BillRobinson
Member
I'm looking through a piece of ladder code put out by Allen Bradley and it's got NOP's all over the place. Is this just left over code from debugging or does is actually do anything in a ladder code?
Clx - nop
- Thread starter BillRobinson
- Start date
mellis
Member
NOP = No Operation
It's an instruction guaranteed to do absolutely nothing.
Which can be a good thing, if you want a rung to hang a comment on. Beats keeping track of dummy bits.
It's an instruction guaranteed to do absolutely nothing.
Which can be a good thing, if you want a rung to hang a comment on. Beats keeping track of dummy bits.
TConnolly
Lifetime Supporting Member
The NOP is also handy when you want to pass parameters into a subroutine but don't want to have to hang output logic on the end of the first rung after the SBR instruction.
Operaghost
Member
So as you can see by the comments already, there is a use for the instruction. But your suspicion of left over code is certainly possible as well.
OG
OG
IMHO that is the only place I want to see a NOP
The SBR instruction is required to accept parameters passed to a subroutine, and is classed as a "Program Control Instruction".
I do not want to mix "Program Control Instructions" with application instructions, so I always put a NOP as the output instruction on the first rung of a subroutine file which uses an SBR - keeps everything in its proper place......
The Plc Kid
Member
@ Alaric and Daba
Can you guys give a example. I have never had to pass paramets into a sub. Where and why would you need to do this. I have only had experience using subs to keep my programs organized and when logic is only needed sometimes.
Sounds interesting but i am lost?
Can you guys give a example. I have never had to pass paramets into a sub. Where and why would you need to do this. I have only had experience using subs to keep my programs organized and when logic is only needed sometimes.
Sounds interesting but i am lost?
TimWilborne
Lifetime Supporting Member
If I'm planning a big change that must be done online, I will use NOPs as place markers. Otherwise passing parameters to subroutines is the only real use I can think of
Why use NOPs as place markers ?
Why not create a BOOL tag called TWControls or your name, put a rung at the bebinning of the code to OTU that tag, and use it as a AFI (use XIC) or AFT (use XIO). You can always cross-reference them instead of having to search for NOPs. And what if someone else puts a NOP in the code ???
And The PLC Kid - very often you will want to use a subroutine multiple times, with different data, you can pass Input parameters, and get back Return parameters - example : You write a subroutine to calculate the area of a circle (i know, easy, but this is an example), you will pass into the subroutine the radius or diameter, and get back the calculated area.
The use of subroutines in this way is being made obsolete in Logix5000 Vers. 16 and up by the Add-On Instruction feature.
Ron Beaufort
Lifetime Supporting Member
here's a quick down-and-dirty example that I use in some of my classes ... this should be enough to give you the basic ideas - but ask if you have additional questions ...
first the "subroutine calls" ...
(continued in next post) ...
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Ron Beaufort
Lifetime Supporting Member
and now the subroutine itself ...
and incidentally, the platforms that use RSLogix500 can't "pass parameters" this way ...
the next step of "elegance/complexity" might be to use Indirect Addressing to change the locations of the "passed parameters" - so that you'd only need one "calling" JSR rather than three ...
of course the more "elegance/complexity" you add, the harder the code gets for the mere mortals to troubleshoot at 3:00 o'clock in the morning ...
and incidentally, the platforms that use RSLogix500 can't "pass parameters" this way ...
the next step of "elegance/complexity" might be to use Indirect Addressing to change the locations of the "passed parameters" - so that you'd only need one "calling" JSR rather than three ...
of course the more "elegance/complexity" you add, the harder the code gets for the mere mortals to troubleshoot at 3:00 o'clock in the morning ...
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Alan Case
Lifetime Supporting Member
I always use an NOP on an otherwise empty first rung of every subroutine.
The comment on this rung details what the subroutine does. Not in exact detail (the individual rung comments are for this)
For example.
SUBROUTINE PURPOSE:
To control and monitor the Waste Water Pump 5 (WWP-5)
This is a VFD controlled 3kw pump that transfers the raw sewage from the main holding tank to the aeration pond.
This little bit of info is so handy later on for either you or whoever follows you.
Regards Alan
The comment on this rung details what the subroutine does. Not in exact detail (the individual rung comments are for this)
For example.
SUBROUTINE PURPOSE:
To control and monitor the Waste Water Pump 5 (WWP-5)
This is a VFD controlled 3kw pump that transfers the raw sewage from the main holding tank to the aeration pond.
This little bit of info is so handy later on for either you or whoever follows you.
Regards Alan
TimWilborne
Lifetime Supporting Member
Since you brought up the what if...what if someone deleted the OTU rung thinking it had been added for some debuging that was no longer used? Then some other person comes along and wonders what the BOOL tag TWControls does and toggles it? NOPs just seem safer to me. More than anything they are just in case someone else adds a rung and throws off my rung numbers in my note by one or two or twenty. Still nothing long term, a week at the max probably
Tag it as "TWControls_DO_NOT_DELETE" ??
One reason I advocate the use of bespoke tags for edit markers, false instructions, true instructions, etc. is that I have worked on projects that had multiple engineers writing code, simulating, and commissioning.
I made them use a tag named with their initials so that they could have their own "markers", and I could police that they were resolving issues by removing them. A quick x-ref on these tags easily showed their progress.
I banned the use of AFI's and shorted branches completely.
I made them use a tag named with their initials so that they could have their own "markers", and I could police that they were resolving issues by removing them. A quick x-ref on these tags easily showed their progress.
I banned the use of AFI's and shorted branches completely.
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