Yokogawa digitalYEWFLO (DY-FF) - Manual (Page 134)

A8-8
IM 01F06F00-01EN
A8.15 Manual Fallback
Manual fallback denotes an action in which a
PID block changes mode to MAN (manual) and
suspends the control action. Manual fallback takes
place automatically as a means of abnormality
handling when the following condition is met:
• IN.status is Bad except when the control action
bypass is on.
To enable the manual fallback action to take place
when the above condition is met, Target to Manual
if Bad IN must be specifi ed beforehand in STATUS_
OPTS.
The table below shows the options in STATUS_
OPTS.
• STATUS_OPTS
Options in
STATUS_OPTS
Description
IFS if Bad IN Sets the sub-status component of OUT.
status to IFS if IN.status is Bad except
when PID control bypass is on.
IFS if Bad CAS IN Sets the sub-status component of OUT.
status to IFS if CAS_IN.status is Bad.
Use Uncertain as
Good Does not regard IN as being in Bad
status when IN.status is Uncertain (to
prevent mode transitions from being
affected when it is Uncertain).
Target to Manual if
Bad IN Automatically changes the value of
MODE_BLK.target to MAN when IN falls
into Bad status.
Target to next
permitted mode if
Bad CAS IN Automatically changes the value of
MODE_BLK.target to Auto (or to Man if
Auto is not set in Permitted) when CAS_IN
falls into Bad status.
A8.16 Auto Fallback
Auto fallback denotes an action in which a PID
block changes mode from CAS (cascade) to AUTO
(automatic) and continues automatic PID control
with the user-set setpoint. Auto fallback takes place
automatically when the following condition is met:
• IN.status (data status of IN) is Bad except when
the control action bypass is on.
To enable the manual fallback action to take place
when the above condition is met:
• Target to next permitted mode if Bad CAS
IN must be previously specifi ed in STATUS_
OPTS.
- AND -
• AUTO must be previously set in MODE_BLK.
permitted.
A8.17 Mode Shedding upon
Computer Failure
When the data status of RCAS_IN or ROUT_IN,
which is the setting received from a computer as
the setpoint SP, falls to Bad while the PID block is
running in the RCAS (remote cascade) or ROUT
(remote output) mode, the mode shedding occurs in
accordance with the settings in SHED_OPT.
• SHED_OPT
The SHED_OPT setting stipulates the
specifi cations of mode shedding as shown below.
Only one can be set.