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Appendix D

Advanced Function Block Behavior

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 National Instruments Corporation

D-3

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.

If a host application (rather than another block) provides the setpoint of a 
block, FOUNDATION 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.

RCAS_OUT

Downstream

RCAS_IN

Host

Application