AB: forced read of IO

I'm using an L33 PLC with an Ethernet I/P Point IO 1734-VHSC24 high speed counter to read in from an encoder, then calculating the counts per cycle. I've been having issues where the PLC cycle time is much faster than the IO update rate, such that in practice, I get several PLC cycles with the same count value, then a jump next time the IO updates.

I see that I can reduce the interval at which the IO updates, but I'd prefer to somehow link it's update to a timed task, or to the logic that reads in the counts, such that the count value is always up to date. Is this possible please?

Create a tag that contains the "last value" of the counter and then compare it to the current value. If they are not equal set a bit enabling your logic. At the end of your logic, copy the current value to the last value.

** keep the scan times as originally set, don't change them

since this is a CompactLogix platform - you might need to consider this ... it's a long shot - but it MIGHT be your issue ...

QUESTION - how many Tasks are you using - and what do you have their Priorities set for? ...

reason for asking: the CompactLogix platform (but not the ControlLogix) uses an "I/O Task" which has a priority of SIX ... so ... if you have other tasks with higher priorities (lower number settings) than SIX then SOMETIMES the Input and Output signals get "lost in the shuffle" when high-speed operations are involved ...

DISCLAIMER: I know that this can be a problem with Version 16 systems - but I have NOT had a chance to explore the same issues with higher versions ...

@OP and jstoraruk ...
There is an instruction that does exactly what you are looking for, look at "DTR"

why not put the calculation in periodic task sub program
You will find that there are may thinks the normal scan time is to fast and you have to put them in a periodic task program PID is one of them

rdrast said:

@OP and jstoraruk ...
There is an instruction that does exactly what you are looking for, look at "DTR"

Click to expand...

Yep that is one way to do it. I prefer NEQ, then I don't have to create a tag for the result.