Duplicate Destructive Bits in PLCs. The BEST Program Scan Lesson.

[@juliollontop4812]

Just because you can, doesn't mean you should. One of the things I try to pass on is make sure it is easy to trouble shoot, point that you show at the end of the video. Thanks Tim

[@TimWilborne]

100%... No where in here did I say we should but we do accidentally do it.

[@WillWilson2068]

When I have a complex condition I like to break it down and set internal bits first. Then have a final rung that says if all are true, THEN set the output bit. Allows you to break down the logic for future troubleshooting as well.

[@theplctech1832]

Very good instructional video! These two concepts (sequence of events and being very careful about using duplicate destructive bits) can be tough to see and caught even by experienced programmers. Thanks for sharing.

[@TimWilborne]

Thanks, I debated on editing the initial video and erasing it but I walked us right into it and felt I needed to explain it.

[@theplctech1832]

I’m glad you didn’t because it’s like I tell my guys, you can’t eat an elephant in one bite, so you have to take little bits and go back to eat some of the bites that you miss. I also told them the other day that a program bug was nothing more than a bite of the elephant that got missed. Like you have said before, “programming doesn’t change” but the programmer may not of thought of every scenario to fully debug it.

[@TimWilborne]

Good saying, I'll remember that

[@cobelali]

Thank you for the great explanation. Coming from a computer programming background where nothing happens if the trigger to a command is false, I had a hard time dealing with multiple commands controlling an output. To me, if the input to a rung was false, I expected the output to be untouched, but as you mentioned in the video, if the input is false, the controller will explicitly write a zero to the output.

[@TimWilborne]

I'm glad it was helpful

[@fellstar]

Hard lesson learned by new engineers the hard way, the world over. Good vid!

[@TimWilborne]

Yes it is whipped me many times. And apparently I still don't learn because I recorded the whole series and didn't notice it 😂

[@chazcampos1258]

Great realistic video about how to troubleshoot logic!

[@TimWilborne]

Glad it was helpful!

[@EthanHunter025]

Hi Tim, At the 4:45-4:50 mark, you mention how the PLC scan still evaluates input6. I’ve asked the same question during a very advanced Rockwell Training Class with a certified Rockwell instructor who said that it does not get evaluated in order to decrease program scan rate times. Is there any sort of official documentation from Rockwell that you could link stating explicitly that the input6 still gets evaluated?

[@TimWilborne]

It evaluates it every time. Look at the text view of the ladder and you can see the exact order.

[@EthanHunter025]

⁠@@TimWilborneis there any documentation that explicitly states that it evaluates every instruction? The ascii text representation of the rung is just another representation of the same rung in ladder. If I had a link to documentation, I’d have something to physically show people when they doubt whether the instruction gets evaluated or if it skips along to writing to the OTE bit location.

[@TimWilborne]

Yes it is in many of the software and processor manuals. I'm not near a computer for the rest of the month so can't site a specific link

[@PROFEDGE]

I watch your video before, and I'm still trying to find how the Micro850 performs its scan. From this video I take it you are saying it uses Scan solve set housekeeping. Duplicate outputs can wreak havoc so I encourage my student to avoid them, or to use setup internal bits to make the decisions and then fire the output once that logic is solved. :b d:

[@TimWilborne]

Here is a video on the scan cycle. The issue with simply stating for them to avoid them is they won't know how to work through them when they run into them. kzbin.info/www/bejne/fYTMc6uPZad7Y8k

[@PROFEDGE]

Good Point.

[@TimWilborne]

Let me know if that video helps with the scan cycle. It is an older video.

[@PROFEDGE]

@@TimWilborne Thx. I'll link your video to the students suggested watch list. There seems to be something unique about the Micro 800 series, in that for themost part t behaves like a SLC or PLC5, and at other times it behaves like a COS Logix PLC. That's why I'm still looking for clarification from Allen Bradley. To be frank most seasoned electricians don't get Change of State scans. I didn't get it until I found in the 5th AB Manual I read a single paragraph explaining that there was 2 CPUs running asynchronously, and the I/O update was on one and the Logic was on the other. If you happen to find out from AB before me let me know, I'll do likewise. Great site and great videos.

[@TimWilborne]

Can you give me an example. I'm not sure I am following you. It does scan similarly to the SLC or PLC5 but it also has timed and event driven interrupts...but so the the older PLCs. Is that what you mean?

[@amel3778]

great ! Thank you sir

[@TimWilborne]

You are welcome

[@enkienki8202]

I would duplicate an output like that, i have seen this done many times by people who say it is fine and what it does is inserts a bug into your program.

[@enkienki8202]

I would not do this, i am correcting what i said below. I have seen this done many times and it works most of the time and then it doesn't. It introduces a bug. I just corrected a long standing problem in one of our programs due to another tech using a duplicate bit. I was warned about this in school to never do this!