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Chapter 7. PID Built-In Function Block
7.2 Reference Array for the PID Function
This parameter block for the PID function occupies 40 words of memory, located at the starting
Instance Variable specified in the PID function block operands. Some of the words are configurable.
Other words are used by the CPU for internal PID storage and are normally not changed.
Every PID function call must use a different 40-word memory area, even if all the configurable
parameters are the same.
The configurable words of the reference array must be specified before executing the PID function.
Zeros can be used for most default values. Once suitable PID values have been chosen, they can be
defined as constants in BLKMOV functions so the program can set and change them as needed.
The LD version of the PID function does not pass power flow if there is an error in the configurable
parameters. The function can be monitored using a temporary coil while modifying data.
7.2.1
Scaling Input and Outputs
All parameters of the PID function are 16-bit integer words for compatibility with 16-bit analog
process variables. Some parameters must be defined in either PV counts or units or in CV counts or
units.
The SP input must be scaled over the same range as the PV, because the PID function calculates
error by subtracting these two inputs.
The process PV and control CV counts do not have to use the same scaling. Either may be -32,000 or
0 to 32,000 to match analog scaling, or from 0 to 10,000 to display variables as 0.00% to 100.00%. If
the process PV and control CV do not use the same scaling, scale factors are included in the PID
gains.
346
PACSystems* RX7i, RX3i and RSTi-EP CPU Programmer's Reference Manual
GFK-2950C
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