SLC5/03 help

I have inherited and must modify a set of programs written by someone else who is no longer available. In the first 2 rungs of all of his programs, he has the following:


There are no other references to these bits anywhere else. I have no clue what ATI and AFI mean (I am working with an old manual for RS Logix 500). It makes no sense to me, and in particular Rung 0001 seems like a waste or a mistake. However being that he seemed to think this was important enough to have in every main ladder program, I thought maybe it is some sort of routine procedure which I am unaware of that may look familiar to others. Any ideas?

Please be gentle, I haven't programmed a SLC in a long time.

It looks to me that the original programmer was setting up an "Always True Instruction" and Always False Instruction" probably for troubleshooting.

Joe

Joe Loescher said:

It looks to me that the original programmer was setting up an "Always True Instruction" and Always False Instruction" probably for troubleshooting.

Joe

Click to expand...

Bingo, and this is very common. I use it too (sometimes more than one), either as a placeholder that I know needs to be replaced/updates at a later date, as a marker for edits, or to jump/block some code without losing the history of what was there before I changed it.

Thanks for the fast responses, excellent. That explains the "illogic" of Rung 0001, it's supposed to always be false then.

So what does it do to help with troubleshooting? Is it a way of seeing if memory got corrupted or something?

I just use an ALWAYS_FALSE_BIT by picking a bit say B3:0/0 and I have a single unlatch function in the last rung of the Main Ladder file (file 2) with that address. i.e. OTU B3:0/0.

I use this instruction to bypass logic and also to prevent logic from executing that I may want to use later. For example, during troubleshooting I may find a a section of logic isn't working but I don't want to completely discard it yet. So I'll put an XIC of my ALWAYS_FALSE_BIT in front of the logic to temporarily not use it. Similarily I may have an output with certain conditions that I want to temporarily bypass so I will branch an XIO of my ALWAYS_FALSE_BIT around the logic.

I also use my ALWAYS_FALSE_BIT for "spare" logic. For example I will have my fault logic set-up to accept 30 faults but I'm only using 20 faults. Instead of deleting the 30 I don't need right now, I will use my ALWAYS_FALSE_BIT to prevent those unused fault bits from turning on. Later when I need another fault the logic is already there; I just remove my ALWAYS_FALSE_BIT and add the appropriate logic.

Wow, excellent strategy. That explains a lot. These programs have a lot of Jumps and interlocks, so having these available was probably his way of being able to easilly execute or stop executing sub routines in order to isolate problems.

Fantastic help, thanks a lot.

I use something similar that I call a "Remove Me Later" bit. Has the same effect as AFIs and ATIs but I never forget to remove the bits when I'm finished testing.

Another tip you can use with AB: if you are making certain changes to a program and you want to be able to go back and forth between what you originally had and your changes, put in a single bit with XIC instructions for the original and XIO for the edits. Then, by toggling the bit you can quickly switch between original and edits.

I often use always on and always off bits for the same reasons above.

They are handy for trouble shooting especially at start up.

Thanks everyone.