BobLfoot

PLC Law

164 posts in this topic

That is a myth that the ignoramt believe, not a law. It applies to motion control in general, not just PLCs. It is pitiful though when the mechanical engineer doesn't understand how his own machine is supposed to work. Good mechanical engineers are hard to find.

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Ok, yes a bit 'tongue in cheek' But then we all have to put up with laws written by others that don't necessarily make sense!

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OK Peter, we all know that PLCs don't actually break the laws of physics - Only Chuck Norris and God can do that. But it is impressive what good controls engineers can do with a PLC, with on the spot, to overcome the dreaded customer requests - changes of scope rather than 'design deficiencies'. Out of curiosity, what do you do? You're obviously an expert in motion control. You seem way too academic (a certain roller coaster physics post comes to mind...and I think I remember you mentioning teaching) to be a lowly controls engineer/integrator, but you've definitely had more field exposure than any profs I've ever had.

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Not sure how it relates to the MrPLC.com T-Shirt idea, but the best I've ever seen was written on the back of this guys T-Shirt it said " I'm a bomb Technician, If you see me running try to keep up !!"" Maybe we could do with a "I'm a programmer,,,

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PLC Law 44 - Before troubleshooting anything make sure the program on your PC is the same as that in the PLC!
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PLC LAW #45: NEVER LEAVE THE RUN/REM/PROG KEY IN A PLC WHERE EVERY TOM DICK AND HARRIET HAS ACCESS TO THE PLC. Edited by BobLfoot

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Incorrect instruction? Check the program? ..... why?? It has been running fine for years! I didn't thought through what happens when a check sum error occurred. I could not clear the error at all. So, I figured I will upload and clear all memory areas, download the program again and see what happens. After I uploaded the memory are tables I compared my program with the one sitting in the CPU. A few minor contact differences were found but we come across this regularly, a lot of engineers of our sister company forget to send the latest changes. Again, I figured I arrived on site off a up till now working installation so I automatically started to match all contacts in my program with those in the CPU. When I came across a contact number difference in the section that handles the Modbus comms I frowned and thought to myself that's strange -never have to change anything here- but I just consequently changed it -it has worked right- and went on. I downloaded the program, downloaded the memory tables, power cycled the CPU, and it was back in business. After a coffee I returned and noticed the Modbus slaves were not polling. They we're being polled looking at the flashing TD on the 232/485 converter that is connected directly with the RS232 port on the CPU, but no response. I automatically assumed it wasn't running before the CPU error and asked the engineer if he had exchanged any sensors of the slaves. He told me he was having problems with a pressure transmitter but his English was very poor and my Polish is even worse so I went and checked the wiring at the pressure transmitter. Everything looked fine. I exchanged the 232/485 converter with that of another working panel but no luck. Triple checked all settings of the RS232 port, polling range setup in the PLC. No errors. Took a new Modbus slave sensor out of the box, put the polling node fixed to 0 (broadcast) and connected it by it's own on the converter. No luck. Now I started to become desperate. Finally I exchanged the complete CPU with one out of another panel. And it worked. What did that tell me? Well, at least the problem is within the CPU. But what could be it? It's running the exact same program it has been for years, same settings, no errors. Then for a moment I remembered this contact I changed in the Modbus comms section which I had long forgotten. As a last resort -at least I found myself pretty stupid, why could this ever work??- I changed the contact number in the Modbus section back to that of the program I took with me, so different from the program when I arrived on site. Power cycle, and BINGO! After a call with Omron, the conclusion was this. At power up, the PLC does a check sum sequence with the program inside. A EPROM memory section gone BAD caused the address of the contact in the Modbus section of the program to change. At the next power up, the check sum failed because the address was different from the program of which the check sum was created. So, although the chance is very small you will ever run into such a problem, don't forget my story about PLC law #22.....

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"I know the problem is mechanical (Hydrulic or even Pneumatic) can't you just fix it in the program?" Ever heard this one (more than a hundred times)? Bud

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The 3 most important words in programming; DOCUMENT, DOCUMENT, DOCUMENT. The amount of frustration that can be saved is definitely worth the initial time investment, DOCUMENT EVERYTHING!

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#46 If no one saw what happened, then they probably had something to do with the failure.
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PLC Law #47: No major programming changes after noon on Fridays. Pat Russell
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A variation on the last two could be:- After spending all day/week/month succesfully commissioning a system always resist the urge to "tidy up" your code just before leaving site. Andybr

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to which I would add: #46a If it happened on night shift then they probably did it on purpose so that they could spend the rest of the night sleeping/drinking tea/reading/talking (pick one!) To back this an incident comes to mind from a few years ago, One machine had a hand control which was poorly designed, and broke when dropped. The operators knew this, and the fact that we only had one spare. So one operator in particular would break it when she thought no one was looking, but it came to our attention (maintenance), and we decided that getting even was better than getting angry, so we made some changes to avoid the problem on the spare, and waited for the inevitable. On being told that there was a problem I dutifully went and got the spare, which bad been 'ruggedised' internally, and fitted it. At the same time we had modified the program so that after every 5 parts you had to press the start button, this worked out at about 4 times a minute. So a quick PLC EPROM change when we fitted the new control and the scene would be set I then told her a tale that 'I have managed to repair it, but you are going to have to keep pressing start as there is still something wrong, I will get the day lads to get on to the machine builder in the morning' She therefore spent the rest of the shift stood at this machine pressing start, and I watched her from a distance and saw her trying to wreck the new control, but no avail. By the end of the shift she was thoroughly p****d off. I then let it drop to her friends after a few days that she had been set up. Apart from some sour looks I never got any more grief off her, but I was well and truly off her Xmas card list.

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Hi All, i read all PLC laws and experiensces.....Gr8 Learnig stuff for me. Realy great lessons for new guys like me. Thanks to all PLC Masters. 2shar

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Great thread, had me in stitches for the last half hour. Heres a few: PLC Law #47.1: No matter how simple it seems or how much the customer begs you - don't make a change 10mins before you're due to leave site for the airport. PLC Law #48: NEVER EVER EVER EVER say "I only need to take the machine offline for 10 minutes to fix that..." Edited by stevec

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PLC LAW 48.1 Never tell the customer "It's just a minor progamming change."

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PLC law #47 should include the definition of the term "plane code". There are two types of plane code. Type A plane code is that code you write in your seat on the plane on the way to commission the machine. Type II plane code is that code you write at the plant so you can catch your plane home.
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PLC LAW #45.1: NEVER MAKE THE OPERATION OF THE MACHINE DEPENDENT ON THE POSITION OF THE RUN/REM/PROG KEYSWITCH. As always, a lesson learned while working on someone else's system. Long before that, I learned to deal with mechanical types who never grasped that the PLC progam would always be running, regardless of whether the machine was running or stopped.

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PLC Law #49: The answer to ANY "Can you make it do that?" question is ALWAYS yes. No matter how much you would love it to be no. PLC Law #49.1: When your boss asks this question, it really means: "You are going to make it do that. At no extra cost. And in the next half an hour." Andy.

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THE 2008 EDITION IS OUT AND AVAILABLE FOR DOWNLOAD AT http://forums.mrplc.com/index.php?autocom=downloads&showfile=817

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Can you believe this thread now has over 100,000 views!

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yes I Can and a couple of Tech Schools use this in their intro courses now. Can you believe it! I also checked with Chakorules and PLC Laws is the most viewed post right now with the remaining top five being. KEWL EH? CX Programmer Improvement Requests 2nd at 44,000 Mitsi FX Cable 3rd at 33,000 Omron FAQ 4th at 17.000 AB FAQ 5th at 14,000

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