In the middle of the ladder logic file I'm trying to understand there's a segment in the form of a statement list:

SEGMENT 59

:A F2.4
:AN F6.6
:= F6.4
:L KT 0.2
:SD T27
:A T27
:= F6.5

SEGMENT 60...

My question is: is this two separate ladder segments, one which sets a flag (F6.4) and another which runs a 0 +/- 1 second timer when the program starts (ie has an always true control input) and sets flag F6.5 on completion, or, does the timer T027 only run when flag F6.4 is true?

 

I am not familiar with Westinghouse, but the code you have written is exactly the same as Siemens S5.

In S5 case, the second logic does nothing, i.e. the timer T27 will not run, since it needs a positive transition to run. With one exception: If T27 is used in another part of the program (for example in a start-up routine), then it may run, but in any case, it is independent from status of F6.4.

 

T27 will run when F6.4 is true, and reset when F6.4 is false. This is like having two outputs in parallel.

F6.5 will turn on when T27 is done. T27 is an "on delay" timer. As you have noted, the timer preset is zero, with a 1 second time base.

You could split this into three very simple rungs of ladder, with "A F2.4, AN F6.6, = F6.4" as the first rung. "A F6.4, L KT 0.2, SD T27" would be the second rung. "A T27, = F6.5" would be the third rung.

This Westinghouse PLC series were just private labelled Siemens S5 PLCs. You may find out more useful information by reading about the Siemens S5.

Programming software for the Siemens S5 is available from several different companies. Some of these packages assemble their ladder format rungs from different combinations of statement list than Siemens Step5 would use, especially when using timers and counters. Some of these would automatically add extra "coils" to timers or counters for "enabled" and "done".

If neither F6.4 or F6.5 are used elsewhere in the program, then they may be safely deleted (together with "A T27") from the rung (segment), leaving you with:

:A F2.4 :AN F6.6 :L KT 0.2 :SD T27


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Michael Griffin
London, Ont. Canada
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