Abstract:
Safety level error detection comparable to that provided by redundant wired safety relays is obtained on a backplane of a programmable logic controller or the like by a combination of error detection methods that in sum provide the requisite level of error detection required without necessitating a particular hardware requirement or duplicative message transmissions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of 60/373,592 filed Apr. 18, 2002, and is a CIP of 09/666,438 filed Sep. 21, 2000, now U.S. Pat. No. 6,631,476 which claims the benefit of 60/171,439 filed Apr. 22, 1999. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     BACKGROUND OF THE INVENTION 
     The present invention relates to industrial controllers used for real-time control of industrial processes, and in particular to high-reliability industrial controllers appropriate for use in devices intended to protect human life and health. “High reliability” refers generally to systems that guard against the propagation of erroneous data or signals by detecting error or fault conditions and signaling their occurrence and/or entering into a predetermined fault state. High reliability systems may be distinguished from high availability systems, however, the present invention may be useful in both such systems and therefore, as used herein, high reliability should not be considered to exclude high availability systems. 
     Industrial controllers are special purpose computers used in controlling industrial processes. Under the direction of a stored control program, an industrial controller examines a series of inputs reflecting the status of the controlled process and changes a series of outputs controlling the industrial process. The inputs and outputs may be binary, that is, on or off, or analog, providing a value within a continuous range. The inputs may be obtained from sensors attached to the controlled equipment and the outputs may be signals to actuators on the controlled equipment. 
     “Safety systems” are systems intended to ensure the safety of humans working in the environment of an industrial process. Such systems may include the electronics associated with emergency stop buttons, interlock switches, and machine lockouts. Traditionally, safety systems have been implemented by a set of circuits wholly separate from the industrial control system used to control the industrial process with which the safety system is associated. Such safety systems are “hard-wired” from switches and relays, some of which may be specialized “safety relays” allowing comparison of redundant signals and providing internal checking of conditions such as welded or stuck contacts. A hallmark of such safety systems is the use of at least two redundant wire sets, each carrying the same signals so that the signals on the wires may be compared to ensure that one signal was not lost or corrupted. In this regard, safety systems may use switches with dual contacts and actuators that are wired to engage only if multiple uncorrupted signals are received. 
     Hard-wired safety systems using duplicated wiring have proven cumbersome as the complexity of industrial processes has increased. This is in part because of the cost of installing the components and redundant wiring and in part because of the difficulty of troubleshooting and maintaining the “program” implemented by the safety system in which the logic can only be changed by rewiring physical relays and switches. 
     For this reason, there is considerable interest in implementing safety systems using industrial controllers. Such controllers are easier to program and have reduced installation costs because of their use of a high-speed serial communication network eliminating long runs of point-to-point wiring. 
     Standard protocols for high-speed serial communication networks commonly used in industrial control are not sufficiently reliable for safety systems. For this reason, efforts have been undertaken to develop a “safety network protocol” for high-speed serial communication providing greater certainty in the transmission of data. Unfortunately, if these new safety network protocols are adopted, existing industrial controller hardware (e.g., network interface cards implementing standard network protocols) may be unusable, imposing high costs on existing and new factories. Such costs may detrimentally postpone wide scale adoption of advanced safety technology. 
     One solution to this problem is described in pending U.S. application Ser. No. 09/666,438 filed Sep. 21, 2000, now U.S. Pat. No. 6,631,476 and entitled “Safety Network for Industrial Controller Providing Redundant Connections on Single Media”, assigned to the same assignee as the present invention and hereby incorporated by reference, in which a safety network protocol is described that may be “encapsulated” in a standard network protocol, allowing standard networks and network hardware to be used. In such a safety network, duplicate messages are transmitted to imitate the duplicate wiring used in a standard “hard-wired” safety system, providing two paths of data communication. The software implementation of redundant physical wiring is both less costly and more flexible. 
     The implementation of a safety network on a backplane of the industrial controller as an extension to a network is described further in U.S. application Ser. No. 10/034,387 filed Dec. 27, 2001, now U.S. Pat. No. 6,549,034 entitled “Programmable Logic Controller for Safety Systems With Reduced Cross-Wiring” also assigned to the same assignee as the present invention and hereby incorporated by reference. In both of these safety network protocol systems, multiple “connected” messages serve as a replacement to separate physical wires used to carry redundant safety information, while allowing flexible reconfiguration without the need for actual rewiring of the safety system. 
     BRIEF SUMMARY OF THE INVENTION 
     The present inventor has recognized that a safety protocol using a standard network or backplane can provide safety operation with a wide variety of error detection protocols whose error detection abilities can be traded off against each other to provide the desired level of error detection. 
     This ability to mix and match error detection methods allows, for example, redundant information in a safety system to be transmitted with a single message much simplifying the design of a programmable logic controller intended to be assembled out of a variety of modular components that must interoperate. This ability to use a single message, in part, recognizes that the network integrity advantages gained by independent wiring can be realized through other message integrity enhancing steps. In part, this ability to use a single message also recognizes that redundant messages provide an increase in availability of the network that is independent of and unessential to safety. 
     Specifically, in one embodiment, the present invention provides a backplane system for interconnecting components of an industrial controller for safety operation comprising and having an input component providing redundant input signals on a first and second dedicated wire conductor and an output component receiving redundant output signals on a first and second dedicated wire conductor. A backplane is provided having at least one conductor for conducting data as digital messages and at least two industrial controller components having connectors allowing connection of the industrial components to the conductor of the backplane and having terminals connectable to receive the redundant signals of the input and output devices on the dedicated wire conductors and to communicate between the input and output devices using at least one message for each set of redundant signals. A safety protocol means enforces a communications protocol over messages communicated over the backplane among industrial controller components to provide an error rate in the transmission of messages between the input device and output device using the backplane no greater than the error rate obtained by direct connection of the dedicated wire conductors of the input and output components directly between the input and output components. 
     Thus it is one object of the invention to provide a protocol-based rather than hardware-based safety system. 
     The industrial controller components may communicate between the input and output devices using only one message for each set of redundant signals. 
     Thus, it is another object of the invention to provide the benefits of redundant signals in reducing errors without the need for the complexity of redundant digital messages on the backplane. 
     More generally, the invention allows an arbitrary level of safety to be obtained by combining a variety of techniques. That is, the invention provides a backplane system for interconnecting components of an industrial controller for safety operation made up of a backplane having at least one conductor for conducting data as digital messages and at least two industrial controller components having connectors allowing connection of the industrial components to the conductor of the backplane for the communication of messages thereon. A safety protocol mechanism which may be implemented in software or hardware, enforces a communications protocol on messages communicated over the backplane among industrial controller components to provide an error indication upon any of: (i) loss of a message transmitted from one component to a second component; (ii) corruption of a message transmitted from one component to a second component; and; (iii) misdirection of a message transmitted from one component, intended for a second component, to a third component. When any predetermined combination of these error indications occurs, the safety protocol means places the industrial controller in a predetermined safety state. 
     Thus, it is an object of the invention to provide safety system error detection using a variety of different protocols suitable for implementation on a backplane independent of specific hardware. 
     The safety protocol may further provide an indication of (iv) repetition of a message previously transmitted from one component to a second component; (v) insertion of a message not transmitted from any component to a second component; and (vi) a change in sequence of multiple messages transmitted from a first component before receipt by a second component. 
     Thus, it is another object of the invention to address a secondary source of errors whose detection can contribute to the necessary overall error detecting level required. 
     The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made to the claims herein for interpreting the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a simplified industrial controller using a standard communication protocol on a backplane linking a central controller with input and output circuits and with a remote configuration terminal, such as may be used in the present invention; 
         FIG. 2  is a schematic block diagram of the control system of  FIG. 1  showing redundant wiring from an input switch to the input circuit of  FIG. 1 , the input circuits having redundant components such as may process the signals from the input switch to send signals over the backplane to the controller of  FIG. 1 , the controller having redundant processors to send signals over the backplane to the output circuit of  FIG. 1 , the output circuit having redundant components to provide outputs to an actuator; 
         FIG. 3  is a fragmentary view similar to  FIG. 2  showing an alternative configuration of the input circuit of  FIG. 2  using conventional control input circuits without redundant components; 
         FIG. 4  is a fragmentary view similar to  FIG. 2  showing an alternative configuration of the output circuit of  FIG. 2  using conventional control output circuits without redundant components; 
         FIG. 5  is a representational view of the dual communication protocols provided by the present invention in which data is first encoded with a safety protocol and then with a network protocol to be compatible with the backplane; 
         FIG. 6  is a schematic representation of a data word transmitted over the backplane showing the embedding of safety formatting data with I/O data within the formatting provided by the backplane; 
         FIG. 7  is a graphical representation having time on the vertical axis and distance along the network on the horizontal axis showing transmission of configuration messages to the input circuit, the controller, and the output circuit, forming the one step of the safety protocol of the present invention; 
         FIG. 8  is a figure similar to that of  FIG. 7  showing the transmission of messages after the configuration process of  FIG. 7  during a start-up and run-time phase of the network; 
         FIG. 9  is a block diagram of the industrial controller of  FIG. 1  showing, in one embodiment, the division of communications between the input circuit, the controller and the output circuit into producer-consumer pairs such as provides redundant communication over a single network and the varied topologies of the implementations of  FIGS. 2 ,  3  and  4 ; 
         FIG. 10  is a flow chart showing the principle stages of the safety protocol of initialization, start-up, and run-time; 
         FIG. 11  is a figure similar to that of  FIG. 7  showing normal protocol operation under the safety protocol of the present invention during run-time; 
         FIG. 12  is a figure similar to  FIG. 11  showing protocol operation with a corrupted producer message; 
         FIG. 13  is a figure similar to  FIG. 11  showing protocol operation with a lost producer message; 
         FIG. 14  is a figure similar to  FIG. 11  showing protocol operation with a corrupted acknowledgement message from the consumer; 
         FIG. 15  is a figure similar to  FIG. 11  showing protocol operation with a lost consumer acknowledgement message; 
         FIG. 16  is a figure similar to  FIG. 11  showing protocol operation with disruption of the connection between the producer and consumer; 
         FIG. 17  is a flow chart of a program executed by the producers of  FIG. 9  in implementing the safety protocol; and 
         FIG. 18  is a flow chart of a program executed by the consumers of  FIG. 9  in implementing the safety protocol of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention can be part of a “safety system” used to protect human life and limb in the industrial environment. Nevertheless, the term “safety” as used herein is not a representation that the present invention will make an industrial process safe or that other systems will produce unsafe operation. Safety in an industrial process depends on a wide variety of factors outside of the scope of the present invention including: design of the safety system, installation, and maintenance of the components of the safety system, and the cooperation and training of individuals using the safety system. Although the present invention is intended to be highly reliable, all physical systems are susceptible to failure and provision must be made for such failure. 
     Referring now to  FIG. 1 , an industrial control system  10  for implementing a safety system with the present invention includes a chassis  11  holding a controller module  12  communicating on a backplane  15  of the chassis  11  with an input module  14  and an output module  16 . Alternatively, the backplane  15  may be a serial network communicating with remote components according to methods well known in the art. The backplane  15  may include multiple parallel traces  17  or a single serial trace joining a number of electrical connectors  13  which may connect to corresponding connectors  13 ′ on the backs of the modules  12 ,  14 , and  16 . The backplane  15  may thus accommodate the connected messaging described below. Although electrical communication for networks or backplanes is common, fiber-optic and wireless communications technologies may also provide the basis for the backplane of the present invention. 
     The backplane  15  may use a standard and commonly available high-speed serial protocol including but not limited to: Ethernet, DeviceNet, ControlNet, Firewire or FieldBus. The backplane  15  may optionally connect to a bridge  21  translating between different of the above standards or other protocols on a serial network  23 . As will be understood from the following, the present invention may be easily adapted to bridge applications. 
     Input module  14  may accept input signals  18  (on like-designated lines) which are communicated over the backplane  15  to the industrial controller  12 . At the industrial controller  12 , the signals  18  may be processed under a control program implementing a safety system (such as a machine lock-out or emergency stop) and further signals sent to the output module  16  which may produce output signals  20  (on like-designated lines) to an actuator  22 . 
     The input signals  18  may come from a switch  19  which may be any of a variety of devices producing safety input signals including but not limited to emergency stop switches, interlock switches, light curtains and other proximity detectors. The actuator  22  may be a relay, solenoid, motor, enunciator, lamp, or other device implementing a safety function. 
     Also connected to the backplane  15  is a standard computer, which may be used as a configuration terminal  24  (not shown) whose purposes will be described below. 
     Redundant System Hardware 
     Referring now to  FIG. 2 , the switch  19  may produce redundant signals  18   a  and  18   b  where signal  18   a  is, for example, from a first contact within the switch  19 , and signal  18   b  is from a second independent contact within switch  19 . The contacts may have the same logic (as shown) both being normally open (e.g., closed with actuation of a pushbutton  26 ) or may be inverted logic with one contact normally open and one contact normally closed. In either case, redundant signals  18   a  and  18   b  are generated so as to provide for higher reliability in determining the state of the switch  19 . 
     The input module  14  may include redundant interface circuitry  28   a  receiving signal  18   a  and interface circuitry  28   b  receiving signal  18   b . Alternatively, but not shown, interface circuitry  28   a  and  28   b  may each receive both signals  18   a  and  18   b  (for internal comparison) or may receive signals  18   a  and  18   b  from a single contact. The contacts, in generating signals  18   a  and  18   b , may each be provided with a separate voltage from the input circuitry  28   a  and  28   b  or from a common voltage source (not shown). Other redundant variations on these wiring systems, known in the art, may also be used. 
     Each of the interface circuitry  28   a  and  28   b  may in turn provide signals to associated microcontrollers  30   a  and  30   b . Microcontrollers  30   a  and  30   b  provide a computer processor, memory and a stored program for executing safety protocol programs as will be described below. Alternatively, or in addition, the safety protocol may be executed by safety protocol circuits  32  with which microcontrollers  30   a  and  30   b  communicate. In this case, the safety protocol circuits  32   a  and  32   b  may be application-specific integrated circuits (ASIC). As it is well known in the art to implement protocols through hardware or software or combinations of each, the term “protocol device” as used herein and in the claims should be understood to embrace generally any combination of software and hardware components implementing the indicated functions. 
     The microcontrollers  30   a  and  30   b  may communicate with each other through an internal bus  34  to compare signals  18   a  and  18   b  as will be described. 
     Microcontrollers  30   a  and  30   b  or safety protocol circuits  28   a  and  28   b  in turn connect to standard network protocol circuits  36   a  and  36   b  of a type well known in the art for handling the low level protocol of the standard backplane  15 . Typically, the standard network protocol circuits  36   a  and  36   b  are implemented by an ASIC whose implementation represents considerable development time and which cannot be easily modified. The standard network protocol circuits  36   a  and  36   b  transmits signals from the input module  14  on the backplane  15  to be received at the controller  12  through similar standard network protocol circuits  38   a  and  38   b.    
     Alternatively, the input module  14  may include redundant interface circuitry  28   a  and  28   b  receiving signals  18   a  and  18   b  connected by a bus to a single associated microcontroller  30   a  which may implement the safety protocol in software and connect to a single standard network protocol circuit  36   a  according to conventional input module architecture. The critical factor is that the non-redundant hardware provides a sufficient level of integrity. In the case where multiple messages are transmitted, these messages are processed by the standard network protocol circuit  38  and provided to redundant safety protocol circuits  40   a  and  40   b , being similar to safety protocol circuits  32   a  and  32   b  described before. These safety protocol circuits  40   a  and  40   b  communicate with processors  42   a  and  42   b , respectively, which include separate memory systems and control programs according to well-known redundancy techniques and which intercommunicate on internal bus  34 ′. Output signals generated by the processors  42   a  and  42   b  may be communicated back through the safety protocol circuits  40   a  and  40   b  to implement the safety protocol, as will be described below (or alternatively, the safety protocol may be handled by the processor  42   a  and  42   b ), and the output signals communicated to the standard network protocol circuits  38   a  and  38   b  for transmission again on backplane  15  to output module  16 . 
     In the case where a single message is transmitted, this message is processed by the standard network protocol circuit  38  and may be provided, in one variation, directly to a single processor  42   a  which generates an output signal communicated back through the standard network protocol circuits  38   a  for transmission again on backplane  15  to output module  16 . The safety protocol may be handled directly by the processor  42   a . The other components, such as processor  42   b  are omitted. When redundant messages are used, output module  16  may receive output data through a standard network protocol circuits  44   a  and  44   b  being similar to standard network protocol circuits  36   a  and  36   b  and  38   a  and  38   b . The standard network protocol circuits  44   a  and  44   b  provide the data to safety protocol circuits  46   a  and  46   b , which in turn provide them to redundant controllers  48   a  and  48   b . As before, alternatively, the safety protocol may be handled by the controllers  48   a  and  48   b  instead. The controllers  48   a  and  48   b  communicate by internal bus  34 ″ and in turn provide signals to output interface circuits  50   a  and  50   b  which provide the output signals  20   a  and  20   b . The output signals may be connected to the actuator  22  so that outputs must be enabled for the actuator  22  to be powered. In this sense, a default safety state is produced (of no power to the actuator  22 ) if there is an inconsistency between the signals received by processors  48   a  and  48   b . A change in the wiring to parallel configurations could create a safety state where the actuator is actuated unless both signals received by processors  48   a  and  48   b  are not enabled. 
     Alternatively, and as will be described, a safety state may be enforced by a safety state signal transmitted from the controller  12  or the input module  14  to the microcontrollers  48   a  and  48   b  of output module  16 , the latter which may respond by producing outputs to output interface circuits  50   a  and  50   b  determined by stored values of desired safety states programmed through the configuration terminal  24  as will be described further below. 
     When a single message is processed along the backplane  15 , output module  16  may receive output data through a single standard network protocol circuit  44   a  and provide it directly to controller  48   a  and, the safety protocol may be handled by the controllers  48   a . The controller  48   a  may in turn provide two signals to output interface circuits  50   a  and  50   b  which provide the output signals  20   a  and  20   b.    
     A bridge circuit  17  per the present invention could use the basic structure shown in the input module  14  but replacing the interface circuitry  28   a  and  28   b  of input module  14  with network protocol circuits  38   a  and  38   b  and safety protocol circuits of  40   a  and  40   b  (where the network protocol circuits  38  and  36  are for different protocols, thereby allowing seamless transmission of safety data per the techniques described below). 
     Alternatively, when a single message is employed, the bridge circuit  17  could use a single network protocol circuit for one network, communicating directly through a single processor to a second single network protocol circuit for the second network in the manner of standard bridge circuits. 
     An alternative embodiment contemplated by the present invention allows direct communication between the input module  14  and the output module  16  without the intervening controller  12  or for systems without controllers  12 . In this case, network protocol circuits  36  (or single network protocol circuit  36 ) communicate directly with network protocol circuits  44  (or single network protocol circuit  44 ). 
     Referring now to  FIG. 3 , specialized redundant input module  14 , in the present invention, may be replaced with two standard input modules  14   a  and  14   b , input module  14   a  holding the equivalent of previously described interface circuitry  28   a , microcontroller  30   a , safety protocol circuit  32   a  and standard network protocol circuit  36   a , and input module  14   b  holding the equivalent of interface circuitry  28   b , microcontroller  30   b , safety protocol circuit  32   b , and standard network protocol circuit  36   b . In this case, the operation of safety protocol circuits  32   a  and  32   b  are implemented in the firmware of the microcontrollers  30   a  and  30   b  and effected via messages communicated on the backplane  15  rather than the internal bus  34 . Or when a single message is used, a specialized redundant input module  14  may be replaced with one standard input module  14   a.    
     Likewise, referring to  FIG. 4 , the redundancy of output module  16  may be implemented by separate output circuits  16   a  and  16   b , output module  16   a  including the equivalent of standard network protocol circuit  44 , safety protocol circuit  46   a , microcontroller  48   a , and output interface circuit  50   a , with output module  16   b  including the equivalents of standard network protocol circuit  44  as  44 ′, safety protocol circuit  46   b , microcontroller  48   b , and output interface circuit  50   b . Or when a single message is used, a specialized redundant output module  16  may be replaced with one standard output module  16   a.    
     As will be described below, the present invention provides a protocol that is indifferent to the exact parsing of the safety components among physical devices having addresses on the backplane  15 . 
     Referring now to  FIGS. 5 and 2 , the operation of the safety protocol circuits  32  (or their software implementation) and standard network protocol circuits  36  in the input circuit  14  is to embed input data  52  from lines  18   b  within a safety-network protocol  54  implemented both as additional data attached to messages sent on backplane  15  and in the management of that data as will be described. The safety-network protocol  54  is in turn encapsulated in the standard network protocol  56  for seamless transmission on the backplane  15 . 
     The Safety Network Protocol 
     Referring now to  FIGS. 5 and 2 , the operation of the safety protocol circuits  32 ,  40  and  46  (or software implementations as will henceforth be understood without further repetition) in conjunction with the standard network protocol circuits  36 ,  38  and  44  for any of these embodiments is to embed safety data  52  (e.g., from lines  18   b ) within a safety-network protocol  54  implemented both as additional data attached to safety data  52  sent on backplane  15  and in the management of the particulars of transmission of that safety data  52 . The safety-network protocol  54  is in turn encapsulated in the standard network protocol  56  for seamless transmission on the backplane  15 . 
     The data encapsulated in the safety-network protocol  54  and standard network protocol  56  can then be received (e.g., by the controller  12 ) and extracted through the successive operation of the standard network protocol circuits  36 ,  38  and  44  and the safety protocol circuits  32 ,  40  and  46  to provide the safety data  52  in its basic state. Note that  FIG. 5  is only symbolic of the process and that the safety-network protocol  54  is not simply an encapsulation of the data  52  within, for example, safety data headers but rather the safety protocol includes timing constraints that may be executed in sequence with the standard network protocol  56  so that the safety-network protocol  54  may operate within the standard network protocol  56  without modification of the backplane  15  or standard network protocol circuits  36 ,  38  and  44 . 
     This dual level encapsulation and de-encapsulation is performed for each transmission of safety data  52  on the backplane  15  that requires a high level of reliability commensurate with safety systems. For non-safety system data, the standard network protocol  56  may be used alone without the safety-network protocol  54  for communication with non-safety elements of the industrial control system  10 . Because all data transmitted on the backplane  15  is embedded in the standard network protocol  56 , the safety-network protocol  54  will work seamlessly with a variety of networks  15  providing they have data transmission capacity suitable for the safety data  52  and sufficient capacity to accept some added safety error detection data  58  of the safety-network protocol  54  as will be described. 
     Safety Message Formatting 
     Generally, the safety protocol may provide a variety of different mechanisms for ensuring that data transmitted is free from undetected errors. A standard for undetected error rate desirably achieved by the invention is less than 10 −7  undetected errors per hour or less than 10 −3  undetected errors per demand. The particular types of errors and the particular methods of detecting these errors may be varied so long as the reliability of the transmitted data on the bridge/backplane matches that of a theoretical hardwired safety system or the error rate demanded by applicable regulations. 
     Among the communication errors that may be detected are: (1) message repetition, (2) message loss including message delay, (3) message insertion, (4) incorrect message sequence, (5) message corruption, and (6) message misdirection. The mechanisms used to detect these errors may include: (a) time stamping the messages, (b) sending messages on a regular schedule and applying a watchdog timer to message receipt, (c) positive identification of the sender and the receiver incorporated into the messages, (d) cyclic redundancy coding (CRC) or other error correction codes embedded in the message including the sending of the compliment of the message, (e) sending of the message multiple times, (f) echoing the message back from the receiver, and (g) the use of a message sequence number. These mechanisms make up the safety message protocol. 
     Message repetition may be caused by operation of a bridge and is not necessarily an error because the I/O circuitry may allow overwriting of data. However, a repeated message will not have an updated timestamp because only the original producer can update the timestamp and thus may register as an error. 
     Message loss is from messages that are never received or those that are received later than the time required. Messages received early are not treated as errors. Message delays are the receipt of the message beyond an expected time interval and are a special case of message loss. 
     Message insertion is an extra message otherwise unaccounted for. It may be detected by an unexpected value of the timestamp or a unique identification of sender and receiver described below (if it is the result of a fault in addressing). 
     Incorrect sequence is caused by messages changing in relative order between the time they are transmitted and the time they are received as may result from network delays. Such messages may be detected by their unexpected timestamps. 
     Message corruption results from the changing of one or more message bits in the message (such as may be caused by interference or hardware problems) during transmission and may be detected by error correcting code, as will be described below, and by sending the complement of the original data together with the original data. 
     Misdirected messages are messages that go to the wrong address. These produce other detectable errors such as message lost, message insertion, message repetition, timestamp errors, and message delay. 
     Referring now to  FIG. 6 , a first aspect of the safety-network protocol  54  is that the safety data  52  (typically I/O data related to a safety application) is attached to safety error detection data  58  to form a safety message  60  that forms the data provided to the standard network protocol circuits  36 ,  38  and  44  to produce a network message  61 . The safety error detection data  58  may include a time stamp (not shown) indicating the time of transmission of the safety data  52  and a sequence count indicating the local order in which the safety message  60  is transmitted with respect to earlier transmissions of safety messages. The sequence count is normally limited in range (0-3) as it is intended to detect the loss of only a single message. 
     Also appended to the safety data  52  and part of the safety error detection data  58  is a cyclic redundancy code (CRC) selected in the preferred embodiment to be eight to 16 bits depending on the size of the message. The cyclic redundancy code is functionally generated from the safety data  52  so that an error in the transmission can be detected when the CRC is recalculated by the receiving device and doesn&#39;t match. A separate CRC may be used for the other parameters of the message, for example, the time stamp and sequence number of the safety error detection data  58 . 
     The safety data  52  may be attached to complementary safety data (not shown) in the same safety message  60 , the complementary safety data having each “1” of the safety data  52  replaced by a “0” and vice versa. A separate CRC is provided for this complementary safety data. The complementary data helps detect low-level hardware errors such as “stuck bits” in buffers and the like. Additional mode and timing data may be attached to the message  60  to indicate its state as single-cast or multicast message, various fault states, and/or to provide a ping counter to be used to measure network delays. 
     Safety message  60  is embedded in the network headers and footers  62  and  64 , which vary depending on the standard network protocol  56  of the backplane  15 . Depending on the backplane  15 , the network header and footer  62  and  64  may include a CRC code and sequence count and other similar safety error detection data  58  operating redundantly with the safety error detection data  58 . Nevertheless, the safety message  60  includes its own safety error detection data  58  to be wholly network-independent to the degree possible. 
     The above features allow detection of (1) message repetition, (2) message loss, (3) message insertion, (4) incorrect message sequence and, (5) message corruption as will be described below. 
     Connected Messaging 
     As mentioned above, the safety error detection data  58  forms only part of the safety-network protocol  54 . The safety-network protocol  54  also includes a configuration step that ensures proper communication under a connected messaging scheme. Referring now to  FIG. 9 , the communications between the controller  12 , input module  14  (or input modules  14   a  and  14   b ) and, the output module  16  (or output module  16   a  and  16   b ) may provide a connected messaging system. As is understood in the art, connected messaging involves opening a connection between pairs of logical devices, one that acts as a “producers” of a message and, one that acts as a “consumers” of the message. The process of opening the connection reserves bandwidth of the network and reserves necessary processing and buffering resources at the producer and consumer to ensure that data of the connection will be reliably transmitted and received. 
     The connected messaging protocol may be implemented as part of the safety network protocol  54  or as part of the standard network protocol  56 , the latter option limiting somewhat the types of standard networks  15  that may be used. Some standard network protocols that support connected messaging are DeviceNet and ControlNet, Ethernet, and ATM. 
     Referring now to  FIG. 9 , under a connected messaging protocol, and in a first embodiment using redundant messaging, the input module  14  provides two producers  80  opening two connections with two consumers  82  of the controller  12 , one for each of the signals  18   a  and  18   b . As a practical matter, these two connections mean that two separate network messages  61  will be sent over the backplane  15  thus decreasing the chance of loss of both messages. 
     For the implementation of  FIG. 3  with separate input module  14   a  and  14   b , two producers  80  are also provided. Even though the producers  80  are now in different devices (having different addresses on the backplane  15 ), the operation of the control program implementing the safety system, above the connection level, need not changed by these changes in implementations. Connected messaging thus makes the safety system largely indifferent to topologies as providing for a natural redundancy over a single network, or multiple links. 
     Controller  12  likewise includes two producers  80  exchanging data with consumers  82  either in a single output module  16  per  FIG. 2  or in separate output module  16   a  and  16   b  per the embodiment of FIG.  4 . Two arrows are shown between each producer  80  and consumer  82  indicating the paring of each message with an acknowledgment message under the safety protocol  54  as will be described below, per FIG.  9 . 
     The bridge circuit  17 , not shown in  FIG. 9 , but as described above, would implement four consumers and four producers (two for each network side) as will be understood to those of ordinary skill in the art. 
     Alternatively, the input module  14  may provide one producer  80  opening one connection with one consumer  82  of the controller  12 , for a single highly reliable signal  18   a  so that only one network message is sent for each input signal. Controller  12  likewise may include a single producer  80  exchanging data with a single consumer  82  for each of the output signals. This approach greatly simplifies implementation of the safety system. 
     Safety Configuration Data and Protocol 
     Referring now to  FIG. 10 , the safety protocol more generally includes an initialization state of which the first step is developing configuration data as indicated by process block  66 . 
     The configuration process involves developing configuration data at the configuration terminal  24  and ensuring that accurate copies of that configuration data are at each of the input module  14 , the controller  12 , and the output module  16 . The configuration data is unique to each connection, provides essential components of the safety protocol, and identifies intercommunicating parties to reduce the possibility of improper connections injecting spurious data into the safety system (message misdirection). This is particularly important in allowing mixing of systems components observing the safety network protocol  54  with standard components observing only the standard network protocol. Devices may support multiple connections in which case multiple configuration data specific to each connection will be used. 
     Generally, the configuration data include data (i.e., a Safety Configuration Consistency Value (SCCV)) uniquely identifying the particular device of the input module  14 , the controller  12 , and the output module  16  holding the configuration data, and particularly the serial number of that device. The serial number is a unique and immutable part of the physical devices and thus together with an internal address of the logical devices within the physical device (which may establish independent connections), the serial number provides each connection with a unique identity eliminating the possibility of crossed connections between different devices once the configuration data is properly disseminated. To augment the serial number, the configuration data may also include a vendor identification number, a device code, a product code, major revision, minor revision, as well as network data including the logical, physical address of the device, all known in the art and identifying the particular device. Similarly, the configuration data within a device may include the serial number of the device to which it is connected. 
     As mentioned, the connection data may also include data necessary for the implementation of the other aspects of the safety protocol as are yet to be described, including variables of “periodic time interval”, “reply timer interval”, “filter count”, and “retry limit”. The configuration data also includes the safety state to which the device will revert in the case of network error and a list of related I/O points indicating other I/O points (related to other connections), which should revert to the safety state if the present connection has an error. This later feature allows selective and intelligent disabling of the safety system upon a communication error as will be described. As will be evident from context, some of this data is dependent on the devices and the system programmer must develop some. 
     Referring to  FIG. 7 , configuration data held within the configuration terminal  24  is sent to each of the input module  14 , the controller  12 , and the output module  16  as messages  66   a ,  66   b  and  66   c.    
     The receiving input module  14 , the controller  12 , and the output module  16  store the configuration and respond with the same configuration message. If the configurations of messages  66   a ,  66   b  and  66   c  exactly match configuration data of messages  66   d ,  66   e  and  66   f , the configuration was successful. 
     The configuration data may be shown to a human operator for confirmation. If the operator finds that the configuration is correct, the configuration is applied as indicated by process  68  shown in FIG.  10  through messages  68   a ,  68   b  and  68   c  from the configuration terminal  24  to the respective input module  14 , the controller  12 , and the output module  16 . The devices must acknowledge these messages via messages  68   d ,  68   e  and  68   f  within a predetermined time interval or the configuration will be cleared and no configuration will be considered to have occurred. The configuration data of messages  66  and  68  may be sent using only the standard network protocol  56 . 
     Once the configuration is complete, the safety protocol enters a start-up phase shown generally in  FIGS. 8 and 10 . During the start-up phase, the necessary safety connections are established and the configuration data is used to verify that the connections expected are those which are in fact made. The purpose of the startup portion of the configuration is to prevent erroneous connections from being opened between: (1) devices in the safety system and other erroneous devices in the safety system, and (2) devices in the safety system and other devices not in the safety system in a mixed system. 
     In this start-up process indicated by process block  70  of  FIG. 10 , the connections are confirmed from the controller  12  to the input module  14  and the output module  16 . In particular, the producers  80  in controller  12  (shown in  FIG. 9 ) send out open connection messages  70   a  and  70   b  to the input module  14  and the output module  16 , respectively. The appropriate consumers  82  respond with connection acknowledgment message  70   c  and  70   d , respectively. The producers  80  in controller  12  and input module  14  then send the configuration data to the consumer  82  in the controller  12  as indicated by messages  70   e  and  70   f . The controller&#39;s consumers  82  check to see that the configuration data matches their configuration data and then send acknowledgment messages  70   f  and  70   g  acknowledging that match. At messages  72   a  and  72   b , conventional I/O data may then commence to be sent. 
     Referring again to  FIG. 10 , the data  72   a  and  72   b  will be transmitted according to the portions of the safety protocol indicated by process blocks  72  involving formation of the safety message  60  incorporating safety error detection data  58  into the network message  61  as has been described above, and according to message handling protocols  74  operating independent of and in conjunction with the content of the safety message  60  which will now be discussed. 
     Message Handling Safety Protocols 
     (1) Normal Transmission 
     Referring generally to  FIGS. 10 and 11 , the message handling protocols  74  provide for message time measurements and respond to errors in the safety error detection data  58  during run-time. These message-handling protocols  74  are implemented in the safety protocol circuits  32 ,  40 , and  46  or may be implemented in software and are different for producers and consumers. 
     Referring now to  FIGS. 11 and 17  for a normal, run-time transmission, the producer  80  upon run-time will send safety messages  84  (encapsulated in the standard network message  61  per safety message  60  as has been described above) to the consumer  82  per FIG.  11 . This sending is indicated generally in FIG.  17 . Optionally, immediately prior to sending the message  84 , a periodic timer is started per process block  89  and a reply timer is started at the moment the message  84  is transmitted per process block  91 . The periodic timer interval  86  is longer than the reply timer interval  88  as set in the configuration process described above. Generally, messages will be sent out at an expected packet rate equal to the periodic timer. 
     Referring now to  FIGS. 9 ,  11  and  18 , the consumer  82  prior to receiving the message  84  is continually checking to see if the periodic time interval  86 ′ of its own periodic timer (started at the consumer&#39;s receipt of the last message  84 ) has expired as indicated in decision block  92 . The periodic timer value  86 ′ is generally greater than the periodic timer value  86 . 
     If the periodic timer has expired, a failure is indicated and the program proceeds to process block  134 , a safety state, as will be described below. 
     If timer value  86  has not expired, then at decision block  90 , the consumer  82  checks to see if the message  84  has arrived. If no message  84  has arrived, the program proceeds back to decision block  92  to again check if the periodic timer  86  has expired. 
     Assuming that a message  84  has arrived prior to expiration of the periodic timer  86 , then the program proceeds to decision block  112  to check the CRC of the message  84  and that the message destination per the SCCV matches the receiving device. In the event that two messages are sent, they may be compared at this point to see if: the data matches in the two messages, that there are two corresponding messages, and that the time stamp does not exceed a predetermined value. If the message is misdirected, that too may be determined at this point. 
     Assuming that the CRC is correct, the program proceeds to decision block  96  and checks to make sure that the sequence count (or alternatively the time stamp) is greater than the sequence count of the last message received. 
     If the sequence count is correct, then the program proceeds to process block  94  and the periodic timer  86  is reset. At process block  95 , the data is applied, for example, to an output or to update variables, and then at process block  98 , and optionally, an acknowledgement message  100  is returned to the producer  80 . 
     Referring again to  FIG. 17 , the producer  80  receiving the acknowledge message at decision block  102 , if such a message is elected and its data matches what was sent, proceeds to decision block  106  to determine if the periodic timer  86  has expired. 
     Assuming that the periodic timer has not expired, the program proceeds to decision block  124  to check the CRCs of the acknowledgement message  100 . The cyclic redundancy code should match the data of the safety message  60  transmitted and the complementary data (when inverted) should match the safety data  52 . 
     Again, assuming that the CRC and data are correct, the program proceeds to decision block  125  to determine whether the sequence count of the acknowledgment message  100  matches that of the message  84  that was sent. 
     If so, then at decision block  127 , the data sent in message  84  is compared to the data of the acknowledgement message  100 . If there is a match, then the program proceeds to decision block  129  where it loops until the periodic timer has expired, and then proceeds to process block  110  to prepare a new message  84 . 
     This process is repeated for multiple transmissions of safety messages  84  and acknowledgement messages  100 . Failure of any of the tests above may optionally cause the controller to enter the safety state. 
     (2) Message Received but Corrupted 
     Referring now to  FIG. 11  in one potential error, the safety message  84  is corrupted, for example, by electromagnetic interference  85 . In this case a message is received at the consumer  82 , as indicated by  FIG. 18  per process block  90 , within the periodic timer value  86 ′ as measured by process block  92 , however, there is an error in the CRC data as determined by decision block  112 . In this case, the program proceeds to process block  114  and no action is taken and in particular, no acknowledgement message  100  is returned. This latter condition will lead to a safety state being adopted if the periodic timer at the producer has expired. In this way, the sensitivity of the system to errors may be adjusted and moderated. In a similar way, a number of errors of a predetermined type, combination, or frequency may be used to trigger the safety state. 
     Referring to  FIG. 17 , in this case there will be no acknowledgment message  100  received by the producer  80  at process block  102 . The program proceeds to decision block  116  to determine if the periodic time interval  86  has expired. If so, the failure is indicated and the program proceeds to the safety state of process block  126 . 
     Optionally, if the periodic timer interval  86  has not expired, the program will proceed to decision block  118  to see if the shorter reply timer interval  88  has expired. If not, the program will loop back to process block  102 . If so, the program will proceed to process block  120  to check if the retry limit has been exceeded. Initially this may not be the case and the program will proceed to process block  122  and a repeat message  84 ′ having the same sequence count will be sent at process block  84  as also indicated by FIG.  12 . If the retry limit has been exceeded, the program proceeds to the safety state  126 . 
     This repeat message  84 ′ will be received at the consumer  82  as indicated by process block  90  of FIG.  18  and assuming that it is correct and that it has arrived within the periodic timer interval  86 ′ based on the previous non-erroneous message, this message  84 ′ results in the sending of an acknowledgment message  100  at process block  98  per the steps described above. 
     Typically, if only one missed transmission has occurred, the acknowledgment message  100  will occur within the periodic timer interval  86  of the producer and messages will continue to be exchanged normally as has been previously described with respect to FIG.  11 . 
     (3) Message Not Received 
     Referring now to  FIG. 13 , in the previous example, the safety message  84  arrived at the consumer  82  to be detected, albeit with errors. It is possible that the safety message  84  will not arrive at the consumer  82 , either as a result of such extreme interference that it is not recognizable as a message under low-level network protocols, or as a result of component failures between the producer and the consumer of an intermittent nature. Under this situation, the producer  80  sends the message  84  but the consumer does not receive a message at process block  90  of FIG.  18 . 
     The “no action” block  114  of  FIG. 18  of the consumer (as described above) is thus not encountered but the result is in any case the same: the consumer  82  takes no action. 
     Thus, as described previously with respect to  FIG. 12  at the expiration of the reply timer at the producer  80 , the producer  80  will produce a second message  84 ′ which if received will result in an acknowledgment message  100  initiating a string of normal communications. 
     (4) Acknowledgment Message Received but Corrupted 
     Referring now to  FIG. 14  the safety message  84  may successfully reach the consumer  82  with no errors but the acknowledgement message  100  may have errors introduced by electromagnetic interference  85 . In this case, the producer  80  reacts as shown in  FIG. 17  by decision block  106  to detect a receipt of an acknowledgment message  100  within the periodic timer interval  86 . But there is an error in the data of the acknowledgment message  100 . 
     If the CRC is correct as determined by decision block  124  and it is the sequence count that is wrong per process block  124 , then the program enters the safety state  126  in which outputs and inputs of the consumer  82  are set to a predefined safety state of the configuration data. The predefined safety state, as was previously provided as part of the configuration data defines that state (outputs and internal state) to which the device will revert in the case of network error. The safety state may be communicated with the list of related I/O points indicating other I/O points (related to other connections), which should revert to the safety state if the present connection has an error. This list was also provided as part of the configuration data described above. The safety state and list of related I/O points may be designated by the system programmer based on the particular application of the control system and states and related groupings of devices that will best provide for failure of individual devices or connections. 
     Similarly, if the sequence count is correct but the acknowledgement data does not match per decision block  127 , the program proceeds to the safety state  126 . If the consumer  82  is the controller  12  messages may be sent to other I/O devices, indicated in the configuration data signaling them to move to the safety state as well. 
     Assuming at process block  124  that the CRC code does not match the safety message  60 , indicating a corruption in the safety message rather than an erroneous duplicate message, the program proceeds to decision block  118  to see if the reply timer has expired as described above. When the reply timer expires, the program proceeds to process block  120  as described above and checks the retry counter to see if the retry limit has been exceeded. If so, the program proceeds to the safety state  126 , however, often this will not have occurred and the program proceeds to process block  122  and a retry message  84 ′ is prepared as indicated in FIG.  14 . 
     Assuming this retry message evokes a non-corrupted acknowledgment message  100 ′ communication continues in normal fashion. 
     (5) Acknowledgment Message Not Received 
     Referring now to  FIG. 15 , it is possible that the acknowledgment message  100  is lost completely either through interference or temporary communication failure. In that case as has been previously described, a duplicate message  84  will be sent from the producer  80 , however, the sequence count will be identical to the sequence count of a message  84  previously received by the consumer  82 . In this case, as shown in  FIG. 18  at process block  112 , the CRC will be correct but as tested at subsequent decision block  96  the sequence code will be wrong. The program, in this case, proceeds to process block  130  to check if the sequence code is one less than that expected. If not, the program proceeds to the safety state  134 . If so, however, the consumer  82  must conclude that the acknowledgment message  100  was lost and an acknowledgment of the previous message is sent again by the consumer at process block  132 . 
     (6) No Messages Received 
     Finally as shown in  FIG. 15 , the producer may be unable to connect with the consumer within the periodic interval  86 ′ of the consumer. In that case, the program proceeds to the safety state  134  directly from decision block  92  as shown in FIG.  18 . 
     The above description has been that of a preferred embodiment of the present invention, it will occur to those that practice the art that many modifications may be made without departing from the spirit and scope of the invention. In order to apprise the public of the various embodiments that may fall within the scope of the invention, the following claims are made.