If you are trying to get rpm then you want to look at the CF instruction which is a frequency counter block.
The MFD has 4 frequency counter blocks. Each is associated with one hard input so you can't do quadrature or anything fancy like that but it will be able to give you rate. In the help for the CF instruction it says that the limits on the frequency are from 4Hz to 5kHz so hopefully, your encoder output will be in this range.
So, if your A signal from your encoder is wired to input 1 then use CF 01 and store the output value in a marker word, lets say MW10. This number will be the number of Hz that the encoder is producing on the A channel.
Then you need to turn this number into RPM. For that I would use the LS or value scaling block and then store the scaled value into another marker word, say MW11. It works a lot like the SLC/500 SCP function except that the output value is clipped as shown below.
As an example, if you have a 100 pulses per rev encoder, then at 4Hz you have 2.4 rpm and at 5000Hz you have 3000 rpm. Since everything here is in integer, use the LS block to scale the 4 Hz to 24 and the 5000 Hz to 30000. Then you can add a decimal point when you display the number.
Note that I haven't tried this but I do have MFD experience. One interesting thing from the help on the frequency counter block states:
Measuring procedure: The pulses at the input are counted for one second, irrespective of the cycle time, and the frequency determined. The measurement result is provided at the function block output QV.
So, you may have to re-trigger the block each second. I don't have the hardware to test this rght now. Also, you may need to flash the scale block with a flash timer to keep the display updating.
I hope this helps.
