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Appendix D

Advanced Function Block Behavior

D-4

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 National Instruments Corporation

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 
control. The upstream block signals to the downstream block that cascade 
initialization has been bypassed by setting its output to a status of 

Good, 

Non-cascade

.

To allow for safe control of a process, even in the event the input sensors or 
control algorithms fail, FOUNDATION Fieldbus provides fault state and 
mode shedding. Fault state is used when an output block is in 

Cascade

 

mode. Mode shedding is used in 

Remote

 or 

Remote Cascade

 modes.

FOUNDATION Fieldbus output function blocks (AO, DO, and CDO) 
support a faultstate to deal with the case where the control of the output 
block has been lost while the block is in 

Cascade

 mode. If the block is in 

Cascade

 mode, and 

CAS_IN

 has a bad quality for longer than the time 

specified in the 

FSTATE_TIME

 parameter, the block enters faultstate. If the 

Faultstate Use Value

 option is set in the 

IO_OPTS

 parameter, the 

block uses the value in 

FSTATE_VAL

 as its output value. If the option is not 

set, it holds the value it had at the time the input went bad until the cascade 
can be reestablished.

For remote modes in which the cascade input is periodically written by a 
host application, FOUNDATION Fieldbus provides mode shedding to 
handle failure of the host application. For the 

RCas

 mode and 

ROut

 modes, 

there are two timeout parameters in the resource block: 

SHED_RCAS

 and 

SHED_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 block 

sheds to a higher priority mode. The action taken by the block when mode 
shedding occurs is defined by the 

SHED_OPT

 parameter, as described in 

Table D-1. The shed condition ends when the host writes the remote input 
parameter. If a normal return shed option is selected, the block attempts to 
enter the original remote mode. If a no return shed option is selected, the