Experimental Physics and
Industrial Control System

The suggestion was to use the calc record to mask the three bits that you want to consider - not to use it to make the comparison. So you would say MBBI.VAL & 0x303

The best way to support any arbitrary bit group would be to allow the mask to be configured directly, as you first suggested. I would not want to hold my breath for that change to go into the database. There are two things that happen in this record that have to be coordinated - which bits to use, and there is also an expectation that they are returned as right shifted numbers - so that the comparisons for the current state are in terms of the device - not the particular screws where the signals come in.

Also, changing that MASK to some arbitrary bit pattern in the device support seems a very obscure modification that future engineers trying to understand the application would likely have a difficult time finding. Personally, I try not to be too tricky - I hate getting phone calls in the middle of the night.

Another approach could be to re-wire the PLC so that the bits are consecutive - as you would expect a single device to be.
Or treat this as two multi-bit devices - one that is the values and the other are the validity bits. This is a bit more obvious and it conforms with the definition and intent of the MBBIx records.

So, there are many ways to change what is there. But it may be most prudent to do something that requires no change to the record - and explicitly shows what is going on with the logic.

Bob