Ladder Requiring Coils on Every Rung

[Jieve]

Hello,



I recently have been programming an ABB AC800M PLC in ladder for a small project. Unlike some of the other PLC vendor software I've used, the Control Builder software for ABB requires a coil output at every rung. Up until now I've just been calling each one "Rung0001EN", "Rung0002EN", etc. as they are never used for anything else in the program, but I'm curious what others would do in this circumstance. Is there any real use to these, and how would you handle them (naming conventions, etc.)?


Thanks!

 

[Ken Moore]

TI required an output on every rung. I would designate a throw away address for the rungs where I did not care about the output. Something called "end of rung", and re-use the same bit, over and over.

 

[I_Automation]

For PLC's that don't have a NOP output I create a NOP tag. Everywhere I need the generic output I OTE NOP.

Also, if no AFI I create an always off tag and force it OFF every scan. If no TRUE create it and set it ON every scan.

For rungs (usually FYI rungs showing values I want to see in the beginning of the routine) I XIC the bits and EQU the number values to themselves, then OTE NOP.

 

[James Mcquade]

AB requires an output coil, timer, counter, math ststement, or some output function to end each rung. what I do is write the logic in order and give the coil name some name that makes since as to what the rung does.
james

 

[Ken Moore]

AB requires an output coil, timer, counter, math ststement, or some output function to end each rung. what I do is write the logic in order and give the coil name some name that makes since as to what the rung does.
james

James,
AB has "output" instructions, such as a move. In other platforms the move would be in the middle of the rung and and output coil that would energize when the move was completed. Often you did not care about the output coil, so you used the same one over and over.

 

[Jieve]

Thanks everyone for the responses.

James,
AB has "output" instructions, such as a move. In other platforms the move would be in the middle of the rung and and output coil that would energize when the move was completed. Often you did not care about the output coil, so you used the same one over and over.

This is exactly the case with the ABB, I've switched to your technique of just using the same coil over and over, makes things easier.

 

[Lare]

Thanks everyone for the responses.






This is exactly the case with the ABB, I've switched to your technique of just using the same coil over and over, makes things easier.

Use set or reset coil, so you don't get warnings of multiple use.
Named waste or trash bit / word?


Damn, From that I remember old plc codes where same bit was used over and over again for temp with reset on code. That way some valuable bits was saved, but troubleshooting was nightmare as same bit had true or false states demending of program flow.

 

[I_Automation]

Talk about duplicate outputs -

Just had to troubleshoot a processing line with a Siemen's S7-300.

EVERY function block checked M190.0 to see if it was allowed to run.

Over half of the blocks reset M190.0 if anything it checked was wrong.

Some blocks turned M190.0 back ON if they felt like it. So half the pumps and heaters were on.

PITA.

 

[rQx]

I encountered the same problem, so I changed programming language to FBD

 

[JesperMP]

I dont get it. What is the need for ladder rungs without outputs coils or other assignment statements ?
Only exception I can imagine is if you want to add rung comments without there actually being any code assigned to the rung. But that is easily fixed with a dummy coil.

 

[I_Automation]

In ControlLogix when you call an AOI (Add-On-Instruction) it is placed in the rung to the left where all the checks are - there has to be an output on the rung. Fortunately there is a NOP command for this.

If there wasn't the NOP to put there, as some platforms don't have it, you would have to put OTE Junk_Bit

 

[JesperMP]

I dont use Controllogix, but it sounds weird to me.
Why isnt the AOI an output instruction ?
What if you call another program, but it is not an AOI, do you still have to place an output coil ?

edit: I dont know AOI, but maybe there is a something similar to Siemens S7 FCs where there is always an output of the block that must be assigned inside the FC. In Siemens S7 you are not forced to actually use this output variable at the call of the FC, i.e. to assign the output to a variable. But maybe you have to in Controllogix.

 

[ohnedich6]

In ControlLogix when you call an AOI (Add-On-Instruction) it is placed in the rung to the left where all the checks are - there has to be an output on the rung. Fortunately there is a NOP command for this.

If there wasn't the NOP to put there, as some platforms don't have it, you would have to put OTE Junk_Bit

Just a slight correction. AOIs that are called default to the right side of the rung and do not need output instructions to "complete" the rung.

 

[daba]

Just a slight correction. AOIs that are called default to the right side of the rung and do not need output instructions to "complete" the rung.

I concur, an AOI is an OUTPUT type instruction, so there is no need for anything else on the rung.

I dont get it. What is the need for ladder rungs without outputs coils or other assignment statements ?
Only exception I can imagine is if you want to add rung comments without there actually being any code assigned to the rung. But that is easily fixed with a dummy coil.

My thoughts exactly, why program a rung that does nothing at all, and if you need to put a rung in just for you to attach a rung comment, a simple NOP or an OTU Dummy_Coil will do the job.

However, I have seen code where they have grouped conditional bits on one rung with a NOP as the output, and I have assumed it was put there for de-bugging purposes, i.e. an animated bit monitor. But at the end of the day, the rung is still being evaluated, so is taking up processor scan time, and memory.