Two rungs writing to one coil

fluoronator · Sep 28, 2006

I know this is a bad practice but what happens when you have two different rungs of logic that write to the same coil? Which rung wins if one is turning it off and the other is turning it on? Is it the one that is scanned last? I'm working with a SLC505.

Ron Beaufort · Sep 28, 2006

in very basic terms, yes, the "last one" wins ...

why are you asking? ... just for general knowledge - or trying to make something work? ...

and welcome to the forum ...

fluoronator · Sep 28, 2006

A contractor installed a piece of equipment which included a PLC. I was just reviewing/commenting their logic and found a single coil being written from multiple rungs, wasn't sure how this would work.

rsdoran · Sep 28, 2006

Are these coils Latch (OTL) and Unlatch (OTU)?

fluoronator · Sep 28, 2006

No, not latching, just -()-.

Sliver · Sep 28, 2006

Look at Ron's ladder signature.

You may get the reference.. To Be (2-b) or (logical OR) not (2-b).

If you had a coil at the far right (call it 'the question'..sorry had to.) you could say that the coil was being written by two rungs.

So now which run turns it on and which rung turns it off?

Brian.

fluoronator · Sep 28, 2006

Isn't that a different situation? In your example the "ON" state would always win.

DJM · Sep 28, 2006

I've seen it done with Set/Reset(latching) coils but never with a non latching output. While using multiple latches isn't my favorite method, I would consider what you are describing as being amateur at best! What would the justification be for this? If you have this many conditions required to turn on an output, use a continuation coil and finish on the next rung.

Using the multiple reference to a single output, the last one wins.

rsdoran · Sep 28, 2006

Are the rungs close to each other and/or in the same ladder? Or are they in separate ladders?

If in separate ladder or files then different conditions may control how the coil is activated.

Is is possible for you to create a report and print it to pdf so we can take a look too? You can get a free pdf program at www.pdf995.com.

fluoronator · Sep 28, 2006

Let me get this straight... in this situation (?) will always be on.
2-B
----+----] [----+----(?)-
| |
| 2-B |
+----]/[----+

In the next situation (?) will always follow the second rungs logic, the first rung does nothing.
2-B
----+----] [----+----(?)-

2-B
----+----]/[----+----(?)-

is this correct?

Clay B. · Sep 28, 2006

rsdoran said:
Are the rungs close to each other and/or in the same ladder? Or are they in separate ladders?

If in separate ladder or files then different conditions may control how the coil is activated.

Is is possible for you to create a report and print it to pdf so we can take a look too? You can get a free pdf program at www.pdf995.com.

Thanks for the pointer to the pdf software. I was looking for something like that.

Ron Beaufort · Sep 28, 2006

as you're quickly finding out, "BASIC" answers usually aren't much good at all in this trade ... EVERYTHING needs to be handled at the "SPECIFIC" level ...

I'll go along with the others who have suggested posting your program file ... the more we know about your system, the better help we can offer ...

fluoronator · Sep 28, 2006

Oops, I garbled the logic...
the first example is supposed to be OR logic and the second example is two seperate rungs.

rdrast · Sep 28, 2006

Yes, the last state will always win... The exception being if there are JMP/LBL instructions to bypass one set of logic or the other, or if they are in seperate conditionally executed subroutines.

In either event, it is in my opinion, an extremely bad practice to use such constructs as I have described above.

Ron Beaufort · Sep 28, 2006

let's set the "should you?" or "shouldn't you?" debate aside for a minute ... if the machine is running "ok" with the extra "coils" in place, then I'd be willing to bet (pocket change only) that each one of those "coils" is located in a separate subroutine/ladder file - and that only one of those files is being scanned at a time ... this is a fairly common approach in programming ... I'm not saying that it's a good practice or a bad practice ... but I've seen it used many times ...

we'll know more as soon as we see that program ...

Two rungs writing to one coil

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