SLC 5/02 & Thermolcouple card error

Hello
My problem of the moment is that on our water chiller, which uses an ALLEN BRADLEY SLC 5/02 processor, The first 2 slots are 1746-NT4 cards (thermocouple).
Last week then again late yesterday afternoon the processor went into fault. When I got connected to the processor last week all I notice was the processor status error read
“Specialty I/O module on slot 1 has generated a generic fault. The card fault bit 1 is set (1) in the module status byte”
Now last week I did not investigate too deeply and simply cleared the major errors.
Yesterday I spent some more time checking thing out when I looked at the status words for the slot 1 module all I found was that the second input (input word 1) had detected an open input. With the same processor error.
Now for the questions.
1st Why would an open thermocouple input cause the processor to fault?
2nd Is there anyway in a SLC 5/02 processor to allow for external resetting of major errors so a PC does not need to be connected. It’s not a problem now but in two weeks when I’m away on vacation it could be catastrophic?
We did replace the Thermocouple as well as the 1746-NT4 card to be safe I am planning on testing the card and thermocouple on the bench to see if another failure occurs. I will however have to use a 5/05 processor since I do not stock spare 5/02’s (only got 2 – 5/02 processors on the chillers the rest are all 5/05’s)
Thanks for the help
Michael

Michael,

I'm speculating that what MAY have happened is that a calculation you may be performing in your program that uses the input from the NT4 card may have caused an overflow and in turn faulted your processor. It wouldn't hurt to put an unconditional unlatch for S:5/0 at the end of your program to avoid this fault. Look for any DIV or DDV or really any math instruction that could potentially set the overflow bit that also use the NT4 input, as this would be a better indicator.

The only way I know of to externally reset a major fault on a 5/02 is with the Hand-Held Terminal (I think the part number is 1747-PT1). If you get one, be sure you don't plug it into a 5/03, as you will probably wipe out the processor and have to re-download the program.

HTH,
Jeff