Publication 1747-RM001G-EN-P - November 2008
Proportional Integral Derivative Instruction 9-27
Application Notes
The following sections discuss:
• input/output ranges.
• scaling to engineering units.
• output alarms.
• output limiting with anti-reset windup.
• the manual mode.
• PID rung state.
• time proportioning outputs.
Input/Output Ranges
The input module measuring the process variable (PV) must have a full scale
binary range of 0 to 16383. If this value is less than 0 (bit 15 set), then a value
of zero is used for PV and the Process var out of range bit is set (bit 12 of
word 0 in the control block). If the process variable is >16383 (bit 14 set), then
a value of 16383 is used for PV and the Process var out of range bit is set.
The Control Variable, calculated by the PID instruction, has the same range of
0 to 16383. The Control Output (word 16 of the control block) has the range
of 0 to 100%. You can set lower and upper limits for the instruction’s
calculated output values (where an upper limit of 100% corresponds to a
Control Variable limit of 16383).
Scaling to Engineering Units
Scaling lets you enter the setpoint and zero-crossing deadband values in
engineering units, and display the process variable and error values in the same
engineering units. Remember, the process variable PV must still be within the
range 0-16383. The PV is displayed in engineering units, however.
Select scaling as follows:
1. Enter the maximum and minimum scaling values Smax and Smin in the
PID control block. Refer to the control block of the PID instruction on
page 9-5. The Smin value corresponds to an analog value of zero for the
lowest reading of the process variable, and Smax corresponds to an
analog value of 16383 for the highest reading. These values reflect the
process limits. Setpoint scaling is selected by entering a non-zero value
for one or both parameters. If you enter the same value for both
parameters, setpoint scaling is disabled.
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