Proficy POSCON issue (Emerson PLC) PAC software

[junkyardgary 0]

 I have been troubleshooting another engineers logic. There is a couple lines that have real world inputs, IE: I00100 and I00101. Each has it own rung, these inputs are POSCON and feed a reset coil each. One line works fine, one does not. I have verify the inputs are transitioning from off to on, but one reset works, and the other does not. 

The processor is a CPE-100. Is there a problem using real world inputs with the POSCON instruction? The results are very reliable, the same issue happens every cycle, never any variation. I'm just trying to make sense out of it before changing a "standard" logic program, without explanation. 

 I am a hired gun engineer here for a few months. Don't want to make unnecessary waves. 

 

[Steve Bailey 118]

I'm not aware of an y issues using a variable associated with an input module on a POSCON instruction. Could there be something else in logic continuously making the variable associated with the RESET instruction true? Is the rung showing weird behavior in a program block that isn't being called?

For debugging purposes, try adding a counter in parallel with the reset coil instruction and see if it increments with each transition of the input point.

[junkyardgary 0]

 Turns out there is a bug with the PAC using "one shot" type instructions fed by real world inputs. I used an intermediary M1500 bit from the I00100 input, then used the M1500 to feed the one shot, everything worked fine. Apparently the "I" inputs do not solidly transition from off to on in the scan, like an internal bit. I can show you logic where the input appears on and off on the same page. Sometimes they work fine, sometimes they refuse to switch. Other than single scan instructions, the inputs seem to work as expected. Tribal knowledge, I guess.

 Perhaps if I used a positive transitioning output POSCOIL, instead of a POSCON it may have worked fine, but I'm not going to rewrite it for giggles, need to move on. Perhaps its a certain PLC, or who knows why some work, some don't. 

Not a fan of the Proficy/PAC software in any event. I can see why so few companies use it.