Installation manual

Publication 1747-RM001G-EN-P - November 2008
Proportional Integral Derivative Instruction 9-3
The PID Instruction
The figure below shows a PID instruction with typical addresses for these
parameters entered.
Place the PID instruction on a rung without any conditional logic. If a PID
instruction goes false, the integral term is cleared.
During programming, enter the Control Block, Process Variable, and Control
Variable addresses after you have placed the PID instruction on a rung.
Entering Parameters
• Control Block is a file that stores the data required to operate the
instruction. The file length is fixed at 23 words and should be entered as
an integer file address. For example, an entry of N10:0 will allocate
elements N10:0 through N10:22. The control block layout is shown on
page 9-5.
Do not write to control block addresses with other instructions in your
program except as described later in this chapter. If you are re-using a
block of data which was previously allocated for some other use, it is
good practice to first zero the data.
TIP
The PID instruction is an integer-only type of PID
algorithm and does not allow you to enter floating point
values for any of its parameters. If you attempt to move a
floating point value to one of the PID parameters using
ladder logic, a floating point-to-integer conversion occurs.
TIP
Use a unique data file to contain your PID control blocks
(for example, N10:0). This avoids accidental re-use of the
PID control block addresses by other instructions in your
program.
PID
PID
Control Block N10:0
Process Variable N10:28
Control Variable N10:29
Control Block Length 23