RSL500 Allows Shorted Branches???

[Aabeck]

I got called to do some updates on a pump control panel for a waste treatment system and found a few lines in the running PLC (SLC5/04 1747-L542B) with shorted branches going around the low level floats that should shut off the pumps & mixer.

When I try to verify the file it shows the 4 errors of shorted branches, so why & how would it let the file be downloaded to the PLC, I didn't think this could be accomplished with online edits either. (Does the PLC go through the branch if the float opens? It must as you can see in the attached picture the low level float is off, but the mixer is still running.)

With the output holding itself on & the low level float bypassed this mixer shown would never turn itself off, even if the Messaging PLC dropped the bit or the messaging watchdog timed out?

This project has 3 pumps written the same way, also.

 

[RoTaTech]

As drawn, the mixer will be started once, and then never turn off (until the PLC is put into Program Mode , or turned off).

 

[nehpets]

Both RSLogix500 & 5000 allow shorted branches, the verification shows it as a warning but will still download and operate regardless. The shorted branches can be programmed in within online edits.

These warnings can be supressed in options.

Steve

 

[Mickey]

Both RSLogix500 & 5000 allow shorted branches, the verification shows it as a warning but will still download and operate regardless. The shorted branches can be programmed in within online edits.

These warnings can be supressed in options.

Steve

Ditto.

People do this for troubleshooting purposes. And unfortunately forget to take them out.

 

[Aabeck]

Thanks for the info guys. I've never come across this, or tried this in 30 years of programming PLC's.

I just tried this on my test 5/05 here & the online edit verified & wrote a shorted branch without any error or warning & let me test & assemble edits with no problem. Then I toggled the test bits & the branch did bypass the instruction (for testing I always branch to a XIC Always On For Testing bit to bypass something.)

I still don't think the software should allow this (I seem to remember working on PLC 2's in the 90's that would not allow this - or am I mis-remembering again?)

 

[Mickey]

verified & wrote a shorted branch without any error or warning

You should have gotten a warning.

 

[Aabeck]

Mickey,

I just went in & tried it again, also checked the options & Verifications / Show Warnings is checked, but it shows no warning & assembles a shorted branch fine. I'm using RS500 ver 9.0.

 

[Doug-P]

I still don't think the software should allow this (I seem to remember working on PLC 2's in the 90's that would not allow this - or am I mis-remembering again?)

As far as the PLC-2 thing, I seem to remember that the software (UPDOC) would allow you to enter, and the processor would correctly execute, an open branch. It was normally used as a diagnostic/testing mechanism.

 

[Mickey]

Mickey,

I just went in & tried it again, also checked the options & Verifications / Show Warnings is checked, but it shows no warning & assembles a shorted branch fine. I'm using RS500 ver 9.0.

HMMM?

My ver 9.00 gives a warning. Click on "verify project"

 

[Mark-]

Hello,

As drawn, the mixer will be started once, and then never turn off (until the PLC is put into Program Mode , or turned off).

Or the output is forced off, right?

 

[PLC GURU]

unlatch as long as it later in the ladder

 

[Aabeck]

Mickey,

If I verify the file or project while offline I do get that error window - but if I add the line in an online edit it doesn't give any error.

Mark & PLC Guru,

The output is neither forced (that would take being online with RSLogix anyway) nor are any of the 4 outputs programmed this way controlled in any other line of the program, on the bit or word level.

 

[redbarron55]

This is often done in troubleshooting and often while trying different logic, not necessarily in the same logic setup.
If done inline then it is probably allowed since the programmer is obviously doing ig for a reason (perhaps). It could have been done to turn the pump on and was an oversight and not removed after the need was gone.