Poor Re-Creation of a Falling Edge Trigger?

theColonel26 · Mar 18, 2020

Found this in an old program I am cleaning up.

Is this a poor re-creation of a Falling Edge Trigger? Thoughts

See the screen shot.

ELake20 · Mar 18, 2020

I do things like this all the time and have seen it done lots of times.

It's not a falling edge trigger, it's a rising edge trigger, true for one scan.

theColonel26 · Mar 18, 2020

But it triggers when SYSTEM.on, turns off, not when it turns on.

RS 5000, has One Shot (ONS), Rising Edge (OSR), and Falling Edge (OSF) Instructions

ELake20 · Mar 18, 2020

Your image description says it is latched when system.on goes true, and you are using a normally open contact.

Steve Bailey · Mar 18, 2020

But it triggers when SYSTEM.on, turns off, not when it turns on.

Perhaps. If SYSTEM.ON is already true when RPTActive goes true, nothing happens until SYSTEM.ON goes false. But if SYSTEM.ON is false when RPTActive goes true, all of the tags are initialized and RPTActive is turned off. By looking at rung 19 alone, you can't be sure which of the two tags triggers the action.

theColonel26 · Mar 18, 2020

I guess I assumed everyone knew what I meant by, "RptActive is Latched when SYSTEM.On goes true"

Which would mean that SYSTEM.On would have to be true before RptActive is true.

kolyur · Mar 18, 2020

From your pic I assume there is another rung somewhere with SYSTEM.On on a normally open contact driving RPTActive on a latching coil. In that case the given rung would work as a falling edge trigger; when SYSTEM.On goes false it would execute the MOV logic for one scan. I don't really see a problem with this. Perhaps using a one shot instruction would be more clear but this logic is fairly self explanatory as long as the other rung is somewhere close by. I've seen much, much worse.

kamenges · Mar 18, 2020

The "benefit" of this method is that it could conceivably scan the rung in two successive scans, depending on how the two bits are controlled. With a true oneshot you would need an off-state scan to allow a retrigger. Might be good, might be bad. That is application specific.

This probably wasn't a consideration but this also makes the rung more platform transportable. Oneshot implementations are not the same across all plcs.

Keith

GaryS · Mar 18, 2020

It looks like it's an FS First Scan bit
only on for the first scan of the program

Poor Re-Creation of a Falling Edge Trigger?

theColonel26

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