PLC book

What is your beef with this exactly?

I get that using latching coils in the ladder is considered poor form by some--and I would agree that in this simple application it is--but they could just be trying to show equivalent statements in the different languages. I'm glad to see that they are at least showing alternative languages instead of just assuming that the world will keep using ladder logic forever and ever amen.

Their ST code is a bit verbose but it would get the job done, provided that the PLC doesn't update outputs until the end of the scan. I might have done this:
K1 := (B1 OR K1) AND B2

but that may be a bit difficult for a beginning student to follow, not to mention harder to troubleshoot.

As for showing the motor directly connected to the PLC output, well that's just silly.
 
Not the worst thing in the world. That could be a tiny, low motor in which case direct from the PLC is fine, especially if just for a classroom demonstration.

Interesting, thanks for sharing.
 
Both Kolyur and Gene Bond have made the point about outputs not being processed until the end of the logic scan.
I fail to see how outcome of the logic would be different in any significant way if an output was serviced immediately upon change of state in the logic. By significant I mean that even if the output were turned on at the conclusion of the statement "K1 := TRUE" and then turned off at the conclusion of the statement "K1 := FALSE", the TRUE state would not last long enough for you to observe it.
Am I missing something?
 
In most cases I would agree. Certainly it would have no effect on a motor. But what if the output device was some sort of high speed apparatus looking for a rising edge on its input? It seems conceivable that this could cause problems depending on the scan speed of the PLC. In this example they would also have to be holding in both buttons for that to occur. To me, it seems like proper programming practice for an output never to be on unless it is on long enough to perform its function (regardless of whether I/O is scanned synchronously or not).
 

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