What kind of problems can happen when using the same coil mor then once in a program

Was just wondering because I seen a PLC program that uses the same coil like 15 or more different rungs. The software said that malfunctions can happen when using the same one more then once.

Greg7683 said:

Was just wondering because I seen a PLC program that uses the same coil like 15 or more different rungs. The software said that malfunctions can happen when using the same one more then once.

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It would depend on the use of the Out (coil).

Are the 15 different rungs using Out or SET/RESET instructions? (This nomenclature would be different for different software).

Are the 15 different rungs all being scanned continuously or perhaps is there 15 subprocedures that are conditionally called and only one would be called a t a time?

Typically if using an out instruction, the last instruction wins.
So if you have 14 rungs of very complex ladder with 14 out instructions for the same address, then on the 15th rung you did this:
1 2 MyBit
-----|>|-------------(out)

MyBit would be false at the end of every scan.

Standard ladder is if-then-else. WHere the Else for a normal OUT is always false.

There is a difference from coil and output coil... just keep in mind that the last one wins (most PLC's) so using a output more than once is not recommended but you can have several contacts that can energise the output

Are you asking about a contact or output ? Do you have a example of the program? zip it and post it or a screen shot would help

What I always do, and is easier to troubleshoot, is have each rung turn on it's own "xxxxModeCallForOutput" bit then at the end check all the bits and turn on the output if any of the conditional bits (and their modes) are true.

That way you can see what mode or subroutine is calling for the output or not.

Some were called at the same time and some at different times when I would monitor the program I ask because sometimes the robots would seem to like get confused and do odd things like push a tray in to a wall or on the floor.

geniusintraining said:

There is a difference from coil and output coil... just keep in mind that the last one wins (most PLC's) so using a output more than once is not recommended but you can have several contacts that can energise the output

Are you asking about a contact or output ? Do you have a example of the program? zip it and post it or a screen shot would help

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No do not think I have a copy I would have to check and they were output

It's not guaranteed to cause a problem if it's programmed exactly how it needs to be, but because of that, it's not considered a good practice.

You can get flickering outputs if rungs aren't arranged exactly correct as far as the scan order is concerned. We have several SLC programs running here in the shop with duplicate destructives (multiple of the same outputs) that don't cause issues, but it gives me a facial tick whenever I have to work on those programs.

Ive had a problem before where a solenoid valve was operating erratically when it was supposed to be ON.

What I had found was; someone had written to the output in 2 different cyclic periods- so it was being written a 1 but then cleared down on the 10ms scan, was very difficult to find and very frustrating.

A perfect example of why it shouldnt be done....


Regards

Daniel

as stated, if the program is done correctly, nothing.
then you modify the program, and the system goes sideways.

multiple outputs of the same address is bad practice.

i spent 2 days at a machine with another programmer looking at the program online. we then had to spend a week looking at thee program at our desks that had 14 b3/86 outputs that stopped the program. the machine ran 24/7. we then went back to the machine and still couldn't figure out what was going on.

we ended up re writing the program.
downtime went from 20-30 hours per week to less than 5.

james