National Instruments FP-3000 - Manual (Page 117)

Appendix D Advanced Function Block Behavior
FieldPoint FP-3000 User Manual D-4 www.ni.com
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 .
Fault State and Mode Shedding
To allow for safe control of a F O U N D A TIO N , even in the event the input
sensors or control algorithms fail, F O U N D A TIO N 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.
Fault State
F
O U N D A TIO N
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.
Mode Shedding
For remote modes in which the cascade input is periodically written by a
host application, F
O U N D A TIO N 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. 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 target mode is changed at
the time the block sheds mode, and the block does not attempt to enter the
original lower priority mode when the shed condition ends.