what happens in most common controllers when power is off

briana banks · Jul 4, 2007

Hi All

what happens in most common controllers when power is off?
what is kept running using the battery?.what is stored in flash memory?.

T

Steve Bailey · Jul 4, 2007

The battery is used to back up RAM memory. When the PLC is running, it uses the program in RAM. Dynamic register values and the on/off status of discrete memory is stored in RAM.

Generally, data only gets stored to flash memory upon a command from a programmer or operator. The PLC can be configured to transfer the program and stored data values from flash to RAM upon power-up.

bmw_apprentice · Jul 4, 2007

I agree with what steve said. although we use Siemens 300 controllers and they use a memory card to hold ther program rather than a memory area built in to the actual controller, this means the program cant be lost when power is lost and so removes the need for battery backups to support the program.

Contr_Conn · Jul 4, 2007

ControlLogix L6x Series B and CompactLogix L43 during shutdown will copy RAM to internal flash NVRAM. L6x uses battery for this, L43 capacitor. Once save completed, battery not used anymore.
If flash is vaild on power up then controller will copy flash to RAM.

BobB · Jul 4, 2007

Omron CS and CJ series use flash RAM for storing the program and a battery is not required to back up the program.

On the other hand, if the PLC carries "volatile" data in memory, a batteri is required to back up the volatile data.

rsdoran · Jul 4, 2007

briana banks said:
Hi All

what happens in most common controllers when power is off?
what is kept running using the battery?.what is stored in flash memory?.

T

I have to say this, nothing happens but the fact that it stops working/scanning and sits idle. I reckon it likes looking at the program offline, its there but not doing anything.

Most plc's have some option of backup memory but for the most part the battery and/or capacitors allow it to retain the RAM memory.

My personal opinion, Steve gave the appropriate answer, the rest just touted what their fav brand could do.

BobB · Jul 4, 2007

That was definately NOT my intention Ron. Just trying to indicate a different way of doing things in this particular brand and models. The older ones required battery to back up the program, or an EPROM, or an EEPROM, as do most older PLCs from virtually all manufacturers.

Also, part of the question asked about flash memory, not just batteries.

In fact, there are a heap of different ways from different manufacturers and I would have thought listing these different ways would have been helpful. I guess we all see things differently.

bmw_apprentice · Jul 5, 2007

Yea, i agree with BobB, i cant speak about anyone else but i work in a factory where 99% of all PLCs used here are Siemens or Allen Bradley so i cant speak for any other models.

Just trying to help Briana banks

GeoffC · Jul 5, 2007

With PLCs with no battery how does the real time clock keep running?

Paulus · Jul 5, 2007

GeoffC said:
With PLCs with no battery how does the real time clock keep running?

Put it this way, it'll tell the right time at least twice a day...

hokie · Jul 5, 2007

GeoffC said:
With PLCs with no battery how does the real time clock keep running?

Most PLC hardware either has a small battery or a small cap that keeps the time while you are powered down.
This would be unrelated to how the nvram is maintain on the unit.

doug2 · Jul 5, 2007

rsdoran said:
I have to say this, nothing happens but the fact that it stops working/scanning and sits idle. I reckon it likes looking at the program offline, its there but not doing anything.

Are you sure? I will look through my old documentation but I believe that the plc continues to scan the program. That is why my first discrete input is a contact off of the control power. That way after power is lost we can safely adjust our output states so that everything will be on its way to a safe position when power comes back on.

jvdcande · Jul 5, 2007

doug2 said:
I will look through my old documentation but I believe that the plc continues to scan the program.

Send the one who told you that over here. He/she clearly needs to be taught the very basics about PLCs. Even if the PLC isn't powered down, but merely in a STOP (e.g. Siemens) or PROGRAM (e.g. AB, OMRON) state, then the program is no longer scanned.

If you switch the engine off, then a car can still run, but only downhill. If you switch the PLC off, then it doesn't do anything anymore. Period.

Kind regards,

doug2 · Jul 5, 2007

jvdcande said:
Send the one who told you that over here. He/she clearly needs to be taught the very basics about PLCs. Even if the PLC isn't powered down, but merely in a STOP (e.g. Siemens) or PROGRAM (e.g. AB, OMRON) state, then the program is no longer scanned.
Kind regards,

You may be correct but I seem to remember teaching the use of the last scan bit many years ago in logicmaster software which to me clearly indicated that the program would continue to scan "after" a power loss long enough to safely position your logic.

OkiePC · Jul 5, 2007

doug2 said:
You may be correct but I seem to remember teaching the use of the last scan bit many years ago in logicmaster software which to me clearly indicated that the program would continue to scan "after" a power loss long enough to safely position your logic.

That particular PLC may be able to make a final pass through the logic, but I seriously doubt that it continued to scan logic on battery power.

what happens in most common controllers when power is off

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