Friday, March 26, 2010

PLC Programming Tips

One of the tips how to do the PLC programming is making and use a “one shot”. A one shot is a coil which goes true each time the enabling rung ahead of it is true, and it stays true for one scan only, no matter how long the enabling rung is true. The one shot is useful when you have some condition that goes on and off and you want to have the PLC take action on that true state for only one scan each time the enabling rung goes from true to false.

For instant you want to count the number of times some event that lasts longer than one scan happens, but the total count will exceed the capacity of PLC built in counter. One way to deal with this problem, if your PLC has floating point registers available, is to add one to a floating point register each time the rung goes true.

A few PLC brands do not have the useful one shot feature. In this case you can make your own one shot with a few rungs of logic:
PLC programming
This is one of few cases in common PLC programming where the order in which you place the rungs is important. From the picture above when you start out, the input I: 3/0, the memory bit B3:0/2, and the one shot output bit B3:0/1 are all false. When the input goes true, the memory bit is still false, so the 2nd rung is true, and both memory bit and the one shot output bit are set by 2nd rung.

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