Rockwell Automation 1747-PT1, D1747NP002 Hand-Held Terminal - Manual (Page 362)

Chapter 26
PID Instruction
26–18
Output Alarms
You may set an output alarm on the control output (CO) at a selected value
above and/or below a selected output percent. When the instruction detects
that the output (CO) has exceeded either value, it sets an alarm bit (bit 10 for
lower limit, bit 9 for upper limit) in word 0 of the PID control block. Alarm
bits are reset by the instruction when the output (CO) comes back inside the
limits. The instruction does not prevent the output (CO) from exceeding the
alarm values unless you select output limiting.
Select upper and lower output alarms by entering a value for the upper alarm
(word 11) and lower alarm (word 12). Alarm values are specified as a
percentage of the output. If you do not want alarms, enter zero and 100%
respectively for lower and upper alarm values and ignore the alarm bits.
Output Limiting with Anti-reset Windup
You may set an output limit (percent of output) on the control output. When
the instruction detects that the output (CO) has exceeded a limit, it sets an
alarm bit (bit 10 for lower limit, bit 9 for upper limit) in word 0 of the PID
control block, and prevents the output (CO) from exceeding either limit
value. The instruction limits the output (CO) to 0 and 100% if you choose
not to limit.
Select upper and lower output limits by setting the limit enable bit (bit 3 of
control word 0), and entering an upper limit (word 11) and lower limit (word
12). Limit values are a percentage (0 to 100%) of the control output (CO).
The difference between selecting output alarms and output limits is that you
must select output limiting to enable limiting. Limit and alarm values are
stored in the same words. Entering these values enables the alarms, but not
limiting. Entering these values and setting the limit enable bit enables
limiting and alarms.
Anti-reset windup is a feature that prevents the integral term from becoming
excessive when the output (CO) reaches a limit. When the sum of the PID
and bias terms in the output (CO) reaches the limit, the instruction stops
calculating the integral output term until the output (CO) comes back in
range.