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8--36
PID Loop Operation
If the process remains out of control for some time, the PV will eventually cross one
of the outer alarm thresholds, named High-high alarm and Low-low alarm. Their
threshold values are programmed using the loop table registers listed above. A
High-high or Low-low alarm indicates a serious condition exists, and needs the
immediate attention of the operator.
The PV Absolute Value Alarms are
reported in the four bits in the PID Mode
and Alarm Status word in the loop table, as
shown to the right. We highly recommend
using ladder logic to monitor these bits.
The bit-of-word instructions make this
easy to do. Additionally, you can monitor
PID alarms using DirectSOFT.
Red Deviation Alarm
Yellow Deviation Alarm
Yellow Deviation Alarm
Maintenance
and Troubleshooting
PID Loop Operation
Red Deviation Alarm
Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
High-high Alarm
High Alarm
Low Alarm
Low-low Alarm
Red
Yellow
Green
SP
PID Mode and Alarm Status V+06
Yellow
Loop Table
V+17
XXXX
Yellow Deviation Alarm
V+20
XXXX
Red Deviation Alarm
Red
The thresholds define zones, which fluctuate with the SP value. The green zone
which surrounds the SP value represents a safe (no alarm) condition. The yellow
zones lie just outside the green zone, and the red zones are just beyond those.
The PV Deviation Alarms are reported in
the two bits in the PID Mode and Alarm
Status word in the loop table, as shown to
the right. We highly recommend using
ladder logic to monitor these bits. The
bit-of-word instructions make this easy to
do. Additionally, you can monitor PID
alarms using DirectSOFT.
PID Mode and Alarm Status V+06
Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Red Deviation
Yellow Deviation
The PV Deviation Alarm can be independently enabled and disabled from the other
PV alarms, using bit 13 of the PID Mode 1 Setting V+00 word.
Remember, the alarm hysteresis feature works in conjunction with both the deviation
and absolute value alarms, and is discussed at the end of this section.
DL350 User Manual, 2nd Edition