PLC Power

I have a customer that we have shipped 3 systems to. Two of the systems sit in a permanent location and always have power to them. The 3rd system is on wheels and is moved in and out of its manufacturing position when they run those parts. When this system is not being run it is disconected from power (sometimes a week at a time).

The question is what effect will this have on the SLC 504 inside and what are the long term consiquences of this action.

Action or reaction

I would imagine that no adverse things will happen as long as the issue of program retention has been addressed. It sounds like it has so other than that, there should be no problems that I am aware of.

What type of PLC is this? I ask because the control logix PLC's I use to use their batteries never lasted very long, we use to have to buy a special battery module for them.

I did not post this at first because I did not want to taint any replies.
Somtimes and I stress somtimes on start up the system behaves irratic and does things it shouldn't.
I don't know if this is a result of being diconnected from power or not, all I know is that the program in all 3 machines is pretty much the same except form minor differences. I am not seeing the same issues with the other machines that are allways connected.

erratic behavior?

Somtimes and I stress somtimes on start up the system behaves irratic and does things it shouldn't.

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This may indeed be happening but it is not a fault of the PLC. The program may have some funny stuff in it that makes certain conditions exist at certain times, but it is not because of being off for some extended period time. The processor will not even go into run in most PLCs if the program is corrupted. Any anomalies that exist are written into the program or exist in hardware. As long as the processor goes to run, the program is intact, but that does not mean it is well written or suitable for the purpose at hand.

Are you also trying to retain variables from one usage to the next, thus many well be battery backed up and not in flash.

The other is does the program restart fresh, no set coils or retentive bits or values left by pulling power. Ensuring the program starts in an "as new state" may help.

Are you also trying to retain variables from one usage to the next, thus many well be battarty backedd up not flash.

The other is does the program restart fresh, no set coils or retentive bits or values left by pulling power. Ensuring the program starts in an "as new state" may help.

Battery Power degrades in a regular, continuous manner when not being used.

The program is residing in RAM. When not powered-up, the RAM is "refreshed" by battery power... the battery degrades more quickly.

Ram consists of transistors and capacitors.

Not all RAM bits are created the same!

As the battery voltage deteriorates some bits will not be "refreshed" simply because it takes just a little more voltage for this particular bit than the battery has at this time. Others will be "refreshed" simply because they don't need as much juice.

Since the thing is a roll-away, roll it away to a plug and plug it in until it's needed again.

Also, if the battery is not recharging under usage, then be sure to keep the batteries fresh.

When they say that batteries last for two years... I believe they mean shelf-life while unused - even so, it still degrades somewhat.

When being used to "refresh", I just don't believe you're gonna get 2 years of adequate "refresh" out of a battery.

If it's a temporary power connection, make sure your ground connection is solid too.

SLC's are really quite miserly on their battery power. There are charts and tables for this sort of thing, but I would put the controllers on an annual battery replacement schedule and not worry about it. I have a 5/05 that's been on 90% battery power for two years and it still powers up with the program inside.

If you can, use an EEPROM and you won't have to worry about the battery at all.

More likely you're seeing undesired operation of the program because it hasn't been debugged well in it's "interrupted cycle" and "startup" modes.

Memory is almost always CMOS Static RAM - SRAM, never Dynamic RAM DRAM. The battery power is just to keep the RAM alive, I dont believe there is a refresh like in a Dynamic Ram refresh for Memory Backup of a PLC.

Very clearly the logic in the PLC is not written to handle Powerup sequences and power down sequences for your application. As mentioned earlier in this thread, some memory retentive bits are causing the untoward things you are seeing/or are happening.

what are the long term consiquences of this action

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Once a week.
52 times a year.

This unit will be warmed and then cooled over 100 times in the next 2 years.

Is it really what you want. This is not a TV set. Its a fine piece of equipment built to run continuously.

With temperature cycles the way you have them going, you run the risk that any small defect in a solder will blast the board out.

Its not certain. You are only augmenting the probability that it will happen. But is this what you want to do?

What do you mean by erratic? Are there settings stored in the data table? Do you have an eprom? If so, is it set to load a power up. Maybe these settings are being overwritten.

SLC's are about as bullet-proof devices as I have seen, I don't think that the power cycling should be a real detriment. I have seen machines that the E-stop drops all power, including the PLC.

I have told my customer that this is not the best thing to be doing. I also asked that the system be pluged in somewhere else when not being used. I was told that it would not be possible ( I knew making this thing portable was a bad idea, its just going to cause to many problems ).
I am using the first scan bit to reset lached bits and variables in the program but next time I am in there I am going to make sure that it is actually working.
I guess I just wanted to know if this repeated action would have an effect and what it would be.

Machines have a life of there owns.
We have built mobile packaging system that are powered by small cheap Honda generators (the home model ones) towed in harvesting fields by a farm tractor.

Those are stored in a barn all winter long (in Canada!!!)

We've had only one or two out of 10 that gave us problems.

This is to say that cycling power On and OFF is not that bad, its just one more thing to add to the variables lowering the MTBF.

... just how deep do you want to go? ...

Greetings, TOMC,

You said:

Somtimes and I stress somtimes on start up the system behaves irratic and does things it shouldn't.

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If you’re just “pulling the plug” and having “erratic” behavior the next time you power back up, here’s a question that comes to mind:

Are you using AC inputs ... or DC inputs?

Most people think that there is no difference ... but there is ... particularly with AC inputs that are “field-wired” as normally closed signals. This can get really weird ...

The trick is that the DC power “holds up” for a few seconds after the power plug is pulled – but the AC power dies instantly.

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This is from a very lengthy post that I placed on another forum some time ago. Warning: If you decide to read the whole thing, you’ll probably need to pack a lunch.

take a trip to a very long post on another forum

Note: You can save a little bit of time by going directly down to the fifth post in the thread.

If you want to take a “preview” of the part most pertinent to your current problem, then scroll down to the paragraph titled “3. RUN-power failure shutdown condition”. You’ll probably still need to start at the beginning to follow everything ... but at least the preview will let you know whether the rest is worth reading or not.

True ... these “AC inputs? or DC inputs?” types of problems don’t pop up very often at all ... but then again, how often do you see people just “pull the plug” on an operating PLC system?

Disclaimer: This might not have anything at all to do with your “erratic start up behavior” problem ... but if this IS the culprit then that long-winded post just might help you track it down.