AO Function Block
March 2006
5-67
Local Override (LO)—The output of the block is
not responding to inputs because the fault state action
is active. OUT [9] freezes or goes to value per
IO_OPTS [14].
The target mode of the block may be restricted to one
or more of the following modes: Man, Auto, Cas,
RCas, or OOS.
Mode Handling
Shed Options—RCAS Mode Only
Automatically changing through the permitted modes
when starting from a remote mode, fall from (or shed)
or climb to a remote mode is determined by the
parameter SHED_OPT [27]. A block climbs and sheds
through the same path. For example, if SHED_OPT
[27] specifies that a block should shed to Auto, then, if
the block target mode is set to RCas, the block goes
through Auto on the way to RCas.
When the block is in Cas mode and the CAS_IN [17]
input goes bad, the block sheds mode to the next
lower permitted mode.
You can configure the shed option as follows:
Shed With Return Options
Remote cascade connection failure shifts actual mode
but keeps trying to restore remote cascade (in other
words, the remote cascade target mode stays in
effect).
Note
During Shed with Return Options, the
actual mode is changed, while the
target mode stays the same.
Normal—On failure of a remote cascade connection
the block attempts to attain the highest permitted
non-remote mode until remote cascade is restored.
Cas is the highest permitted non-remote mode and
Auto is is the next permitted non-remote mode. If Cas
or Auto are not available, the block will shed by default
to Man.
Retained Target—The retained target mode is the
previous target mode before it was changed to RCAS
or ROUT. On failure of a remote cascade connection
the block attempts to attain the retained target mode.
Auto—On failure of a remote cascade connection the
block attempts to attain Auto, if permitted, until remote
cascade is restored.
Man—On failure of a remote cascade connection the
block sheds to Man until a remote cascade connection
is restored.
Shed With No Return Options
Note
During Shed with No Return Options,
the target mode is changed, while the
actual mode stays the same.
For any shed with no return option, the target mode
changes as determined by the option. Therefore, there
is no attempt to restore the connection following
failure. The behavior on change to the remote cascade
target mode is identical to that for Shed With Return
Options.
Normal—On failure of a remote cascade connection
the block sets the target mode to the highest permitted
non-remote mode. Cas is the highest permitted
non-remote mode and Auto is is the next highest
permitted non-remote mode. If Cas or Auto are not
available, the block will shed by default to Man.
Retained Target—The retained target mode is the
previous target mode before it was changed to RCAS
or ROUT. On failure of a remote cascade connection
the block sets the target mode to the retained target
mode.
Auto—On failure of a remote cascade connection, the
block sets the target mode to Auto, if permitted.
Man—On failure of remote cascade connection, the
block sets the target mode to Man, if permitted.
The user may configure SHED_OPT [27] so that it
calls for a target mode that is not permitted. When
doing this, the mode logic uses the following rules as
applied by the remote logic:
Shed logic never results in a non-permitted target
mode.
Shed logic never attempts to attain an actual
mode of Auto or Cas if that mode is not permitted.
Status Handling
Output or readback fault detection is reflected in the
status of PV [7], OUT [9], and BKCAL_OUT [25]. A
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