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Daryl

Proper power down

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Is there any particular method to powering down a CompactLogix (or any plc for that matter) other than just using the isolator in the control panel? Thanks... Daryl

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Just shut off power to the PLC. This is one of the nice thing about PLCs over PCs and other devices, they do not require any special power down sequence. Contrologix is like any other PLC just remove power. Now, in your machine or system you may want to program a power down sequence to properly power down the machine or system and you can do that within the PLC logic. Just keep the PLC alive during the machine power down sequence.
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The challenge with PLC's is not stopping them. The challenge is making sure you have logic in place to restart correctly.
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Thanks folks, power down as expected! Much appreciated. cheers... Daryl

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this isn't exactly what you asked – but I'll go ahead and mention it anyway – since once in a great while you'll run into a tricky debugging problem if you don't take this effect into account ... suppose that you have a system with an input wired Normally-Closed ... this input could be a pushbutton, a limit switch, or a sensor, etc. ... suppose that the input is NOT being actuated at the present time ... specifically, the electrical circuit is CLOSED – so we have a status of ONE in the PLC's input bit/box ... now suppose that when you "power down" you just throw a Master Power Switch to the OFF position ... specifically, you've just killed the power to the PLC processor - AND – to the input circuit AT THE SAME TIME ... here's the trick: the PLC (in most cases) doesn't just slam on brakes and stop instantly ... instead it CONTINUES TO SCAN for a certain amount of time ... usually this "hold up" time, as it's called, amounts to one or two seconds ... even if it's just one or two scans, that can still be enough to cause a problem in some systems ... think about it this way: while the processor is still making those last "power off" scans, it sees NO CURRENT flowing through that input circuit ... in simplest terms, the PLC processor suddenly thinks that you've PUSHED THE BUTTON ... specifically, the program (still scanning merrily along) now sees a status of ZERO in the input bit/box ... now in some (most?) systems this effect doesn't amount to a hill of beans ... most people never consider it – and it never causes a problem ... BUT ... in some systems, this ARTIFICIAL "actuation of the input" can cause some significant issues ... problems related to this effect can be especially tricky to troubleshoot and diagnose – because quite often the reason that the "power down" occurred was due to a plant-wide power failure ... and after the power comes back on again, most people just naturally assume that any "abnormal" operation of the machinery is due to the "power surge" that occurred ... so ... to answer your basic question: most people do indeed just "throw the switch off" to power down the PLC – BUT – if that switch also kills the power to the INPUT circuitry at the same time, it's POSSIBLE (not likely – but POSSIBLE) to have the system act "weird" because of this "hold up time" effect ... NOTE: the Allen-Bradley PLC-5 processors don't usually (never?) suffer from this effect ... there's some type of "magic" involved in their power supply system that does indeed allow them to instantly "slam on brakes" and come to a screeching halt with no "extra scans" ... TIP: the next time you get a chance to experiment, watch the "Run" LED on your processor while you kill the power to it ... for most systems that LED will remain green for at least a full second or so ... the question then becomes: "Just what is the PLC processor doing during that last second or so before it finally goes to sleep?" ... the answer COULD be that the processor (bless its little heart) suddenly THINKS that you've pushed a button that really and truly did NOT get pushed ... (or maybe a limit switch got tripped – or maybe a sensor got actuated, etc., etc.) ... if that phantom "actuation of the input" could cause an "issue" in your operation, then you need to plan for the situation before it becomes an embarrassment ... if you find this subject "entertaining" you can read more about it in some of the PLC Quizzes I've posted on my website ... go to the "Sample Lessons and Videos" page – and then scroll down to find the link ... no registration is required to download the material ... going further: it can actually make a difference whether the input in question is wired using AC power – or DC power ... that subject is covered in the "quizzes" so you might want to look for it – if you don't already have enough to worry about ... party on ... Edited by Ron Beaufort

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Like cutting steel belt at a tire assembly machine B&T station too short if they e-stopped it or stepped on the safety mat too early. That action would kill power to the photo sensors that the logic was waiting for as it measured out the next length of stock. It took a lot of trial and error to figure out it was happening because, although we were shutting off both AC and DC power at the same time, the DC signal from the sensors was dropping out a couple scans before the AC input telling the PLC it had lost power was dropping to zero, and then several (I counted them) scans took place before the PLC shut down. So we had to change how those sensors were configured and invert them in the program to solve it without altering the safety circuit. This happened after replacing a SLC and relay logic with Logix I/O and my new program, and was intermittent, it depended on the operator more than anything, whether or not they were tripping the safety circuit getting on the mat too early, and some of them used the e-stop button during every splice, and would hit reset and start afterward. This was still not causing waste unless the machine was still measuring out the next piece. Edited by OkiePC

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Ron, Some very interesting information there, I appreciate you going the extra mile to educate on the subject. All of my PLCs run 24/7 and I've been thinking it might be prudent to check all inputs that are normally closed to see their effect of becoming '0'. cheers... Daryl

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