designer

[DLTimmons]

I seeing something I not sure what to think of

if you have several rungs with only out put coils when you accept them they are merged with the rung above it. They only to stop is to leave a nop rung.

Am I thinking wrong here or do you have to have a control element in every rung?

Donnie

[BobO]

Stack based PLC...that's just the way they work. The merged result is the reality of how it executes. If you want something unconditional in a place that it can't be, I always use a contact with $On.

The only places that you can have no control elements in a rung is at the top of certain scopes...program blocks, task blocks, stages, loops, etc.

[franji1]

They only to stop is to leave a nop rung.

And you have to be careful with this one.  Technically, NOP means NUL-OPERATION or NO-OPERATION, hence

STR X0
OUT Y0
NOP
OUT Y1

when drawn, it looks like this:

Code: [Select]

.X0        Y0
-] [-----( OUT )

---------( NOP )

           Y1
---------( OUT )

Y1's state is NOT "ALWAYS ON" (even though it's tied to the power rail), but on the STATE of X0!!!

Do-more WARNS you with our Program Check utility:
W101 Rung mmm#rrr is missing input logic, powerflow state will be based on previous rung's power flow state.

see attached screen shot of Program Check output

[DLTimmons]

I'm not getting that warning with a check. Accept just shoves everything to the rung with the last control element.  Now that I know how it works I will put a NO element in every rung like that.



Just to many different PLC programs out there and everyone has something different.

[franji1]

I'm not getting that warning with a check. Accept just shoves everything to the rung with the last control element.  Now that I know how it works I will put a NO element in every rung like that.
Just to many different PLC programs out there and everyone has something different.

You'll only get the warning if you try your work-around and stick a NOP between the rungs.  If you DON'T stick a NOP, it WILL MERGE and will NOT generate that warning.

Oh, you also have to Accept the rungs F8 for Program Check to check the rung.

[DLTimmons]

That's going to drive the AB guys nuts in they use rungs without control elements all over the place. That is why I tried it over the last 12 month been doing more AB than AD I hope the Do-More changes that.

Donnie

[BobO]

It's not a matter of whether we'd like to, it's a function of the controller architecture. Stack-based execution just works that way. Koyo's CPUs are stack based and have worked that way forever. Texas Instruments (where we cut our teeth) was also stack-based. We'd love to change it...but we can't.

[PLCGuy]

Correct me if I am wrong, but if you organize your rungs with the uncontrolled elements first you should be fine. I put my uncontrolled rungs up on top then follow with others. Also using stage helps a great deal with this. I am doing a AB project, by force, and it is driving me nuts I can not use stage programming to separate out this condition. Get away from grouping all your rungs in one big heap and start to use stage.

[plcnut]

I try to put them on top, but if I need them lower for whatever reason, I use ST1.

[deep6ixed]

Wouldn't good practice be to always use ST1.  That way down the road someone who reads the project later can see that its meant to be on.

[ATU]

What I do for rungs that have to be on all the time is put them all at the top. However, I leave the first 10 stage numbers unused. Stage 0 only has variable initializations and jump instructions that turn on all other stages.  Then toward the end of the project, I look to see if there is any structure to those rungs that makes sense, then copy groups of them into Stages 1-10 setup as categories. Its more of an organizational tool.  You just have to be mindful to not put instructions in stages that shouldn't be.

[PLCGuy]

yes I sometimes use the ST1 but some I-boxes will not allow that.

[franji1]

yes I sometimes use the ST1 but some I-boxes will not allow that.

There are a small number of instructions/IBoxes that "always" execute (that is they are not dependent upon power flow), hence they MUST NOT have any input logic.

[Greg]

And, I believe that all those IBoxes have their titles and instruction names bolded & italicized and bolded respectively. e.g....