National Instruments FieldPoint FP-3000 User Manual
Also see for FieldPoint FP-3000: Operating instructions
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Appendix D Advanced Function Block BehaviorFieldPoint FP-3000 User ManualD-4 ©National Instruments CorporationBypassing Cascade InitializationIf cascade initialization is unnecessary, you can bypass it by leaving theback calculation input of the upstream block unwired. By manually writinga status of Good, Non-cascade to the back calculation input, the blockbypasses cascade initialization and immediately transitions to automaticcontrol. The upstream block signals to the downstream block that cascadeinitialization has been bypassed by setting its output to a status of Good,Non-cascade.Fault State and Mode SheddingTo allow for safe control of a process, even in the event the input sensors orcontrol algorithms fail, FOUNDATION Fieldbus provides fault state andmode shedding. Fault state is used when an output block is in Cascademode. Mode shedding is used in Remote or Remote Cascade modes.Fault StateFOUNDATION Fieldbus output function blocks (AO, DO, and CDO)support a faultstate to deal with the case where the control of the outputblock has been lost while the block is in Cascade mode. If the block is inCascade mode, and CAS_IN has a bad quality for longer than the timespecified in the FSTATE_TIME parameter, the block enters faultstate. If theFaultstate Use Value option is set in the IO_OPTS parameter, theblock uses the value in FSTATE_VAL as its output value. If the option is notset, it holds the value it had at the time the input went bad until the cascadecan be reestablished.Mode SheddingFor remote modes in which the cascade input is periodically written by ahost application, FOUNDATION Fieldbus provides mode shedding tohandle failure of the host application. For the RCas mode and ROut modes,there are two timeout parameters in the resource block: SHED_RCAS andSHED_ROUT. If the block is in a remote mode and the block input being used(RCAS_IN or ROUT_IN) is not updated within the timeout period, the blocksheds to a higher priority mode. The action taken by the block when modeshedding occurs is defined by the SHED_OPT parameter, as described inTable D-1. The shed condition ends when the host writes the remote inputparameter. If a normal return shed option is selected, the block attempts toenter the original remote mode. If a no return shed option is selected, the |
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