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Chapter 7. PID Built-In Function Block
7.3.3
Time Interval for the PID Function
The start time of each CPU sweep is used as the current time when calculating the time interval
between solutions of the PID function. The times and time intervals have a resolution of 100 µs.
When an application uses multiple PID functions, all of them use the same time value.
The PID algorithm is solved when the current time is equal to or greater than the time of the last PID
solution plus the Sample Period or 10 ms; whichever is larger. If the Sample Period is set for
execution on every sweep (value = 0), the PID function is restricted to a minimum of 10 ms between
solutions. If the sweep time is less than 10 ms, the PID function waits until enough sweeps have
occurred to accumulate an elapsed time of at least 10 ms. For example, if the sweep time is 9 ms,
the PID function executes every other sweep, and the time interval between solutions is 18 ms. If a
specific PID function is executed more than once per sweep (by referencing the same reference array
location in multiple PID function blocks), the algorithm is solved only on the first call.
The longest possible interval between executions is 65,535 times 10 ms, or 10 minutes, 55.35
seconds.
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PACSystems* RX7i, RX3i and RSTi-EP CPU Programmer's Reference Manual
GFK-2950C
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