SLC500 IIM Instruction

Good Morning,

I came across a new instruction and would like some help better understanding how it works. I am trying to add a new 24vdc input to a system that only has one 24v input card. Only one input is currently being used by a prox sensor that is acting as a shaft encoder.

There is an IIM instruction on the input card and my understanding is that every time this rung is scanned it will interrupt the normal scan and update the status of the input before continuing.

My new input doesn't need this but I do not want to cause any unintended side effects by using this input card. Can I just change the mask like I have detailed below to exclude it? I am not too familiar with the mask concept either. Existing input is in red and my new input is in blue.

Old Mask
0FFFFh
(1111 1111 1111 1111)

New Mask
00FFFh
(0000 1111 1111 1111)

Is your new input only being used in that routine after the IIM, or is it being used elsewhere? The concern with the IIM is that your data could change mid scan.

If you only want to remove the one input from the IIM you would use 7FFF (07FFh). That would then be: 0111 1111 1111 1111

OG

See here for the technical details on how an IIM instruction works.

The mask controls which inputs are updated when the instruction is run. Any bit that is '1' in the mask will be updated, any bit that is '0' will not be. As such 00FFFh would set inputs 12 through 15 to not be updated by the instruction.

It shouldn't be necessary to change anything, unless having your new input update in mid-scan presents a problem for your logic.

The original mask allows all 16 inputs to be updated by the IIM instruction. Your proposed change to the mask would prevent your new input's status (along with the status of input points 12, 13 and 14) from being updated mid-scan, and will have no effect upon the input (I:9.0) that is currently being referenced after the mid-scan update.

Basically, any bit that is set to 0 in the mask will not be updated mid-scan by the IIM instruction, but will still update normally with the rest of the IO.

Thanks everyone for the responses, I changed the mask but it should not matter either way as the new input is an acknowledge button for an unrelated part of the equipment in a different ladder.

I did learn a lot about masks in the process, I've never really understood what they did or how they worked.