plc - corrupt ?

[Janus]

hi

i have a PLC where the inputs appear stable,

but, the internal memory coils appear to be operating erratically ?

causing the output relay to operate in chatter mode for period of time, then no output at all, until PLC is re-booted.

( the PLC hardware has been changed, no improvement )

the PLC is programmed with flash device, we do have a small hunch that the flash device has become corrupted,

and hence the software program in this PLC is corrupt,

the DC supply appears stable, all inputs are digital,

does anyone have other experience, ideas ?

thanks

 

[Ken Roach]

Welcome to the Forum !

PLCs are designed from the ground up to detect memory corruption and stop processing (often actually deleting the corrupted program).

I'm not saying it's *impossible*, but it is extremely unlikely, to the point where it has never been observed to occur.

It is far more likely that wiring or other electronic components have degraded over time. Loose or corroded common or neutral wires, or a power supply that cannot deliver its rated power are frequent contributors to "chattering relay" conditions.

Can you describe the make and model of this controller, and some information about the control circuit for this relay ?

 

[Bit_Bucket_07]

Sounds like a program error. Try running a different program in the PLC. If you are able to monitor internal bit status, then I assume that you have the ability to view and/or edit the entire program. I'd look for a data move to the register(s)in which your internal logic bits are located. Perhaps a sequencer is attempting to run on a timer pulse and the timer's preset has a value that is too low?

Difficult to evaluate without seeing the program.

 

[Ned_Flanders]

hi

i have a PLC where the inputs appear stable,

but, the internal memory coils appear to be operating erratically ?

causing the output relay to operate in chatter mode for period of time, then no output at all, until PLC is re-booted.

( the PLC hardware has been changed, no improvement )

the PLC is programmed with flash device, we do have a small hunch that the flash device has become corrupted,

and hence the software program in this PLC is corrupt,

the DC supply appears stable, all inputs are digital,

does anyone have other experience, ideas ?

thanks

My money is on a faulty field device dragging the voltage down.
Make and model of plc?

 

[Janus]

thanks for replies, have tested flash memory device here , all is ok,

my gut feel now, is the dc source to the plc ( this is a IMO device ).

Measures 24v dc, but, if there is a high impedance connection, path in this supply, then hunch is the IMO cannot process program and switch the outputs due to current constriction

plan is to re-route dc supply at mo

 

[Ned_Flanders]

I actually had a similar fault this morning. After a power outage, the Siemens plc power supply would not restart correctly and kept switching on and off quickly (and hence the outputs.

It needed "Fixing with a new one"!

 

[Janus]

latest tests show that running PLC from pure DC source ( fully charged lead acid batteries ), the PLC outputs 'chatter' with no external wiring,

this is the second unit to fail on this installation

is this a harware batch issue with PLC or something odd about the 230v/27v ac then 24v dc supply at site ( although if PLC outputs are shorted out, the function of the rest of the plant operates ok )

 

[OkiePC]

is this a harware batch issue with PLC or something odd about the 230v/27v ac then 24v dc supply at site

Unlikely.

What brand/model PLC?

Can you post the logic?

 

[ceilingwalker]

I had this problem just yesterday. Don't know if this will apply to your situation. I changed my CPU from a SLC-5/03 to a SLC-5/05 and I was getting the same error message. It turned-out to be my version of RSLogix500 was to old for the new processor. Created a new program with nothing on it, just to confirm RSLinx was able to see the processor. Performed a successful autoconfigure in RSLinx, unplugged the RS-232 cable and confirmed with the loss of communication error. That left only the software. Hope this helps.

 

[Goody]

After all you have described, I would suspect a programming error.

If it is just one output, I would trace the function of every 'bit' in that line until I found the culprit.

 

[Bit_Bucket_07]

hi

i have a PLC where the inputs appear stable,

but, the internal memory coils appear to be operating erratically ?

causing the output relay to operate in chatter mode for period of time, then no output at all, until PLC is re-booted.

( the PLC hardware has been changed, no improvement )

the PLC is programmed with flash device, we do have a small hunch that the flash device has become corrupted,

and hence the software program in this PLC is corrupt,

the DC supply appears stable, all inputs are digital,

does anyone have other experience, ideas ?

thanks

FWIW: I've seen this symptom more than once (over the past 31 years) due to a damaged backplane. I've also seen similar dysfunction caused by a damaged I/O module pulling down the DC bus voltage. You could remove all I/O modules and then replace them one-by-one to determine if a bad module is causing the problem, although that might require reconfiguration of your I/O map, depending upon make and model of your PLC. (This also assumes that you have a modular PLC system, rather than a "brick".)

I'd create a very simple test program and attempt to run it. If that didn't clear up the blinking output issue, then I'd certainly assume a hardware failure of some sort.

 

[Lancie1]

The PLC is programmed with a flash device, and we do have a small hunch that the flash device has become corrupted.

Is the PLC a Siemens? If so, you might have got the Stuxnet trojan from your flash drive. It was targeted at specific plants, but there may be exceptions. I think Siemens offers fixes for that virus now.

 

[Baranyk]

Is the PLC a Siemens? If so, you might have got the Stuxnet trojan from your flash drive. It was targeted at specific plants, but there may be exceptions. I think Siemens offers fixes for that virus now.

This would be hard, because Stuxnet looked for a particular hardware configuration. So Janus' Siemens PLC (if it is a Siemens) would have to be in the same configuration as an uranium enrichment facility's... which I find hard to believe, if he is troubleshooting on this forum

 

[Lancie1]

This would be hard, because Stuxnet looked for a particular hardware configuration.

The originators tried to eliminate all other facilities, but that was not guaranteed. Nothing in war is guaranteed. The enemy has cloned Stuxnet and fed it back as a changed version that attacks who-knows-what-all. Watch out about using off-the-street flash drives to move programs around in your PLCs.

 

[Baranyk]

Is the PLC a Siemens? If so, you might have got the Stuxnet trojan from your flash drive. It was targeted at specific plants, but there may be exceptions. I think Siemens offers fixes for that virus now.

They tried to eliminate all other facilities, but that was not guaranteed. Nothing in war is guaranteed.

Except that the battle plan will never survive contact with the enemy.