The FOR loop and the watchdog

The construct

FOR i := 1 TO 10 DO
  Sum := Sum + i;
END_FOR

The loop counter i is an integer you declare in VAR. The loop starts at 1, runs the body, adds one, and stops once the counter passes 10. So 10 is included.

The BY step

FOR i := 0 TO 100 BY 2 DO      (* even numbers only: 51 iterations *)
FOR i := 10 TO 1 BY -1 DO      (* counting down *)

The default step is 1. To count down the step must be negative, otherwise the loop body never executes at all.

The bounds are read once

Start, end and step are evaluated when the loop is entered. Changing them in the body does not extend the loop:

FOR i := 1 TO N DO
  N := 1000;        (* no effect: the iteration count is already fixed *)
END_FOR

And do not rely on the counter's value after END_FOR. Some vendors leave it at the limit plus the step, others at the limit.

What FOR is good for in a PLC

• One operation repeated over 64 motors or 200 valves.
• A sum, an average or a maximum over a set of values.
• Searching a recipe table.
• Numeric approximation: several passes of an iterative formula in one scan.

And why you should fear it

The whole program must finish inside one scan cycle. If the cycle is 10 ms and the loop body costs 2 microseconds:

At 10,000 we are already past the cycle time. At a million the controller stops: the watchdog sees that the scan did not finish in the allowed time, raises a fatal fault, and drops every output. In a real plant that is a stopped line and a phone call at three in the morning.

The lab on this site mimics it: any loop that runs more than 10,000 iterations in one scan is aborted with an error. Try it, then remember that a real PLC would have faulted instead.

How to handle it

1. Keep the upper bound a known constant, not a variable an HMI can write.
2. Spread heavy work across scans: do twenty items per scan, remember where you were, resume next scan.

Last := Cursor + 20;
FOR i := Cursor TO Last DO
  (* only twenty items in this scan *)
END_FOR
Cursor := Cursor + 21;
IF Cursor > 999 THEN Cursor := 0; END_IF

3. Put heavy maths in a slow task, every 500 ms rather than every 10 ms.