G
Hi,
There have been a number of posts like 'scan should be less than 20 msec.' which are somewhat meaningless without further definition - akin
to 'how long is a piece of string?'.
PLC manufacturers make statements like '1 msec per K of (ladder) program'. This is, of course, only vaguely indicative because each
instruction has a different execution time for true, false, integer, float, word, file, etc. The longest (program) scan I've experienced was on a PLC3 - 350 msec. It was a large and complex program, but the only obvious effect of the scan time was that the operators needed to be very
definite about pressing a push button.
The upshot is - if your application is large and requires low scan times, then you need multiple PLC's.
A propos: I determined empirically many years ago that I could easily operate a push button and beat a 100 msec scan, I could often beat a 50
msec. scan, but I could not beat a 20 msec. scan.
Gerry
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There have been a number of posts like 'scan should be less than 20 msec.' which are somewhat meaningless without further definition - akin
to 'how long is a piece of string?'.
PLC manufacturers make statements like '1 msec per K of (ladder) program'. This is, of course, only vaguely indicative because each
instruction has a different execution time for true, false, integer, float, word, file, etc. The longest (program) scan I've experienced was on a PLC3 - 350 msec. It was a large and complex program, but the only obvious effect of the scan time was that the operators needed to be very
definite about pressing a push button.
The upshot is - if your application is large and requires low scan times, then you need multiple PLC's.
A propos: I determined empirically many years ago that I could easily operate a push button and beat a 100 msec scan, I could often beat a 50
msec. scan, but I could not beat a 20 msec. scan.
Gerry
_______________________________________________
LinuxPLC mailing list
[email protected]
http://linuxplc.org/mailman/listinfo/linuxplc