Safety Considerations Guide, Tricon v9.0 - Tuv-fs.com

46 Module DiagnosticsMain Processor and TriBusEach Main Processor (MP) Module uses memory data comparison between itselfand the other MPs to ensure that the control program executes correctly on eachscan. Each MP transfers its input point data to the other two MPs via the TriBusduring each scan. Each MP then votes the input data and provides voted data to thecontrol program. The results of the control program (outputs), including all internalvariables, are transferred by the TriBus. If a mis-compare is detected, specialalgorithms are used to isolate the faulting MP. The faulting MP enters the fail-safestate and is ignored by the remaining MPs. Background diagnostics test MPmemory and compare control program instructions and internal status.The integrity of the TriBus is continuously monitored and verified independentlyby each MP. All TriBus faults are detected within the scan associated with theTriBus transfer. Fault isolation hardware and firmware causes the MP with thefaulting TriBus to enter the fail-safe state.An independent watchdog ensures that the control program and diagnosticsexecute within 500 milliseconds seconds (the watchdog period). If an MP fails toexecute the scan, the watchdog forces the MP to the fail-safe state. The I/Oprocessor adds a sequential element to the MP watchdog. If an MP fails to reportthe proper sequence of execution, the I/O processor causes the MP to enter the failsafestate.External CommunicationLoss of external communication is not indicated by a system alarm. However,alarms can be generated by using:• Semaphore flags• System attributes! CAUTIONExternal communications are intended for transporting non-safety-critical data.The guidelines outlined in section “Using Serial Communication” on page 32should be followed in SIS applications.Semaphore FlagsEstablish a semaphore between a controller and an external device by using a timerfunction block to evaluate the changing state of semaphore flags.Tricon Safety Considerations Guide