller includes a fault handler which checks the messages received from the other Computers. The fault handlers send and receive error messages, over the communication links, to assist in the identification of a faulty Computer. Subsequent messages from the Computers deemed to be faulty are ignored, and the tasks assigned to the faulty Computer are executed by alternate Computers in the system. Claims What is claimed is: Description CROSS REFERENCE
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ARCHITECTURE OF THE FAULT-TOLERANT
MULTIPLE COMPUTER SYSTEM
TABLE DESCRIPTION
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Table I Tables used in the System
Messages
Table II-A
*Inter-Computer Messages
Table II-B
Internal Messages
Fault Handler
Table III-A
Message Format Checker
Table III-B
Reasonable Limits Checker
Table III-C
Redundant Data Table
Table III-D
*Redundant Value Voter
Table III-E
*Check Agreement
Table III-F
*Find Values That Agree
Table III-G
*Record Voted Value
Table III-H
*Task Unselected/Selected Message Module
Table III-I
*Task Completed/Started Message Module
Table III-J
*Watch Dog Timer Checker
Table III-K
*Start Watch Dog Timer Module
Table III-L
Sampling Data Table
Table III-M
*Start Synchronizer Module
Table III-N
*Check Sampling Timer Module
Table III-O
*Find Sampling Number Agreement Module
Table III-P
*Find Computers That Agree
Table III-Q
*Reset Sampling Timer
Table III-R
*Record Voted Sampling Number
Table III-S
Fault State Table
Table III-T
*Send Good Message Module
Table III-U
*End Time Period Module
Table III-V
*Check Error Message Agreement Module
Table III-W
*Record Error Module
Table III-X
*Display Faulty Computer
Table III-Y
*Start Fault Handler Module
Scheduler
Table IV-A
Task Status Table
Table IV-B
Task Index Table
Table IV-C
Scheduling Status Table
Table IV-D
Awaiting Task Table
Table IV-E
*Record Data Ready
Table IV-F
*Find Awaiting Execution Number
Table IV-G
*Test If Health Check Selected
Table IV-H
Special Tasks Table
Table IV-I
*Record Special Tasks
Table IV-J
*Task Selector
Table IV-K
*Record Task Selected By Own Computer
Table IV-L
*Completed Task Recorder
Table IV-M
*Test If Last Completed Task
Table IV-N
*Unselected/Selected Task Recorder
Table IV-O
*Record Task Selected
Table IV-P
*Test If Selected Task
Table IV-Q
*Task Unselector
Table IV-R
*Task Releaser
Table IV-S
*System Status Monitor
Table IV-T
*Start Scheduler Module
Task Communicator
Table V-A Data Values Table
Table V-B Task Input Table
Table V-C Task Output Table
Table V-D *Store Data Value Module
Table V-E Task Data Table
Table V-F *Task Dispatcher
Table V-G *Release Task Module
Table V-H *Task Results Message Sender
Table V-I *Starter
Table V-J *Counter
Table V-K *Start Task Communicator
Applications Computer
Table VI *Applications Computer Executive Program
Microprocessor Based Implementation of Operations Controller
Table VII-A
*General Executive Program
Table VII-B
Conditions for Module Execution
Table VII-C
*General "Task" Program
Table VII-D
*Fault Handler Executive Program
Table VII-E
Module Modifications
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The tables indicated with asterisks (*) are pseudo code programs
DETAILED DESCRIPTION OF THE INVENTION ARCHITECTURE OF THE FAULT-TOLERANT MULTI-COMPUTER SYSTEM The architecture of the disclosed Fault-Tolerant Multi-Computer System is illustrated in FIG. 1. The system comprises a plurality of Computers 10 connected by means of input lines 12 to various sensors and manual inputs collectively represented by block 14. The outputs of Computers 10 are transmitted by means of output lines 22 to a Combiner/Voter Network 24, which selects and/or combines the output data generated by the various Computers. The Combiner/Voter Network 24 distributes this data, by designated line 26, to the appropriate actuators and displays collectively represented by block 28. Each Computer 10 has its own private communication link, such as Communication Links 16, 18, and 20, over which it can transmit messages containing data to every other Computer. For example, messages originating in Computer A are transmitted to all the other Computers via Communication Link 16. All the other Computers connected to Communication Link 16 can only receive messages over Communication Link 16. To transmit a message back to Computer A, they must use their own communication link, i.e., Computer B would use Communication Link 18 and Computer N would use Communication Link 20. The messages and data sent over the communication links are sent in serial form; therefore, each link may be a single pair of wires or other serial transmission medium such as an optical fiber. Each communication link is also connected back to the transmitting Computer, permitting verification that the message sent on the communication link is correct. This is part of the fault detection features of the system to be discussed later. Each Computer such as Computer 10a through 10n consists of one or more computers (or processors), depending upon the number of tasks to be executed by that particular Computer for a particular application and upon the fault-tolerant sophistication of the system. Each Computer 10a through 10n is hereinafter referred to as Computer 10 without the identifying subscript. Each Computer has an assigned set of tasks which it is capable of executing, where the set of tasks assigned to each Computer 10 is less than the total set of tasks to be executed by the system. One feature of the system, however, is that each task to be executed is assigned to at least two different Computers. Certain tasks critical to the operation of the system are assigned to several or possibly all of the Computers. Each Computer in the system is capable of individually executing each assigned task. For example, consider a relatively simple system having three Computers and required to execute fifteen (15) different tasks, of which the tasks designated Tasks 7 and 11 are critical to the operation of the system. Further, consider each Computer in the system to be capable of executing at least eleven (11) of the required tasks. In this example, Computer A may be assigned Tasks 1 through 11, Computer B assigned Tasks 5 through 15 and Computer C assigned Tasks 11 through 15, Tasks 1 through 5, and Task 7. In the example, Tasks 7 and 11 are assigned to each Computer; however, in a system having more than three Computers, Tasks 7 and 11 would have been assigned to more than two Computers but not necessarily to all of them. The execution of each assigned task in each of the Computers 10 is data driven, i.e., when all of the data required for the execution of a particular task is available, each Computer to which the task is assigned is capable of selecting and executing the task. The data is usually the results of one or more previously executed tasks. When execution of a task is completed, the task results are communicated to each of the computers by sending messages via the communication link. When the task results are received by Computers which require the particular data for the performance of a subsequent assigned task, the receiving Computer will store the received data. The Computers which have no assigned task requiring the particular data may discard the received data. The selection of each task to be executed by each Computer is made dynamically by each Computer. This is done in such a manner that all Computers assigned a task will not necessarily proceed to execute that task. Stated alternatively, a Computer may not execute all tasks which are assigned to that Computer. Each Computer makes its own decisions, based upon knowledge of previous decisions by all Computers, as communicated in messages received via the communication links. The task selection is performed by a Schedular described in detail hereinafter with reference to FIG. 24. Briefly, the selection of each task to be executed by each Computer is made dynamically on a priority basis. To this end, a priority number is assigned to each Computer and a priority number is assigned to each task within a given Computer. When a given Computer needs to select a task, the task status information is scanned to determine which of the assigned tasks are ready for execution. A task is ready for execution when all of the data necessary for the execution of the task is available. The Computer selects the ready task having the highest task priority and sends out a message on its communication link signifying to the other Computers that it has selected the task. When a Computer receives a message indicating that another Computer has selected a task, the selected task is removed from the ready status in all of the other Computers capable of performing the same task. In the time interval between task selection and starting the execution of the selected task, the computer checks to determine if another Computer has selected the same task. If the Computer which selected a task does not receive a message indicating that another Computer has selected the same task, the Computer initiates the execution of the selected task. In the event another Computer selects the same task before the first Computer initiates the execution of the task, the priority of each Computer which selected the task is analyzed, and the task remains selected by the Computer having the highest priority. The remaining Computers unselect the previously selected task, and proceed to select the next highest priority task ready for execution. When it is desirable that certain identified tasks be executed by more than one Computer, the same functional task is duplicated for scheduling purposes, one copy for each execution desired. Fault detection in the system is accomplished by a combination of methods. Faults may be detected by comparing the results of each executed task with stored range limits, by comparing the results of the same task executed by two or more different computers, by error detecting codes on information communicated, by analyzing the scheduling sequence or by the use of watch-dog timers. The system may embody all five fault detection methods, any lesser number of the above methods in combination, or in special applications, any one of the above listed methods. The messages sent by a Computer are received and analyzed in every Computer in the system to determine if an error exists. If an error is detected, each Computer detecting the error sends out an error message via its communication link to all of the other Computers. An error message signals the detection of an error and identifies the Computer which made the error. The error messages received are analyzed in each Computer. When a Computer receives error messages from two or more Computers, the computer which is identified as making the error is assumed to be faulty. When a Computer is deemed faulty by another Computer, the messages signaling task selection and containing data results of any task executed by the faulty Computer are discarded or ignored. The receipt of messages signifying an error detected by two or more Computers also reinstates the ready status of the tasks presently selected and being executed by the Computer which is deemed to be faulty. The tasks selected and being executed by the faulty Computer are subsequently re-selected and re-executed by other Computers capable of executing those tasks. In the disclosed system, a Computer determined to be faulty is not turned off or disabled, but is permitted to remain active and to continue to execute each of the assigned tasks, if it can. The remaining Computers continue to check the messages sent by the faulty Computer to determine if the malfunction is temporary or permanent. If the malfunction is temporary, the faulty Computer will eventually return to normal operation and the results of the tasks executed in that Computer will be correct. After a Computer deemed to be faulty correctly executes its assigned tasks for a predetermined period of time, the malfunction is assumed to have been temporary and the excluded Computer is restored to full participation in task selection and task execution. If a faulty Computer sends incorrect information to Actuators and Displays, the faulty information is corrected by the Combiner/Voter Network 24. The output or task results of any task used for actuator activation or display purposes are generated by the Computers 10 to which the specific tasks are assigned. The output of each Computer 10 is transmitted on lines 22 to the Combiner/Voter Network 24. The Combiner/Voter Network 24 combines the appropriate output data for actuator activation or display purposes as required. When duplicate outputs are provided by multiple Computers, the output data to be used is selected by a voting process. FIG. 2 shows in greater detail the architecture of the multi-computer system, shown in FIG. 1. Each Computer 10 comprises an Applications Computer 100, such as Applications Computers 100a through 100n, and an Operations Controller 200, such as Operations Controllers 200a through 200n. Each Applications Computer 100 and its associated Operations Controller 200 are interconnected by a buss 30, as indicated by busses 30a through 30n. The data from the sensors and manual controls, indicated by block 14, are received directly by the Applications Computers 100. Similarly, the data to the Actuators and Displays 28, via the Combiner/Voter Network 24, are obtained directly from the outputs of the Applications Computers. Only the Operations Controllers are interconnected by the communication links 16 through 20. The Applications Computers 100 are of conventional architecture as shown on FIG. 3. Each Applications Computer comprises a Power Supply 102, a Central Processing Unit (CPU) 104, a Memory 106, and an Input-Output Network 108. The Operations Controllers 200 each comprise a plurality of Receivers 202, a Fault Handler 204, a Scheduler 206, a Task Communicator 208, and a Transmitter 212, as shown on FIG. 4. The structure of the Operations Controllers 200a through 200n shown in FIG. 2, including the interconnecting communication links 16 through 20, represent the novel aspects of the disclosed Fault-Tolerant MultiComputer System. The system does not contain a master controller to determine or control which Computer 10 will execute a designated task. Further, the system is not a fully redundant system wherein each Computer 10 is capable of executing and does execute every task. APPLICATIONS COMPUTER FIG. 3 shows the structure of a typical Applications Computer 100. Each Applications Computer 100 has a Power Supply 102, which supplies electrical power to the Central Processing Unit 104, the Memory 106, the Input-Output Network 108, and the associated Operations Controller 200, as indicated. The Central Processing Unit 104, Memory 106 and Input-Output Network 108 are connected by the buss 30. The Input-Output Network 108 is further connected to the Sensors and Manual Controls 14 by line 12, and to the Combiner/Voter Network by line 22. As previously indicated, the Operations Controller 200 is also connected to the buss 30. The Central Processing Unit 104 may comprise one or more microcomputers, such as Central Processor 8086 manufactured by Intel Corporation of Santa Clara, Calif. The Memory 106 may comprise one or more read only memories, such as Erasable PROM 8708 also manufactured by Intel Corporation, which store the programs to be executed by the Central Processing Unit. The Memory 106 may also include one or more read-write (RAM) memories, such as Static RAM 8102A also manufactured by Intel Corporation. The Input-Output Network 108 may comprise one or more commercially available integrated circuits, such as Programmable Peripheral Interface 8255A manufactured by Intel Corporation, with attendant A/D and D/A converters. Alternatively, the Central Processing Unit 104, Memory 106 and Input-Output Network 108 may be incorporated in a single integrated circuit such as Microcomputer 8748 manufactured by Intel Corporation. The operation of the Applications Computer 100 is as follows. The Operations Controller 200 generates a task signal indicative of the task to be executed by the Central Processing Unit 104, and sends the task signal along with the requisite data to the Central Processing Unit over Buss 30. The Central Processing Unit 104 responds to the task signal, accesses the appropriate program in the Memory 106, executes the task with the provided data, and outputs the results on Buss 30. Data from the Sensors and Manual Controls are received over lines 12 at the input of the Input-Output Network 208, which makes the received data available for use in the execution of an assigned task. The results of those executed tasks which are used for actuator control or display are output to the appropriate actuator and/or display through the Input-Output Network 108. Other results from a task executed by the Central Processing Unit 104, which are required for further computation within the system, are transmitted to all other Computers via the Operations Controller 200 and the communications link. When the execution of the task is completed, the Central Processing Unit initiates execution of the next task selected by the Operations Controller. OPERATIONS CONTROLLER FIG. 4 shows the structure of the Operations Controller 200 in block diagram form. The Operations Controller 200 has a plurality of Receivers 202a through 202k, each connected to a communication link associated with one of the Computers 10. There may be as many Receivers 202 as there are Computers 10, or there may be fewer Receivers if the computer associated with this Operations Controller has no need to receive communications from one or more other Computers in the system, i.e., the results of none of their tasks are needed by this Computer for the execution of its assigned tasks. The input to one of the receivers, designated Receiver 202k, is connected by means of line 214 to the output of Transmitter 212, which sends the messages and data over the communication link from the associated Operations Controller. This feedback connection between the Transmitter 212 and Receiver 202k is part of the fault detection system to check the message sent over the communication link, and also permits the task results to be input back into the generating Computer for subsequent task execution. This feedback connection may be direct, or preferably in the form of a loop connection from Transmitter 212 to the appropriate Receivers in each other Computer and finally back to Receiver 202k in the same Computer. The Receivers 202a through 202k receive the messages serially transmitted over the communication links and convert them to a parallel format for subsequent utilization in the Operations Controller and Application Computer. As used hereinafter, the term "messages" will include all messages, such as Task Completed/Started, Task Unselected/Selected, Error, Task Data Values, and other messages communicating information between the Computers via the communication links. The Receivers 202 also include circuits which establish message protocol and perform other necessary format conversions. The Transmitter 212 performs the reverse function, receiving parallel data and converting it to a serial format for transmission over the communication link. The format conversion may strip or add carriers, provide padding or add special codes for transmission error control as is known in the art. The Receivers 202 and Transmitter 212 each contain buffers permitting a message to be received some time before it can be output. Each buffer is capable of holding more than one message; for example, each buffer may be capable of holding up to ten (10) messages. Receivers 202 and Transmitter 212 may be commercially available integrated circuits incorporating both a receiver and transmitter, such as the Programmable Communications Interface (PCI) 2651 manufactured by Signetics of Sunnyvale, Calif. or the SDLC Protocol Controller 8273 manufactured by Intel Corporation of Santa Clara, Calif. These circuits are supplemented with additional buffering using commercially available integrated circuits such as FIFO 33512 manufactured by Fairchild Corporation of Mountain View, Calif. The parallel data outputs of the Receivers 202 are transmitted to a Fault Handler 204, where each received message is analyzed to determine if it is good or faulty. The Fault Hander 204 may be a micro-computer having storage capabilities, a part of a micro-computer, or a special purpose circuit. The Fault Handler 204 performs one or more of the following fault detection checks: 1. Compare the received data value with predetermined limit values to determine if it is reasonable, i.e., has a value between predetermined minimum and maximum values. 2. Compare the received data with the results of other Computers performing the same task, to determine the most probable value, and to identify Computers providing values which differ significantly from the most probable value, 3. Determine if the scheduling information was received in a proper sequence, 4. Determine by means of watch-dog timers if the task execution was completed within a predetermined time period after the execution was started, or 5. Check error detecting codes, determined over other information communicated and included in each message. In addition to performing fault detection checks, the Fault Handler 204 also performs the following functions: 1. Transmits an Error message to the Transmitter 212 when an error is detected. 2. Stores Error messages received from all Computers. 3. Decides if one or more of the Computers is faulty. 4. Discards all messages received from the Computers determined to be faulty. 5. Transmits to the Scheduler 206 error-free messages from non-faulty Computers. 6. Generates a fault display indicating the Computers which have been determined to be faulty. 7. Decides that a Computer is no longer faulty and readmits the Computer previously determined faulty, after the faulty Computer sends good messages for a predetermined period of time, 8. Generates the required input/output sampling commands, and 9. Initiates startup of the Applications Computer and Operations Controller, when the Computer is first turned on or power is returned after a temporary power failure. The function of the Scheduler 206 is to schedule the task to be executed by its own Applications Computer 100. The Scheduler performs the following functions: 1. Keeps track of the status of all assigned tasks and determines which of the tasks are ready for execution, i.e., all the data needed for execution is available. 2. Selects the ready task having the highest task priority for next execution and generates a signal indicative of the task selected, and 3. Unselects the selected task and selects the next highest priority task when it receives a task selection message for the same task from another Computer having a higher assigned Computer priority. The Scheduler 206 may be implemented by means of a micro computer or a part thereof, or with special purpose hardware, depending upon the number of assigned tasks and complexity of the system. The Task Communicator 208 stores the current values of the data required for the execution of each task assigned to the associated Applications Computer 100. The Task Communicator responds to each task signal generated by the Scheduler 206 and makes available to the associated Applications Computer 100 the data required for the execution of the task identified by the task signal. Upon completion of each task, data values produced by the executed task, or an error message if an error was detected in the execution of the task, are sent by the Task Communicator 208 to the Transmitter 212. The Transmitter 212 also receives the Task Complete/Started messages from the Task Communicator, Task Unselected/Selected messages from the Scheduler 206, and Sampling Number and Error messages from the Fault Handler 204. The Transmitter 212 converts the received messages to a serial format which is sent to the other computers via the associated communication link. The data sent over the associated communication link is also received by Receiver 202k over line 214. The messages received by the Fault Handler 204 from Receiver 202k are treated in the same way as any other message received from the other Computers in the system. In this way, the data generated by the associated Applications Computer, required for the execution of a subsequent task, is communicated to and stored in the associated Task Communicator 208 of each Computer 10. The operation of the Operations Controller 200 requires the maintenance of various tables of information. These tables store the recent actions of all Computers, including itself. Table I below lists the various tables used in the system and the elements to which these tables are assigned.
TABLE I
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TABLES USED IN THE SYSTEM
TABLE ELEMENT
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Redundant Data Fault handler 204
Computer Status Fault Handler 204
Sampling Data Fault Handler 204
Fault State Fault handler 204
Scheduling Status Scheduler 206
Task Status Scheduler 206
Data Values Task Communicator 208
Internal Watch-Dog Timer
Task Communicator 208
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MESSAGES The operation of the Fault-Tolerant Multi-Computer System requires that various items of information be transmitted in messages between the multiple Computers in the system. Table II-A is a tabulation of the message types used in the following description of the system. Each message is assumed to comprise a fixed integer number of 8-bit bytes or characters. It is recognized that the various items of information in the messages listed in Table II-A may be presented in various other ways and may use different numbers of bytes and/or bits. The message types given in Table II-A, and their contents, represent a specific format that may be used.
TABLE II-A
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INTER-COMPUTER MESSAGES
Message Type Byte No. Byte Contents
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Task Data Value
1 Message Type
2 Sending Computer
3 Data I.D.
4 Sequence number
5-12 Data Value
13-14 Error Detecting Code
Redundant Data
1 Message Type
Value 2 Sending Computer
3 Data I.D.
4 Sequence Number
5-12 Data Value
13-14 Error Detecting Code
Task Completed/
1 Message Type
Started 2 Sending Computer
3 Completed Task
4 Completed Execution
Number
5 Started Task
6 Started Execution
Number
7-8 Error Detecting Code
Task Unselected/
1 Message Type
Selected 2 Sending Computer
3 Unselected Task
4 Unselected Execution
Number
5 Selected Task
6 Selected Execution
Number
7-8 Error Detecting Code
Error 1 Message Type
2 Sending Computer
3 Faulty Computer
4 Error Type Code
5-6 Null (not used)
7-8 Error Detecting Code
Sampling Number
1 Message Type
2 Sending Computer
3 Sampling Number
4 Starting Flag
5-6 Excluded Bits
7-8 Error Detecting Code
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The first two and last two bytes of all the intercomputer messages listed on Table II-A contain similar information. The first and second bytes of each message identify the message type and sending Computer respectively. The last two bytes are an error detecting code determined and checked over all other bytes of the message. The form of error detecting code used depends upon the communication link protocol selected; a 16 bit Cyclic Redundancy Check (CRC) code or any other code having similar error detection coverage may be used. In addition to these error detecting code bytes, each byte or character may be transmitted with additional bits which are used solely for error detection and/or correction. The error detecting bits and bytes are generated by the Transmitter 212 and checked by the Receivers 202, and are not passed along with the rest of the message for subsequent handling in the Operations Controller. Task Data Value and Redundant Data Value messages differ only in whether or not the data values contained in the messages are redundantly computed by more than one Computer, and thus must be processed by majority voting as discussed hereinafter. Task Data Value Messages and Redundant Data Value Messages are sent by a Computer after completing the execution of a task, in which new values for some task data variables have been computed. A Task Data Value or Redundant Data Value message comprises 14 bytes as indicated on Table II-A. The first byte identifies the message as a Task Data Value or Redundant Data Value message, which contains a new data variable value. The second byte identifies the Computer in which the new data value was computed. The third byte identifies the particular data variable for which a new value was computed by the sending Computer. The fourth byte provides the sequence number of the new data value. The sequence number distinguishes this particular value of the data variable from previous and subsequent values of the same data variable, computed by the same Computer or by any other Computer in the system. The sequence numbers are assigned sequentially (0 to 255 decimal) in circular fashion, i.e., 0 follows 255. The next 8 bytes, bytes 5 through 12, contain the new value for the data variable. The final two bytes contain the error detecting code. The Task Completed/Started message is sent after a task has been completed, and follows the Task Data Value and Redundant Data Value messages from the completed task. The Task Completed/Started message informs the other Computers in the system that the sending Computer has completed the execution of the task identified in Byte 3, and identifies the new task started in Byte 5. Bytes 4 and 6 give the execution numbers of the completed and started tasks, respectively. Each execution number distinguishes the particular execution of a task from previous and subsequent executions of the same task. The execution number corresponds to the sequence number of the data values being used or being computed in the execution of the task. The Task Unselected/Selected message is sent when the Scheduler has selected the next task to be executed by the Applications Computer. Bytes 5 and 6 of the Task Unselected/Selected message identify the newly selected task and its execution number. Bytes 3 and 4 identify the previously selected task and its execution number; this task is now unselected and replaced by the selected task. When a Computer starts executing its selected task, it tentatively selects a known, fixed task, namely the Health Check task, so that a task is always selected. The selection of this Health Check task is not explicitly communicated to other Computers; its selection is assumed by all Computers when a Task Completed/Started message is received. Later, if the Computer selects another task, it sends out a Task Unselected/Selected message. Bytes 3 and 4 identify the unselected Health Check task, and bytes 5 and 6 identify the task selected in place of the Health Check task. If prior to initiating the execution of the selected task, the Operations Controller receives a Task Unselected/Selected message from another Computer having a higher priority, indicating that it also has selected the same task (not Health Check) with the same execution number, the Operations Controller of the lower priority Computer unselects the task and selects a new task. The Operations Controller then generates a Task Unselected/Selected message informing all of the other Operations Controllers that it has unselected the previously selected task and identifying the newly selected task and its execution number. An Error message is generated when an Operations Controller detects an error in a message received from another Computer, or detects an error committed by its own Computer. The first byte identifies the message as an Error message. The second byte identifies the Computer which detected the error, while the third byte identifies the Computer from which the erroneous message originated. The fourth byte contains an error type code which identifies the type of error detected. The fifth and sixth bytes contain null codes (not used). As previously indicated, bytes 7 and 8 contain an error detecting code. It should be noted that null bytes are included in some messages so that most message types are the same length and thus simplify message handling. Alternately, these null bytes could be omitted from the messages. A Sampling Number message is sent by each Computer at the end of each sampling period. The first byte identifies the message type, and the second byte identifies the Computer sending the message. The third byte provides the new sampling number, which distinguishes the present sampling period from previous and subsequent sampling periods. Like the data value sequence numbers and task execution numbers, the sampling numbers are assigned sequentially (from 0 to 255 decimal) in circular fashion, i.e., 0 follows 255. The fourth byte is a starting flag signifying if the sending Computer is starting or restarting operation. The fifth and sixth bytes contain one bit for each possible Computer in the system, and indicate if the Computer associated with each bit is currently excluded by the sending Computer or not. The seventh and eighth bytes contain the error detecting code. As previously stated, these messages are transmitted between the multiple Computers of the system. The same messages are also transmitted between some subsystems of the Operations Controller. Within one Operations Controller, not all bytes of a message may be transmitted. In particular, the error detecting code bytes are not communicated beyond the receivers. Within each Operations Controller, additional internal messages are used to communicate information between the subsystems or modules of the Operations Controller. These messages are listed in Table II-B and will be discussed in conjunction with the modules that produce and/or use such internal messages.
TABLE II-B
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INTERNAL MESSAGES
MESSAGE TYPE BYTE NO.
BYTE CONTENTS
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EXCLUDE COMPUTER
1 MESSAGE TYPE
2 EXCLUDED COMPUTER
3-4 EXCLUDED BITS
INITIATE SPECIAL TASKS
1 MESSAGE TYPE
2 TASK TYPE
3 EXECUTION NUMBER
RESTART 1 MESSAGE TYPE
DISPATCH TASK 1 MESSAGE TYPE
2 TASK
3 EXECUTION NUMBER
RELEASE TASK 1 MESSAGE TYPE
2 COMPLETED TASK
3 COMPLETED EXECUTION
NUMBER
4 STARTED TASK
5 STARTED EXECUTION
NUMBER
TASK DONE 1 MESSAGE TYPE
2 TASK
RECORD ERROR 1 MESSAGE TYPE
2 NEW FAULTY COMPUTER
3 ERROR INDICATOR
TASK INPUT 1 MESSAGE TYPE
2-3 TASK ADDRESS
THE FOLLOWING SET OF BYTES ARE REPEATED FOR
EACH DATA VARIABLE USED AS A TASK INPUT. SEE
TASK COMMUNICATOR DISCUSSION FOR MORE DETAIL.
4-11 INPUT VALUE
12 ACTUAL DELAY INTEGER
TASK OUTPUT 1 MESSAGE TYPE
THE FOLLOWING SET OF BYTES ARE REPEATED FOR
EACH DATA VARIABLE COMPUTED AS A TASK OUTPUT.
SEE TASK COMMUNICATOR DISCUSSION FOR MORE DETAIL.
2 DATA I.D.
3 REDUNDANT DATA
4-11 OUTPUT VALUE
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FAULT HANDLER The details of the Fault Handler 204 are shown in FIG. 5. The Fault Handler 204 comprises a Message Format Checker 216, Reasonable Limits Checker 218, Redundant Value Voter 220, Message Sequence Checker 222, Execution Time Checker 224, Synchronizer 226, Fault Tolerator 228, Fault Status Display Panel 230, and Start Fault Handler Module 231. The Message Format Checker 216 receives the outputs from the Receivers 202a through 202k, merges the messages received into a single stream of data, and performs selected message format checks. The Message Format Checker 216 checks each received message to determine if the message type is valid, if the sending Computer identified in the message corresponds to the Receiver that received the message, and if the error detecting code is correct (checked in conjunction with the Receivers). A Record Error message is sent to a Fault Tolerator 228 when the message type is not valid, when the Computer identified in the message does not correspond to the Receiver receiving the message, or when an error is detected through use of the error detecting code. The error-free messages passed by the Message Format Checker are received by one of a plurality of error detection modules or checkers, such as the Reasonable Limits Checker 218, Redundant Value Voter 220, Message Sequence Checker 222 or Execution Time Checker 224. The error detection module to which a message is communicated is determined by the message type; each message is usually further checked for errors by only one of the error detection modules. The Reasonable Limits Checker 218 checks if the data value of a Task Data Value message is between predetermined minimum and maximum limits. It generates a Record Error message when the data value is outside the predetermined limits. Error-free Task Data Value messages are forwarded to the Fault Tolerator 228. The Redundant Value Voter 220 receives the Redundant Data Value messages and generates a "voted data value" when a predetermined number of Redundant Data Value messages are received having the same sequence number and same data value for a given task data variable. The "voted data value" is the value of that data variable that will be used in the execution of any subsequent task requiring this data. The "voted data value" obtained is communicated in a Redundant Data Value message forwarded to the Task Communicator via the Fault Tolerator and Scheduler. After the "voted data value" is determined, a Record Error message is generated for any received message having a data value which does not agree with the "voted data value" for that sequence number of that data variable. The Execution Time checker 224 comprises a plurality of "watch-dog timers", one for each Computer 10. Each "watch-dog timer" is started in response to a Task Completed/Started message received from the associated Computer. The "watch-dog timer" monitors the execution time of the task started by that Computer. A Record Error message is generated when the "watch-dog timer" expires before a subsequent Task Completed/Started message is received, which indicates that the previously started task has been completed and another task has been started. Expiration of the watch-dog timer indicates that the task was improperly executed. The Task Completed/Started messages are always forwarded to the Message Sequence Checker 222. The Message Sequence Checker 222 checks that the Task Completed/Started and Task Unselected/Selected messages are received from each Computer in a correct sequential order. For example, a Task Completed/Started message, indicating that a particular task has been started, should have been preceded by a Task Unselected/Selected message from the same Computer indicating that the same task with the same execution number had been selected. In a like manner, a Task Completed/Started message should be preceded by a Task Completed/Started message from the same Computer in which the started task and execution number of the first message are the same as the completed task and execution number in the subsequent message. If the task numbers or execution numbers do not agree, a Record Error message is generated. Error-free Task Unselected/Selected and Task Completed/Started messages are forwarded to the Fault Tolerator. Each Record Error message generated by the various fault detection modules is sent to the Fault Tolerator 228. Each Record Error message includes the identity of the Computer 10 which sent the message, and an identification of the particular error detected. The error-free Sampling Number messages, after passing through the Message Format Checker, are received by the Synchronizer 226. The Synchronizer generates "initiate input/output tasks" messages in synchronization with the Synchronizer modules in other Computers in the system. At the end of each sampling period, the Synchronizer generates a Sampling Number message containing the current sampling number of the associated Computer. The Sampling Number messages are sent to all of the Computers in the system via the Transmitter 212, and are used to synchronize operations of like Synchronizers 226 in the other Computers 10. In the event the Synchronizer's own Computer is starting after a momentary power interruption or other failure, the Synchronizer will also generate an "initiate start-up task" message and "initiate fail-safe task" messages. The "initiate input/output tasks", "initiate start-up task" and "initiate fail-safe task" messages are internal messages used by the Synchronizer's own Operations Controller. These messages are sent to the Scheduler 206 and are not communicated to the other Computers. Each of these messages is a particular version of the Initiate Special Tasks message listed in Table II-B. The "initiate input/output tasks" message is sent to the Scheduler 206 to initiate scheduling of the input/output tasks assigned to its own Computer, in synchronization with all of the other Computers in the system. These input/output tasks perform sampling of system inputs and outputs, where the sampling must be synchronized between Computers. The sampling number generated by the Synchronizer becomes the execution number of the input/output tasks. The "initiate start-up task" message initiates scheduling of the system start-up task(s) assigned to its own Computer, in synchronization with all the other Computers in the system. These start-up tasks perform any functions needed to properly start the operation of the other application tasks. Finally, the "initiate fail-safe task" message initiates scheduling of the fail-safe task or tasks assigned to the Synchronizer's own Computer. The fail-safe tasks send out "safe" data values during a start or restart, to all actuators and displays connected to the Computer. In addition, the Synchronizer 226 and Fault Tolerator 228 generate Restart messages when operation of the associated Operations Controller needs to be restarted. The Restart messages initiate start-up procedures within the Scheduler 206, Task Communicator 208, and Fault Handler 204, which initializes the variable data used within those units. Within the Fault Handler, the Restart messages are sent to the Start Fault Handler Module 231, which initialize variable data within the checkers and the Fault Tolerator 228. The error-free Task Data Value messages, the Redundant Data Value messages which convey a "voted data value", the Task Completed/Started messages, the Task Unselected/Selected messages, and the Error messages are received by the Fault Tolerator 228. The Fault Tolerator also receives the Record Error messages generated by the Message Format Checker 216, Reasonable Limits Checker 218, Redundant Value Voter 220, Execution Time Checker 224, Message Sequence Checker 222, and Synchronizer 226. The function of the Fault Tolerator 228 is to pass on to the Scheduler 206 only those error-free messages received from Computers which are not deemed to be faulty. The Fault Tolerator maintains, for each Computer in the system, an indication of whether or not that Computer is currently deemed to be faulty. Whenever an error-free message is received from a Computer which is not considered faulty, that message is forwarded to the Scheduler. Messages from faulty Computers and erroneous messages are discarded. These actions are performed for Task Data Value, Task Completed/Started, and Task Unselected/Selected messages. Redundant Data Value messages which convey a "voted data value" are always forwarded to the Scheduler, even though the sending Computer may be deemed faulty. Error and Record Error messages are used and not forwarded by the Fault Tolerator. When a Record Error message is received from the Message Format Checker 216, Reasonable Limits Checker 218, Redundant Value Voter 220, Message Sequence Checker 222, Execution Time Checker 224, or Synchronizer 226, the Computer which sent the erroneous message is recorded as being faulty, and an Error message is generated identifying the Computer which sent the message. The Error message is sent out to all Computers via the Transmitter 212. An internal Exclude Computer message identifying the faulty Computer is sent to the Scheduler 206. The Fault Tolerator 228 also responds to the Error messages received from other Computers, and will conclude that a Computer is faulty when a predetermined number of Computers have sent Error messages identifying that particular Computer as faulty, even though an error has not been detected by an error detection module in its own Computer. As before, when the Fault Tolerator decides that a Computer is now faulty, it sends an Exclude Computer message to the Scheduler. If the number of Computers sending Error messages identifying a particular Computer as faulty is less than the predetermined number, the Computer is assumed to be healthy since the received Error message(s) may be the result of malfunctions in the Computers sending the Error messages or their associated communication links. The Computer or Computers which sent these Error messages will discard messages from the Computer deemed faulty; however, the remaining Computers will treat that same Computer as healthy and will accept the messages as if no Error messages were received. In all cases where one of the Computer's own checkers or the Synchronizer send an internal Record Error message indicating a detected error or fault, that Computer will deem the Computer faulty and will discard all messages received from that Computer; this continues until it is concluded that the fault was temporary and the faulty Computer has recovered. Although the Fault Tolerator 228 will discard messages received from Computers deemed to be faulty, the Message Format Checker 216, Reasonable Limits Checker 218, Redundant Value Voter 220, Message Sequence Checker 222, Execution Time Checker 224, and Synchronizer 226 will continue to check each message received from all Computers. The Fault Tolerator continues to monitor the messages received from the Computer deemed to be faulty. The Fault Tolerator will decided that a Computer is no longer faulty when, during a predetermined time period, its own checkers do not detect an error and simultaneously the number of Computers generating Error messages identifying the faulty Computer is less than the required predetermined number. When it is determined that a Computer is no longer faulty, the Fault Tolerator will generate an "Exclude Computer" message which shows that the previously excluded Computer is no longer excluded. The "Exclude Computer" message is communicated to the Scheduler 206, where it cancels the current exclusion status of the identified Computer, and the previously excluded Computer is thus readmitted to full participation in the system. The Fault Tolerator 228 further generates signals activating a Fault Status Display Panel 230 identifying the Computers deemed to be faulty or excluded. The Fault Status Display Panel 230 may be an externally mounted display panel readily visible to the operator, and/or may be placed inside the Computer cabinet adjacent to the particular Operations Controller hardware. Each Computer in the system has its own display panel, and each display panel has at least two lamps or indicators for each Computer in the system. Both of the lamps are activated when the corresponding Computer has been deemed to be faulty by the Operations Controller associated with the particular display, and the faulty Computer is presently excluded from the system. The first lamp is turned "off" when the Computer is readmitted; however, the second lamp is left on indicating that the Computer had previously been excluded. The in-cabinet mounting of the display panel is desirable, since the display will be conveniently available to service personnel during maintenance or servicing of the system. The operation of the Fault Handler 204 is as follows: Messages from the Computers in the Fault-Tolerant Multi-Computer System are received by the individual Receivers 202 connected to the respective communication links. The Receivers 202 check the error detection code, the length of the message, etc. The received message is then forwarded to the Message Format Checker 216, along with information identifying the Receiver which received the message. If an error is detected by a Receiver, information identifying the type of error detected is communicated to the Message Format Checker 216. Because the messages are randomly received at the individual Receivers 202, and may be received at a rate too fast for immediate processing by the Message Format Checker 216, the messages are placed in a temporary storage buffer associated with each Receiver, until they can be checked by the Message Format Checker. Each temporary storage buffer is able to store about ten messages at any time. Each received message contains additional bytes or bits of information, such as the message error detecting code, start of message and end of message codes, and character error detecting/correcting codes, which are only used by the Receivers. These additional bits of information are stripped from the message before it is forwarded to the buffer and Message Format Checker 216. The Message Format Checker 216 interrogates the buffers associated with each Receiver 202 in a cyclical manner, and checks each received message. It checks if an error was detected by the Receiver, if the message type is a valid message type, and if the Receiver which received the message is associated with the particular Computer which originated the message. If the Message Format Checker detects an error, it sends a Record Error message to the Fault Tolerator 228. If no error is detected, the received message is forwarded to the appropriate Fault Handler module. Subsequent operation of the Fault Handler depends upon the message type. Operation will thus be discussed for each message type. Error-free Task Data Value messages, passed by the Message Format Checker 216, are forwarded to the Reasonable Limits Checker 218. The Reasonable Limits Checker checks each Task Data Value message and forwards it to the Fault Tolerator 228 if no error is detected. The Fault Tolerator checks if the Computer which sent the message is currently considered to be faulty. If that Computer is not faulty, the Task Data Value message is forwarded to the Scheduler 206; otherwise, the message is discarded. If the Reasonable Limits Checker detects an error, it sends a Record Error message to the Fault Tolerator 228. Each error-free Redundant Data Value message, passed by the Message Format Checker 216, is forwarded to the Redundant Value Voter 220. The Redundant Value Voter compares the value of the data variable contained in the received message with the values of that data variable contained in previously received Redundant Data Value messages. If the data value contained in the received Redundant Data Value message agrees with the values in a predetermined number of previously received Redundant Data Value messages, a "voted data value" is obtained. The Redundant Data Value message containing the "voted data value" is forwarded to the Scheduler 206 through the Fault Tolerator 228. When a "voted data value" is obtained, and the value contained in a previously received Redundant Data Value message disagrees with the "voted data value" just obtained, a Record Error message is also transmitted to the Fault Tolerator identifying the Computer which sent the disagreeing data value. If the Redundant Data Value message does not produce a "voted data value", the Redundant Data Value message is discarded. If after a "voted data value" is obtained, the value of the data variable contained in a subsequent Redundant Data Value message disagrees with the "voted data value", a Record Error message is transmitted to the Fault Tolerator 228. Each error-free Task Unselected/Selected message, passed by the Message Format Checker 216, is forwarded to the Message Sequence Checker 222. The Message Sequence Checker checks the message for scheduling sequence errors, and forwards it to the Fault Tolerator 228 if no errors are detected. The Fault Tolerator checks if the Computer which sent the message is currently considered to be faulty. If that Computer is not faulty, the error free Task Unselected/Selected message is forwarded to the Scheduler 206; otherwise, the message is discarded. If the Sequence Checker detects an error, it sends a Record Error message to the Fault Tolerator 228. Each error-free Task Completed/Started message, passed by the Message Format Checker 216, is forwarded to the Execution Time Checker 224. The Execution Time Checker starts a watch-dog timer and forwards the message to the Message Sequence Checker 222. The Message Sequence Checker checks each message and forwards it to the Fault Tolerator 228, if no error is detected. The Fault Tolerator checks if the Computer which sent the message is currently considered to be faulty. If that Computer is not faulty, the Task Completed/Started message is forwarded to the Scheduler 206; otherwise, the message is discarded. If the watch-dog timer for a Computer expires before it is restarted by a subsequent Task Completed/Started message, the Execution Time Checker 224 sends a Record Error message to the Fault Tolerator 228. If the Message Sequence Checker detects an error, it sends a Record Error message to the Fault Tolerator. Each error-free Sampling Number message, passed by the Message Format Checker 216, is forwarded to the Synchronizer 226. The Synchronizer compares the Sampling Number messages. Sampling Number messages are not passed on to the Fault Tolerator 228. However, the Synchronizer periodically generates a new Sampling Number message, sending it to the Transmitter 212. The Synchronizer compares the sampling number contained in each received Sampling Number message with the sampling numbers contained in previously received Sampling Number messages and with the previously determined "voted sampling number". If the sampling number contained in the received Sampling Number message agrees with a predetermined number of sampling numbers contained in previously received Sampling Number messages, a new "voted sampling number" is obtained and an "initiate input/output tasks" message is sent to the Scheduler 206. If the Sampling Number message produces a new "voted sampling number", and if the sampling number given in a previously received Sampling Number message disagrees with the "voted sampling number" just obtained, a Record Error message is sent to the Fault Tolerator 228. Each error-free Error message is forwarded directly to the Fault Tolerator 228 from the Message Format Checker 216. The Fault Tolerator compares this message with previously received Error messages. If the Fault Tolerator decides that a particular Computer is faulty, based upon a predetermined number of Error messages naming that Computer, the Fault Tolerator will thereafter consider that Computer to be faulty. If that Computer was not previously considered to be faulty, the Fault Tolerator sends an internal Exclude Computer message to the Scheduler 206. The Fault Tolerator also activates the lamps in the Fault Status Display Panel 230 associated with the Computer which is now considered to be faulty. The Display Panel indicates those Computers which are presently excluded, as well as any Computer which was at one time excluded but has subsequently been readmitted into the system. When a Record Error message is received by the Fault Tolerator 228, from the Message Format Checker 216, Reasonable Limits Checker 218, Redundant Value Voter 220, Message Sequence Checker 222, Execution Timer Checker 224, or Synchronizer 226, the Fault Tolerator thereafter considers the Computer identified in the Record Error message to be faulty. If a specified time interval has passed since an Error message was sent regarding that Computer, an Error message is sent to the Transmitter 212 for transmission to all Computers. If that Computer was not previously considered to be faulty, the Fault Tolerator sends an Exclude Computer message to the Scheduler 206. The Fault Tolerator also activates the lamps in the Fault Status Display Panel 230 associated with the Computer which is now considered to be faulty. When the Fault Tolerator excludes a Computer, it checks for certain abnormal conditions. If the excluded Computer is the Fault Tolerator's own Computer, it restarts its own Computer. Similarly, if the number of excluded Computers exceeds a predetermined number, it restarts its own Computer. The number of excluded Computers could exceed the predetermined when its own Computer is faulty, or when some common fault produces errors in many Computers. To restart its own Computer, the Fault Tolerator sends a Restart message to the Start Fault Tolerator Module 231 and to the Scheduler 206. The Fault Tolerator also monitors the elapsed time since a Computer was last deemed to be faulty, in response to either an internal Record Error message or matching Error messages received from other Computers. When a faulty (excluded) Computer transmits error-free messages for a predetermined length of time, the Fault Tolerator reverses the excluded status for that Computer and readmits that Computer into active participation in the system. When such a decision is made, the Fault Tolerator sends an Exclude Computer message to the Scheduler 206. The Exclude Computer message shows the readmitted Computer as not (presently) excluded. The Fault Tolerator also deactivates the presently excluded lamp in the Fault Status Display Panel associated with the Computer no longer excluded. However, it leaves on the lamp indicating that the Computer was excluded at one time. When the Computer is starting after being turned on, or restarting after a momentary power failure or interruption, the Synchronizer 228 starts its sampling period timer, and transmits an internal Restart message to the Start Fault Handler Module 231 and the Scheduler 206. The Start Fault Handler Module initilizes internal data for the Fault Tolerator 228, Redundant Value Voter 220, Message Sequence Checker 222, and Execution Time Checker 224. The Synchronizer then generates an internal "initiate fail-safe task" message which is transmitted to the Scheduler 206. The Synchronizer continues to generate the "initiate fail-safe task" message at periodic intervals until a predetermined number of Computers are operating and their sampling period timers and sampling numbers are synchronized. When the sampling period timer expires, the Synchronizer restarts the sampling period timer and generates a Sampling Number message containing its current sampling number. This message is sent via the Tansmitter 212 to all of the Computers in the system. Concurrently, the other Computers are generating similar Sampling Number messages, whether they are also starting, or are operating normally. The Synchronizer accepts the Sampling Number messages received from all Computers and attempts to determine the current sampling number of the system. The Sampling number is determined by a voting process, i.e, a sampling number on which at least a predetermined number of Computers agree. Once this "voted sampling number" is determined, the Synchronizer uses the "voted sampling number" as its own sampling number and synchronizes its sampling period timer with all the other sampling period timers in the system. When the "voted sampling number" is first obtained and the sampling period timer is synchronized, the Synchronizer sends an internal "initiate start-up task" message to the Scheduler 206. The "initiate start-up task" message causes the Scheduler 206 to initiate scheduling of special start-up task(s) assigned to the Computer. The Synchronizer also generates an internal "initiate input/output tasks" message when a "voted sampling number" is obtained, which is sent to the Scheduler 206. As previously indicated, the "initiate input/output tasks" message initiates scheduling of the input/output tasks which sample the system inputs and outputs. Sampling is done by the input/output tasks using the Input/Output Network 108 of the Applications Computer, to receive input data from the Sensors and Manual Controls 14 and to output data to the Actuators and Displays as shown on FIG. 3. The execution number used for the initiated input/output tasks is the current sampling number of the Synchronizer. The Computer thereafter receives messages from the other Computers, and new input data from the sensors and manual controls, and assumes normal active participation in the Fault Tolerant Multi-Computer System. The preferred implementation of the Fault Handler is one, or possibly several, microprocessors having adequate storage and computational capabilities, such as the 8080A Microprocessor manufactured by the Intel Corporation of Santa Clara, Calif. or any other microcomputer of similar type. However, if desired, the Fault Handler may be made from commercially available discrete electronic components, as shall be shown by way of example in the following description of the individual modules of the Fault Handler. The individual modules of the Fault Handler will be described in the following sections by means of Psuedo Code computer program listings. Psuedo Code is used for the program listings because it is not dedicated to a particular microprocessor or type of microprocessor, and is universally applicable to different types of computers and computer languages. A programmer having ordinary skills in the art would be able to translate the presented Psuedo Code program listings into actual program listings for a particular computer. MESSAGE FORMAT CHECKER The Psuedo Code program for the Message Format Checker 216 is given in Table III-A and a comparable flow diagram is shown on FIG. 6. The Message Format Checker module checks all messages received from all Computers 10, via the Receivers 202. The portions of the received message that are checked by the Message Format Checker are the first byte of the message which identifies the message type, the second byte which identifies the Computer sending the message, and the special bits generated by the Receiver which identify the Computer connected to that Receiver and any errors detected by the Receiver. As previously discussed, each Receiver 202 receives messages from a particular Computer and the Operations Controller has a plurality of Receivers 202, each receiving only the messages sent by a specified Computer in the system. In the given example, it is assumed that a special byte generated by the Receiver 202 is identical to the expected second byte of the message, which identifies the Computer which sent the message.
TABLE III-A
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MESSAGE FORMAT CHECKER
______________________________________
/* IF ERROR DETECTED BY RECEIVER */
IF ERROR DETECTED BITS NOT = 0
THEN
ERROR INDICATOR =
FUNCTION OF (ERROR DETECTED BITS)
ELSE /*IF MESSAGE TYPE CODE NOT VALID*/
IF MESSAGE TYPE > MAXIMUM TYPE
ORIF MESSAGE TYPE = 0
THEN
ERROR INDICATOR = MESSAGE TYPE ERROR
ELSE /*CHECK SENDING COMPUTER CODE*/
IF SENDING COMPUTER NOT = RECEIVER
THEN
ERROR INDICATOR =
SENDING COMPUTER CODE ERROR
ELSE
ERROR INDICATOR = 0
ENDIF
ENDIF
ENDIF
IF ERROR
INDICATOR NOT = 0 /* IF ERROR WAS DETECTED*/
THEN
CALL: SEND MESSAGE TO FALUT TOLERATOR
INPUT DATA:
MESSAGE TYPE = RECORD ERROR TYPE
NEW FAULTY COMPUTER = RECEIVER
ERROR INDICATOR = ERROR INDICATOR
OUTPUT DATA: NONE
ELSE /*FORWARD RECEIVED MESSAGE*/
/*CASE OF MESSAGE TYPE*/
IF MESSAGE TYPE = TASK DATA VALUE TYPE
THEN
CALL: SEND MESSAGE TO
REASONABLE LIMITS CHECKER
INPUT DATA: MESSAGE = RECEIVED MESSAGE
OUTPUT DATA: NONE
ELSE IF MESSAGE TYPE =
REDUNDANT DATA VALUE
TYPE
THEN
CALL: SEND MESSAGE TO REDUNDANT VALUE
VOTER
INPUT DATA: MESSAGE = RECEIVED MESSAGE
OUTPUT DATA: NONE
ELSE IF MESSAGE TYPE =
TASK COMPLETED/STARTED TYPE
THEN
CALL: SEND MESSAGE TO EXECUTION TIME
CHECKER
INPUT DATA: MESSAGE = RECEIVED MESSAGE
OUTPUT DATA: NONE
ELSE IF MESSAGE TYPE =
TASK UNSELECTED/SELECTED TYPE
THEN
CALL: SEND MESSAGE TO
MESSAGE SEQUENCE CHECKER
INPUT DATA: MESSAGE = RECEIVED MESSAGE
OUTPUT DATA: NONE
ELSE IF MESSAGE TYPE = SAMPLING NUMBER TYPE
THEN
CALL: SEND MESSAGE TO SYNCHRONIZER
INPUT DATA: MESSAGE = RECEIVED MESSAGE
OUTPUT DATA: NONE
ELSE /*MESSAGE TYPE = ERROR MESSAGE TYPE*/
CALL: SEND MESSAGE TO FAULT TOLERATOR
INPUT DATA: MESSAGE = RECEIVED MESSAGE
OUTPUT DATA: NONE
ENDIF ENDIF ENDIF ENDIF ENDIF
/*END CASE*/
ENDIF
RETURN
END:
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Referring to the Psuedo Code program in Table III-A and flow diagram of FIG. 6, the Message Format Checker 216 first checks if an error was detected by the Receiver, as shown in the flow diagram by block 232. The symbols "/*" and "*/" are used in the first line of Table III-A and thereafter to indicate that the enclosed text is a comment in the Psuedo Code and not part of the actual code. The enclosed text is only a comment explaining the following line. For example, the enclosed text on line one of Table III-A identifies the "ERROR DETECTED BITS" of line two as the error detected signals generated by the Receiver. If the error detected bits obtained from the Receiver are not equal to zero (0), where zero values of the error detected bits are indicative of no error detected by the Receiver, then a Record Error message is generated as indicated by block 234, identifying that an error was detected by the Receiver and the checking is terminated (third ENDIF). The error indicator code designating the type of error detected is generated as a function of the error detected bits obtained from the receiver. If no error was detected by the Receiver, the Message Format Checker proceeds (ELSE) to check the message type code as indicated by block 236. If the message type code is a number greater than the constant maximum inter-computer message type number used in the system, or if it is equal to zero as checked by block 237, then a Record Error message is generated as indicated by block 238, and the checking is terminated (second ENDIF). The error indicator code is set equal to the fixed value which identifies the error as a message type error. If the message type code is not equal to zero (0) and is not greater than the maximum type number, the program proceeds (ELSE) to compare the sending Computer byte of the message with the Computer code generated by the Receiver, as indicated by block 240. If the sending Computer code contained in the message does not agree with the Computer code generated by the Receiver, a Record Error message is generated as indicated by block 242 and the checking is terminated (first ENDIF). The error indicator is set equal to the fixed value which identifies the error as a sending Computer code error. If no error in the sending Computer code is found, the error indicator is set to zero (0) as indicated by block 244, and the checking is ended. The error indicator value of zero indicates that no error was detected. In the Psuedo Code program Table III-A and flow diagram FIG. 6, a Record Error message is "generated" when an error is detected by making the error indicator non-zero. Following the checking (the third ENDIF), the error indicator is tested to determine if a Record Error message must be sent, as indicated by block 233. If the error indicator is not zero, (THEN) a Record Error message is sent to the Fault Tolerator, as indicated by block 235. If the error indicator is zero (ELSE), the received message must be forwarded to the proper checker module. The message type code is then tested to determine the message type. If the message type is a Task Data Value message as tested by block 259, the received message is sent to the Reasonable Limits Checker 218, as indicated by block 239. If the message type is a Redundant Data Value message as tested by block 241, the received message is sent to the Redundant Value Voter 220, as indicated by block 243. If the message type is a Task Completed/Started messag | ||||||