Yokogawa digitalYEWFLO (DY-FF) - Manual (Page 135)

<APPENDIX 8. PID BLOCK>
A8-9
IM 01F06F00-01EN
Available Setting
for SHED_OPT
Actions upon Computer Failure
Normal shed,
normal return Sets MODE_BLK.actual to Cas*, and
leaves MODE_BLK.target unchanged.
Normal shed, no
return Sets both MODE_BLK.actual and MODE_
BLK.target to Cas*.
Shed to Auto,
normal return Sets MODE_BLK.actual to Auto**, and
leaves MODE_BLK.target unchanged.
Shed to Auto, no
return Sets both MODE_BLK.actual and MODE_
BLK.target to Auto**.
Shed to Manual,
normal return Sets MODE_BLK.actual to Man, and
leaves MODE_BLK.target unchanged.
Shed to Manual,
no return Sets both MODE_BLK.actual and MODE_
BLK.target to Man.
Shed to retained
target, normal
return If Cas is in MODE_BLK.target, sets
MODE_BLK.actual to Cas*, and leaves
MODE_BLK.target unchanged.
If Cas is not set in MODE_BLK.target, sets
MODE_BLK.actual to Auto**, and leaves
MODE_BLK.target unchanged.
Shed to retained
target, no return If Cas is set in MODE_BLK.target, sets
both MODE_BLK.actual and MODE_BLK.
target to Cas*.
If Cas is not set in MODE_BLK.target, sets
MODE_BLK.actual to Auto**, and MODE_
BLK.target to Cas.
* The modes to which a PID block can transfer
are limited to those set in MODE_BLK.
permitted, and the priority levels of modes are
as shown below. In fact, if Normal shed, normal
return is set for SHED_OPT, detection of a
computer failure causes MODE_BLK.actual to
change to CAS, AUTO, or MAN, whichever is
set in MODE_BLK.permitted and has the lowest
priority level.
MAN Higher priority level
AUTO
CAS
RCAS
ROUT
Lower priority level
FA0807.ai
** Only if Auto is included in MODE_BLK.
permitted. If the block upstream of the PID
block in question is a control block, mode
transitions of the PID block to CAS occur in the
following sequence due to initialization of the
cascade connection: RCAS or ROUT
 AUTO
 CAS.
A8.18 Alarms
There are two kinds of alarms generated by a PID
block: block and process alarms.
A8.18.1 Block Alarm (BLOCK_ALM)
The block alarm BLOCK_ALM is generated upon
occurrence of either of the following errors (values
set in BLOCK_ERR) and notifi es the content of
BLOCK_ERR.
Value of
BLOCK_ERR
Condition
Input Failure IN.status of the PID block is either of the
following:
• Bad-Device Failure
• Bad-Sensor Failure
Out of Service MODE_BLK.target of the PID block is O/S.
A8.18.2 Process Alarms
There are six types of process alarms. Only one
process alarm can be generated at the same time,
and the process alarm having the highest priority
level from among those occurring at the same
time is generated. The priority level is set for each
process alarm type.
Process
Alarm
Cause of Occurrence
Parameter
Containing
Priority Level
Setting
HI_HI_ALM Occurs when the PV increases
above the HI_HI_LIM value.
HI_HI_PRI
HI_ALM Occurs when the PV increases
above HI_LIM value. HI_PRI
LO_ALM Occurs when the PV decreases
below the LO_LIM value. LO_PRI
LO_LO_ALM Occurs when the PV decreases
below the LO_LO_LIM value.
LO_LO_LIM
DV_HI_ALM Occurs when the value of [PV -
SP] increases above the DV_HI_
LIM value.
DV_HI_PRI
DV_LO_ALM Occurs when the value of [PV
- SP] decreases below the DV_
LO_LIM value.
DV_LO_PRI