ONS and OTU instruction

I have a question about using a one shot bit in front of a unlatch instruction. Is this ok to do? I typically put them in from of the latch instruction , but was told its not a good idea to use in front of the unlatch instruction. I noticed when I had a ONS in front of the OTU the OTL instruction would some times not unlatch. I m trying to find a way to use the unlatch bit for one rung pulse. Because this rung stays true through a sequence. Thx

As long as you aren't using the same tag as the one for the OTL bit, you should be fine.

Can you post an example?

Exactly. Make sure each ONS has it's own unique tag assigned to it. If you do that, then there is no issue with using the ONS and the OTU together.

OG

I saw a situation recently that failed to unlatch a bit on power up. The latched bit was on when a PLC 5 had power restored.
Rung 1 XIC ONS OTL
Rung 2 XIO ONS OTU

Same input for XIC and XIO, different bits for ONSs and same bit for OTL and OTU.

In my case, it would be unusual to have power fail while the input was true, so it was not an issue usually.
The problem is the Pre-Scan sets any ONS bit true, so the XIO can't trigger the one shot to unlatch the bit on the First-Scan.

mavrick said:

I m trying to find a way to use the unlatch bit for one rung pulse.

Click to expand...

See attached 2 rungs.

I would do an unconditional unlatch, and do the latch with your condition and an ONS. This gives you just under 1 PLC scan, but all of your other logic should see Thisbit on for just one scan

That is what I think you asked for - was I right?

Mike Nash said:

I saw a situation recently that failed to unlatch a bit on power up. The latched bit was on when a PLC 5 had power restored.
Rung 1 XIC ONS OTL
Rung 2 XIO ONS OTU

Same input for XIC and XIO, different bits for ONSs and same bit for OTL and OTU.

In my case, it would be unusual to have power fail while the input was true, so it was not an issue usually.
The problem is the Pre-Scan sets any ONS bit true, so the XIO can't trigger the one shot to unlatch the bit on the First-Scan.

Click to expand...

You are totally missing the point of the the OTL and OTU instructions.


They are classed as "retentive" instructions, and the state of the bits associated with those instructions are NOT affected by the pre-scan. If you specifically require a bit that has been OTL'd to be turned off if the processor enters the Run state, then you have to write code to do it, during the First Scan.


ONS bits are deliberately set during pre-scan so that the logic has to see a false to true transition again before affecting any following output instructions.

Mike Nash said:

In my case, it would be unusual to have power fail while the input was true, so it was not an issue usually.

Click to expand...

Wow !! You have the ability to predict when the power will fail ?? ! ??

Thingstodo that is similar to what I’m doing but I would have an input before the otu instruction with a ons also before otu instruction .
So my sequence goes latch a valve
Go through a cooling process
Unlatch the same valve
Hold for a minute
Then latch the valve
Once sequences are finished all is reset. But inputs stay true throughout process, until reset

daba said:

You are totally missing the point of the the OTL and OTU instructions.

They are classed as "retentive" instructions, and the state of the bits associated with those instructions are NOT affected by the pre-scan. If you specifically require a bit that has been OTL'd to be turned off if the processor enters the Run state, then you have to write code to do it, during the First Scan.

ONS bits are deliberately set during pre-scan so that the logic has to see a false to true transition again before affecting any following output instructions.

Wow !! You have the ability to predict when the power will fail ?? ! ??

Click to expand...

Sorry for quoting all of it, but... it wasn't my program and has been in operation for at least 6 years this way based on file dates. I don't know precisely what was being done when the power anomaly occurred, I just happened to be the guy that saw what had happened.

And yes, I "get" latches and retentive. I was pointing out that you can't assume behavior based on what another PLC would do. And I have seen mention that even Rockwell's description of Pre-Scan actions on the various one shots is not always correct.

I had initially decided not to post on this thread at all, but since I had seen this just last week, and was able to find it quickly because I spend a lot of time learning from these forums, such as here: http://www.plctalk.net/qanda/showpost.php?p=30622&postcount=23
I overcame it.

I'm sorry to have ruffled feathers.