Appendix D Advanced Function Block Behavior©National Instruments CorporationD-3FieldPoint FP-3000 User ManualTo prevent windup, the control loop needs to be aware when it is unable tocontrol the process. If the downstream block can no longer control theprocess, it reports a status of Bad to the upstream block. This breaks thecascade until automatic control can be resumed, in which case cascadeinitialization takes place again.Remote CascadesIf a host application (rather than another block) provides the setpoint of ablock, FOUNDATION Fieldbus provides the Remote Cascade mode. Theremote cascade mode is equivalent to Cascade mode, except that thecascade input parameter is RCAS_IN instead of CAS_IN, and the backcalculation output is RCAS_OUT instead of BKCAL_OUT. Unlike CAS_IN andBKCAL_OUT, which are input/output parameters, RCAS_IN and RCAS_OUTare contained parameters and can only be written by a host application. Toallow the controlled block to enter Remote Cascade mode, the hostapplication must act as the upstream block in the cascade initialization andimplement the status handling described above.Figure D-2. Remote Cascade ModelThere is a second remote mode in which a cascade must be initialized:Remote Output (ROut mode). Unlike RCas mode, where the block setpointis set remotely, in ROut mode the block output is set by the host application.The back calculation output for ROut mode is ROUT_OUT, and the cascadeinput is ROUT_IN. Otherwise, cascade initialization proceeds normally.RCAS_OUTDownstreamRCAS_INHostApplication
