I have a question about powering down SLC 504. Does it do any good? I do not believe that it helps on the SLC 504. I have used it to clear errors on TI/Siemens. But remember the SLC is not locked up nor does it have an error.
I'm not sure what your issue is. My only comment is the SLC's battery is not accessible while it resides in a chassis. Therefore, the processor card must be removed to replace it. Therefore, the SLC has a capacitor to retain its memory while the battery is being replaced. I've had to tape a piece of solder wire across this cap overnight to dry out the memory of an SLC 5/05 whose IP address was unknown and serial port configured for ASCII.
I should have stated earlier that our technicians believe that powering down the PLC clears errors. It is my position as the Plant(s) Engineer that it will not clear any errors.
Cycling power always clears out cob-webs - even in well-written systems and especially poorly-written systems. Most programmers try to minimise their use of retentive latches and pure sequence logic to avoid locking up their control system. Also, there is an RTOS running in the background, well proven with years of service, but who knows ...
Point 1:
If you are looking for the first scan to initialise something e.g.,
XIC S:1/15 JSR 3
then it might help to power-down and power-up your PLC if your system is acting up.
Point 2:
You can lock up a SLC to fault mode with certain types of errors e.g., numerical overflow error - that are not being trapped.
For example, somewhere in your main scan it would be a good idea to put at least the following:
XIC S:5/0 ... OTU S:5/0
I always log S:21 and S:20 too - you might wish to signal the HMI or SCADA system that your maths is wonky.
If you fail to put this and you generate an overflow, then your PLC would lock up.
In that case, and my memory is foggy here, a restart might help - it has been a long time since I have not used this trap, so you will have to check and see if S:5/0 is cleared or not on a cold start.
I am sure there are more reasons to cycle the power on a troublesome system, but those are 3 that come to mind.
It doesn't help any more or less than switching to PROG mode and back to RUN mode. UNLESS, your SLC is set to always load the program from an EEPROM. In which case cycling power would reset back to the state stored in the EEPROM.
> ... Therefore, the SLC has a capacitor to retain its memory while the battery is being replaced. I've had to tape a piece of solder wire across this cap overnight to dry out the memory of an SLC 5/05 ...
Most SLC's have a couple of solder points on the main board (I think labled VDD and GND) that if shorted, AFTER the battery has been disconnected, will discharge that cap (I usually hold a screwdriver across these points for about 30 seconds). It's in the SLC manual.
