Ladder logic open circuit detector

I would like to be able to see if the value of any one of a group of 384 integers goes Negative or drops below 20 degrees. This is how we are detecting open circuit condition to our thermocouples. I was wondering if there was a better way to do that other than writing 384 LES instructions?

FTSE 8.2
24 MLX 1100
3 PV+ 600

There are other ways, using looping and indirect addressing, for example, but the hard-graft long-winded way usually ends up the fastest execution, and is always clearer to see what is being done.

If you are using -IT4s on the Micros, They come w/ an open circuit flag bit for each channel you can also use.

Im using the Spectrum controls 1762sc-IF8u

daba said:

There are other ways, using looping and indirect addressing, for example, but the hard-graft long-winded way usually ends up the fastest execution, and is always clearer to see what is being done.

Click to expand...

It's not clearer if you fill up the program with 384 identical blocks of code.
And the chance of making a manual mistake on line number 132 and 297 has increased a lot.
And if you decide to add another check or change something you have to do that to 384 places.

No, a loop is the way to go when it's so many.


PS. If you absolutely want to do it without a loop in the PLC, because perhaps some of the values can't be indexed or whatever. My advice is to make a small program on your PC or use a spreadsheet or whatever that generates the code in a text file. Then copy and paste it in.
That allows you to make changes easily (just change the code generating program) and it avoids manual mistakes.

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If you can tolerate a worst-case delay of 384 program scans, you can check one value per scan as a way to implement the loop.

I'm assuming the MLX 1100 does not include the FSC instruction, though not necessarily a great option anyway.

FTSE 8.2
24 MLX 1100
3 PV+ 600

Click to expand...

You said you have 24 ML1100's. I will assume the 384 integers(inputs) are also spread out among the 24 ML1100's. As there is no way one ML1100 can have that many analog inputs. That would mean only 16 compare instructions pre PLC. Not that bad to me.

Or am I missing something here?

If using either of the engineering unit data formats, you can have 384 XIC instructions pointing at the sign bit, eg. I:4.0/15.
If you want it to look a bit neater you can have 8 xio in series turning on a 'card ok' bit, and then putting these in series as XIC to create an all ok bit, which you can negate for an alarm.