Always on bit on AB

Hi

On Omron i would usually use an "always on" bit infront of these moves

My question is, what is the equivalent on AB, or do you just skip them?

The AB PLCs support unconditonal rungs, therefore no bit is necessary on those rungs and inclusion of one would be superfluous.

The always on bit is necessary in Omron PLCs (and a few others) because they do not allow unconditonal rungs. In those PLCs you must have a state to LOD first to begin a rung.

That said, some AB programmers prefer to create their own always on and always off bits. Ironically, you would programatically set these bits with an unconditonal rung. Personally, I don't see the need for them except for debugging. Once the program is debugged an always on and always off bit are unnecessary. [I'm putting on a flame proof suit now]

Okay, thank you very much.

And gl with your suit

Always on and off bits are great for debugging. I always use them in alarm circuits until things are sorted out.

Removal depends on confidence level of your programming and laziness. Probably more are left in because of the later.

How many times have you come across a 3rd party program that has a contact/rung/section that doesn't make sense or do anything? See above.
In the old days with lack of memory (silicone based) a programmer had to be efficient. Now with plenty of memory and speed available you can get sloppy.

John Soltesz said:

How many times have you come across a 3rd party program that has a contact/rung/section that doesn't make sense or do anything? See above.

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What do you mean that it doesn't do anything?

That was strange. I just did a reply and it went to a login screen.

Sometimes a programmer will test a circuit and find that it doesn't work. He finds a solution and forgets to remove the parts that don't work or he was using for test. I have seen this many times in the last couple of decades I have been programming. There will be a rung using I/O with nothing wired to them. So the rung does nothing but take up space and scan time.

Ok, i see. I just didn't understand the "See above" comment, and thought something was wrong with my MOV'es

Alaric said:

[I'm putting on a flame proof suit now]

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What are you saying Alaric

The reason I do use them in AB's are not only for trouble shooting but also to keep all of our programs similar... AD's, Mitsubishi, and Siemens use a 'bit' in front of the end result, so I try and keep them uniform

I guess platform to platform consistency might be one valid reason.

For debugging I actually use a sequential series of bits that are just for that purpose, usually one or two reserved words. I can set/clear the bits in the data table as needed to do the debugging and don't specifially treat them as always on or always off bits. Once thats done I remove them from the code by simply searching on the reseverd word addresses.