AB SLC 505 First Scan Bit

I just changed out an AB 5/04 to a 5/05 on one of my lines. I had the program all configured offline, loaded into a new 5/05 chasis, powered down the rack, swapped processors, and then started back up. I even went to program mode and then run mode.

Things looked like they were going OK except all my products were rejecting. Since I really didn't change the ladder, I thought of a BTW or MSG that I missed was doing something. Well after about 10 minutes of pulling out my hair (along with the production manager wanting to know how long), I found in my logic that the first scan bit was on and staying on. This was causing my product to reject since I clear the conveyor lines on a startup of the PLC (so nothing is lost or misrouted).

I have never seen the first scan bit just stayed on. I had to toggle it to get it to clear. A search has turned up no similar posts. Has anyone seen this or is this a known feature or a certain firmware?

I would search on the Rockwell site, but I can't get into right now.

I don't see an issue with this in the Knowledgebase.

I have never seen or heard of such a thing, but I wonder if it is possible to toggle on that bit after first scan...would the system just leave it on? Could a glitch in comms write a 1 to that address and leave it there?

Can you address that bit with an output instruction ?

I have never seen or heard of such a thing,

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me either ...

I wonder if it is possible to toggle on that bit after first scan...

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yes, it is ...

would the system just leave it on?

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no, I just tested it in the lab and it appears that it automatically gets turned OFF at the end of the scan ...

Can you address that bit with an output instruction?

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yes, you can ...

Just to make sure since it hasen't been explicitly mentioned, Bruce you are talking about S:1/15 correct?

There's also a note that "This bit is set during execution of the startup protection fault routine. Refer to S:1/9 for more information". Could there be anything there?

Just to make sure since it hasen't been explicitly mentioned, Bruce you are talking about S:1/15 correct?

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excellent point, Bernie ... let's make SURE that we're all talking about the same thing ... I've seen a few people program a "roll your own" first scan bit in the past ...

Has anybody ask him to post the program yet ? If not post it and tell us the location of your first scan bit.

Another possibility is that comms were lost and the bit isn't really still on, but Bruce would have probably noticed that.

Bernie's info about the Fault routine makes me think that is possible, that the fault routine is repeatedly running and turning on the bit. I have so little experience with Fault Routines, I am not sure this is possible without hitting a scan time watchdog error.

Could the fault routine clear the error, then it gets set again at the end of scan (along with the watchdog timer) and then errors again on the next scan?

Mystery Solved

OK, slightly embarrassing here, but Bernie and Ron sent me to the right direction. I am posting the part of the code that was involved for all to see (including a search for the correct first scan bit - in a small bid to try and redeem myself).

Now to give some more background information, this was a 5/04 programmed about 13 years ago and all I changed was the comms because of the processor type. I guess 13 years ago, I (or someone else very early on) fat fingered the first scan bit on this rung. You can see that I have plenty of other rungs where I used S:1/15 but on this rung I somehow used S:2/15. I'm not sure how I've missed this all these years, but I definately did. I guess I even typed over the default description somehow because this is a comms bit. Not sure why, but it's been working for quite a long time.

Now I need to check out my other 100+ 5/04s and see if I copies this problem.

Thanks to all for making me look again to show what was happening. Standing there with the entire factory down trying to figure out what was happening made me put blinders on and completely miss the facts - I hate it when that happens.

Thanks for letting us know the answer, and don't be embarrassed...we've all done stuff like that.

It's always surprising when I find a mistake I made years later, but for all the ones I have found, I wonder how many others are still out there...