Studio 5000 Latch Coil does signal pass thru

[stayclashy80]

So I'm having issues with a certain rung and one of my coworkers mentioned it may not allow the signal past the latch coil. For example in the picture if the bottom branch with the latch coil is true will it also turn on the end coil called cfg_VFD.PCmd_Stop?

 

[drbitboy]

Only if the input rung to, not the bit operand of, the OTL is True. See documentation below (from here).

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[stayclashy80]

ok so thats how i thought it worked, we cant figure out why the coil is not turning on when the bottom branch is all true. i should have attached this screen shot first. my thought was something is overwriting the coil or one of the bits is changing state so quickly we just cant see the graphical representation of the coil turning on. I was thinking of running a trend to catch and data that's happening too quickly to see on the graphical representation. Any ideas?

 

[plvlce]

my thought was something is overwriting the coil or one of the bits is changing state so quickly we just cant see the graphical representation of the coil turning on.

These are indeed the things you should start by checking for -- what you see is not necessarily the state of the bits at the time that specific rung ran. Run a cross-reference on your OTE bit -- is it used in any other destructive (ie output) instructions? If nothing else is setting it then one of the XIC/XIO instructions must not be true at the time the rung is scanned; if you can't figure out from looking at the logic why one might be off then you can add troubleshooting bits immediately prior to the rung to check nd see which one is preventing the OTE from coming on.

 

[Steve Etter]

If you haven't already, do a cross reference on you Cfg_VFD.PCmd_Stop tag to make sure you don't have a redundant coil somewhere.

 

[stayclashy80]

dang ok, yeah I've done all my normal research for anything overwriting but still getting stumped. i guess i need to setup a trap or a trend to try and figure it out. First I wanted to make sure the signal does pass thru the latch coil to turn on the end coil. thanks everyone as always for your help

 

[Steve Etter]

As a sanity check, write another line of logic and move the Cfg_VFD.PCmd_Stop tag to it. See if it comes on that way.

 

[stayclashy80]

the top branch does turn the coil on so i think it has to be something with the bottom branch bits messing it up

 

[drbitboy]

As good as or better than trending for diagnostics:

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[I_Automation]

the top branch does turn the coil on so i think it has to be something with the bottom branch bits messing it up

As mentioned earlier the bottom branch its might not all be TRUE during the scan of that rung, but coming on later and then showing as TRUE in Studio5K


After the latch add OTE TestBit1, then add a rung before this rung of OTU TestBit1 to tun it off every scan before this rung (hopefully) turns it on.


If it (TestBit1) doesn't come on then the branch isn't true, if it does show on then something else is shutting off the output.


As another test after the output add OTE TestBit2 with another OTU on the previous rung.

 

[Operaghost]

...in the picture if the bottom branch with the latch coil is true will it also turn on the end coil called cfg_VFD.PCmd_Stop?

So to get back to the original question...Yes it would turn on. Here is an example of how this logic would flow. I should have put the right-most AND higher to show it affects both the top and bottom steps in the branch.

OG

 

[stayclashy80]

oh nice, i like that diagnostic logic Drbitboy, I'm gonna try that out. Thanks everyone!

 

[cardosocea]

So to get back to the original question...Yes it would turn on. Here is an example of how this logic would flow. I should have put the right-most AND higher to show it affects both the top and bottom steps in the branch.

OG

How does that rightmost AND work?? Wouldn't that not turn on the coil?

 

[parky]

I suppose it depends how RW treat a coil i.e. does it have an effective out connection or does it treat it as a cold side connection i.e. common.
This one does as expected.

 

[I_Automation]

How does that rightmost AND work?? Wouldn't that not turn on the coil?

The right BND (BranchEnd) would logically be an OR



[Top Branch] OR [Bottom Branch] OTE


In RSLogix5 & RSL500 if the top branch was TRUE it would NOT scan the next branch - just bypass the NXB and jump to the BND, but in 5K it does scan all instructions even if the lower branches are redundant because it already had a true branch.


EDIT:
Parky,


RW scans exactly as your example, so something else is going on after this rung is scanned.