National Instruments FP-3000 - Manual (Page 116)

Appendix D Advanced Function Block Behavior
© National Instruments Corporation D-3 FieldPoint FP-3000 User Manual
To prevent windup, the control loop needs to be aware when it is unable to
control the process. If the downstream block can no longer control the
process, it reports a status of Bad to the upstream block. This breaks the
cascade until automatic control can be resumed, in which case cascade
initialization takes place again.
Remote Cascades
If a host application (rather than another block) provides the setpoint of a
block, F
O U N D A TIO N Fieldbus provides the Remote Cascade mode. The
remote cascade mode is equivalent to Cascade mode, except that the
cascade input parameter is
RCAS_IN instead of CAS_IN , and the back
calculation output is RCAS_OUT instead of BKCAL_OUT . Unlike CAS_IN and
BKCAL_OUT , which are input/output parameters, RCAS_IN and RCAS_OUT
are contained parameters and can only be written by a host application.
To allow the controlled block to enter Remote Cascade mode, the host
application must act as the upstream block in the cascade initialization and
implement the status handling described above.
Figure D-2. Remote Cascade Model
There is a second remote mode in which a cascade must be initialized:
Remote Output (
ROut mode). Unlike RCas mode, where the block setpoint
is 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 cascade
input is ROUT_IN . Otherwise, cascade initialization proceeds normally.
Bypassing Cascade Initialization
If cascade initialization is unnecessary, you can bypass it by leaving the
back calculation input of the upstream block unwired. By manually writing
a status of
Good, Non-cascade to the back calculation input, the block
bypasses cascade initialization and immediately transitions to automatic
Downstream
RCAS_IN
RCAS_OUT
Host Application