Answer to why micrologix 1500 was losing the program.

kracin

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Jan 2017
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omaha
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had an issue tonight, program was being lost in a micrologix 1500. reloaded it several times, every time it went out it was running, never while off.



the issue ended up being 24v power to a solenoid valves coil had come lose in the connection and was shortcircuited. this probably caused some kind of surge in the lines back to the plc, causing it to either clear its memory on its own for safety, or corrupting it where it wouldn't read as having anything in memory.


i saw the papers on using diodes to protect against surges, would these have any effect on an outputs wires grounding out and causing the program to wipe? i was thinking of pushing to get the time and resources to set up surge protection on all of the inductive load outputs just to be safe, but don't want to be wasteful either.


thanks.
 
this field issue would be unlikely to cause the loss of program, i cannot remember if the ML1500 has a battery for backup or if its capacitive. sounds like the CPU lost supply due to the short and the program was not retained due to loss of supply and a volatile memory.


if battery then its likely to be flat, if capacitive maybe that's failed.

i suggest upgrade it to ML1400 as this will give you Ethernet comms and on-line editing features.the ML1200/ML1500 were a bad products when released due to no on-line editing.
 
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I've done a little work with ML 1500 and some of the others. We had interposing relays in the output wiring. It helped a lot because the relay coil is just milliamps, and also the terminations are easier accessed and more reliable than just connections inside a coil termination enclosure. Plus you don't have all that inrush current going thru the contacts in the output relay in the micro logix. Also when I worked on a PLC 5 system years ago at a utility, all our outputs not only had the relays, but many of them had their own circuit breakers, so a shorted wire or coil wouldn't take down the whole station when it tripped everything in the panel.
BTW, welcome to the forum!!
 
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this field issue would be unlikely to cause the loss of program, i cannot remember if the ML1500 has a battery for backup or if its capacitive. sounds like the CPU lost supply due to the short and the program was not retained due to loss of supply and a volatile memory.


if battery then its likely to be flat, if capacitive maybe that's failed.

i suggest upgrade it to ML1400 as this will give you Ethernet comms and on-line editing features.the ML1200/ML1500 were a bad products when released due to no on-line editing.


battery was good, i was stumped at first and figured it was something bad in the controller itself. so replaced the 1764-lrp with a 1764-lsp with a new power supply as well to see if we could eliminate the problem quickly. it had the same result. the battery of the old and the new both were 3.5vdc so the battery itself was not dead.


the reason i was asking how it causes the loss of a program is because grounding problems were issues others were having when the plc would lose it's program. and the PDF from AB talking about preventing problems due to bad grounding puts surge protection in the same category as bad grounding
 
I've done a little work with ML 1500 and some of the others. We had interposing relays in the output wiring. It helped a lot because the relay coil is just milliamps, and also the terminations are easier accessed and more reliable than just connections inside a coil termination enclosure. Plus you don't have all that inrush current going thru the contacts in the output relay in the micro logix. Also when I worked on a PLC 5 system years ago at a utility, all our outputs not only had the relays, but many of them had their own circuit breakers, so a shorted wire or coil wouldn't take down the whole station when it tripped everything in the panel.
BTW, welcome to the forum!!


Appreciate it, i've used the forum for knowledge here and there, never posted anything because i haven't needed to yet.



i'd love to get this thing upgraded but it's not in the cards to change out the entire thing to a new PLC. if we did it would go to a compactlogix and 5000 software.
 
I had an issue with a slc 5/04 and plc5 over the years.

the units were in production and running.
in one plant, the lights would sometimes flicker, not often.

the plc would loose its program.

The cause.
sudden power requirements from other machines starting (200+ hp air compressors for example) will cause a dip in the line voltage causing the lights to flicker (brownout).

the plc sees the power going off and is trying to do a shut down procedure (for lack of a better term). half way into the shutdown, the plc then realizes the power is ok and tries to restart by reloading the program, BUT, the shutdown process has not finished. so you get an erased program.

Don't laugh, I was there the second time and restored the program. I was packing up when the lights flickered and the plc lost its program.

we put a corcom line filter on the plc and never had the issue again on the slc 5/04. the plc5 was not on the light circuits, but was on the air compressor line. that fixed the issue except for when the power went off for several seconds.

Since then I specify line filters on all plc panels.
regards,
james
 
... the plc sees the power going off and is trying to do a shut down procedure (for lack of a better term). half way into the shutdown, the plc then realizes the power is ok and tries to restart by reloading the program, BUT, the shutdown process has not finished. so you get an erased program...
Don't know for certain, but suspect this should not happen. The CPU should have plenty of time to handle a simple brownout.
There is a threshold setting on the 5 Volt supply. When the system sees the 5 volts dropping, it halts the CPU and stores retentive variables. etc. It gets that done before the voltage drops below 3 volts.
The battery backup of the program should not come into play here. The memory will hold the program as long as it has it's minimum voltage. About 3 volts. When the supply is up, memory is getting 5 volts, and not using the battery.

I suspect the line spikes are so severe that the filtering in the PLC supply can't handle it. Also, it's dumping all of the spikes to it's ground connection. Probably a small green wire on that little screw.
It sounds like a spike is making it's way to the memory, and changing one or more bits. That's all it takes to clobber it. Checksum fault. If allowed to continue, there will be a point where chip damage occurs. You will see it as a CPU fault that can't be cleared.

The Corcom is an excellent idea. I use them wherever I can. It has an elaborate filtering system, and dumps all of it's spikes to it's case, which you have bolted to the frame.
 
I suspect the line spikes are so severe that the filtering in the PLC supply can't handle it. Also, it's dumping all of the spikes to it's ground connection. Probably a small green wire on that little screw.
It sounds like a spike is making it's way to the memory, and changing one or more bits. That's all it takes to clobber it. Checksum fault. If allowed to continue, there will be a point where chip damage occurs. You will see it as a CPU fault that can't be cleared.

Exactly so. Semiconductors are susceptible to noise and spiking. If I had a dollar for every time I found where coils weren't suppressed, low voltage wires were ran close to AC voltage, a radio antenna was mounted next to the PLC, grounds missing, etc, etc, I could be retired now.

Everyone thinks their power is fine because the multi-meter says so. What you miss with a multi-meter are all of the sub millisecond transients that are happening. Some of these transients reach tens of thousands of volts. Transients are bad JuJu for any semiconductor device no matter how well protected it may be from the factory.

Surge protection, suppressing coils and proper fusing are necessary for a quality stable control system. It's just the right thing to do.

Maybe Phil will cover this in one of his next emails.
 
Probably overkill for the OP application, but we supply every PLC from an online UPS.

+1 Surge protection, suppressing coils and proper fusing are necessary for a quality stable control system. It's just the right thing to do.
 
this field issue would be unlikely to cause the loss of program, i cannot remember if the ML1500 has a battery for backup or if its capacitive. sounds like the CPU lost supply due to the short and the program was not retained due to loss of supply and a volatile memory.


if battery then its likely to be flat, if capacitive maybe that's failed.

i suggest upgrade it to ML1400 as this will give you Ethernet comms and on-line editing features.the ML1200/ML1500 were a bad products when released due to no on-line editing.


I disliked the ML1500 as well, but with later revisions you could online edit. I think it was Rev. C, but I'm not sure. I'd replace with a CompactLogix, unless software makes that cost prohibitive.
I'm always confused when I see new MicroLogix installations. The 1400 was ok, but I wasn't a big fan of the entire line.
 

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