SLC 500 intermittently faulting

brijacq

Member
Join Date
Oct 2002
Location
Cumbria
Posts
44
We have just installed a new heat treatment furnace using SLC 500, the main fault is that intermittently the processor will fault, it resets without any problems but then maybe two hours later it will fault again. When i look at the fault code it says "a runtime error has ocurred on i/o module 2". The card which is in i/o module 2 is a thermocouple input card. Any help will be appreciated.
 
You may want to look at the rack, it may have a crack in the solder traces. I have had this problem several times with AB racks. At times one module will show an intermittent fault, while at other times all of the inputs on a module or the whole PLC will light up even though there is no power to the inputs. This may not be what your problem is but it could be a good place to start looking. If it is an intermittent problem, which is caused by heat it could be hard to test for, but if you have another rack handy you may want to swap it out and see if the problem persists.
 
Most of the I/O module runtime errors I've seen are from noise conducted into the controller system from an outside source, usually through the shield on the serial port or through an I/O wire, like the output to an unsuppressed inductive load.

If this is an inductive furnace, there are huge magnetic fields nearby; it wouldn't take much of a weakness in a shielding and grounding system to cause a significant induced voltage in some of the SLC I/O wiring.

It might be a defect in the Thermocouple module itself, but because it's intermittent I suspect it's a spike getting into the T/C wire or shield and zapping the A/D converter inside the module.

Get a really thick green wire for your chassis ground; make it bigger than the gauge of your power wires. 8 gauge isn't too big for a ground wire.
 
Thanks for the replys, but i think we have sorted the fault. We noticed that on th 8 channel tc i/p card you need to earth the connection point (tb1) which is actually on the card (not at the front of the card) and we have not had the processor fault for about 12 hours now. Hope this is good info for future reference.
 

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