*Onsite right now* RSLogix500, Unlatch with no contact on the rung???

[plvlce]

"Find All" for that all 3 returns three incidences... A latch, an unlatch, and XIC.

All 3 have an unlatch without a contact on the rung.

This sounds to me like someone trying to force an OTL to emulate an OTE. As long as the bit is only evaluated after the OTL and before the OTU (which from the picture is the case), it will be seen as true when the OTL rung conditions are met and false otherwise (exactly how an OTE works).

No idea why anyone would program it that way instead of just using an OTE, but it would work.

EDIT: The one difference from an actual OTE I can think of would be on transitioning to run -- it would not be forced off during the prescan

 

[celichi]

I'm troubleshooting a machine for a customer right now, and the output that is not working needs this bit to go high to activate the coil. But the unlatch coil doesn't have a contact before it. How does it ever latch???

This looks to be an Auto Request to Extend in Auto.

Having an unconditional unlatch is a way of doing a one shot request.

If that bit is on, and the device is in AUTO, it would latch up the Extend output and indicator, and then the auto request bit is cleared.

If the PLC is scanning this rung, and there is an auto request to extend, and the device is in manual, the auto request bit is cleared so that it is not "hanging around" when the device is put back in Auto.

 

[AutomationTechBrian]

Having an unconditional unlatch is a way of doing a one shot request.

Ok... that's a good thought to chew on. I'll look into that.

 

[mhertwig]

To me it looks like this is probably set before rung 66 and the unlatch on 68 is to reset B10:5.14 in case the set condition drops out. what is the set condition? What is the OTL? Could also be HMI as others have mentioned. You should be able to go live with the PLC and see what condition is missing.

 

[arpus4KM]

the bigger question might be, what were you called out for that requires knowing why this will/won't latch?

did you ever trace where B10:5/14 is coming from?

like it was stated, if you make the bit true, it will be true for one scan, then unlatch after that. a one shot.

if you want to see if it's actually turning on, add a counter or something in to the program and then activate b10:5/14 again.

 

[AutomationTechBrian]

the bigger question might be, what were you called out for that requires knowing why this will/won't latch?

I was called to assist their in-house PLC tech in figuring out why the machine won't complete a cycle in "Auto".

 

[arpus4KM]

I was called to assist their in-house PLC tech in figuring out why the machine won't complete a cycle in "Auto".

sorry, I guess I didn't word it right,

but what has happened since it was working correctly.... they said nothing has changed and it just suddenly wouldn't run in auto I assume then?

Also, you don't need a latching output bit if the actual valve does not have any kind of spring return to center position on it. all it takes is a momentary on state to kick the spool over, and there it sits until commanded again.

 

[AutomationTechBrian]

Yes, that's right.

 

[arpus4KM]

Yes, that's right.

so can you command the bit on with some kind of "auto run" button on the hmi or panel? does the valve match the type with no spring return? 2 position 2 way, or 3 position 2 way with no return?

 

[sharkyh2o]

If b10:5/14 latch is after the unlatch, it will function and always unlatch when the rung that latches is not true. Last scanned state wins. Is it on the rung right below? Should have been written with just an OTE instead.

 

[arpus4KM]

If b10:5/14 latch is after the unlatch, it will function and always unlatch when the rung that latches is not true. Last scanned state wins. Is it on the rung right below? Should have been written with just an OTE instead.

there probably isn't simply because the OTU is sitting there by itself.

I've seen programming from co-workers who have found it's best to unlatch any kind of input that could get stuck on and keep a machine running, whether a panelview gets stuck with it on internally, or physically stuck on. It's basically just a oneshot. which means the process stays in that position a different way.

 

[sharkyh2o]

OP states that there is a latch in post 5

 

[ASF]

Usually when I see something like this it's because the latch bit is set during a sequence. The sequence sets an OTL, and each time the logic is scanned during that step, it will open the valve because while that rung is scanned the auto open bit is true. Two rungs later the auto open bit is false, but it doesn't matter by then - it's been received and acted on and is now irrelevant.

As soon as the sequence moves on from that step, it stops latching the bit, and it will remain unlatched.

There are definitely better ways of doing it, but it works.

 

[arpus4KM]

OP states that there is a latch in post 5

Thanks, I didn't see that.


seems to me like there's not a lot that can be done unless he shows a significant amount of the program, or can give some more detail about it and what's been checked.

 

[AutomationTechBrian]

Just a quick note as I start preparing for tomorrow morning...

I'm going to take a little time reviewing the rungs that control the cylinder outputs. I'll probably put together a couple of custom data monitor files to utilize. I also need to review the "Trend" function to see if it works with discrete, one-shot captures... it's been a very long time since I've played around with that.

This program is surprisingly large and complex for the size of the machine. I'm trying to focus on the I/O because 90% of the issues seem to come from issues like a bad sensor or solenoid. This machine has an earlier model right next to it, and I've been called in twice in the past to troubleshoot that. One time it was a prox sensor that got moved out of place, the other time it was a stuck pneumatic actuator.

A lot of things you guys suggested I already know, so I haven't responded to everything. The unconditional unlatch confused me, though. I still need to process how I want to test it... maybe add a rung right below the latch rung with a counter. Or maybe the trend function would capture the quick one-shot. One thing I know almost for certain... it doesn't come from the HMI. It's a repeating function of the program's cycle. One component thrust into position with the middle solenoid, then the left and right cylinders push components to either end of that component and the shaft holding those parts spin the components into the threads of the center component.

I'm at home, and not under the gun right now. So if I see something that needs to be talked-through, I'll post a screen capture here. Thanks for the input so far. It's helpful.