Encoder Inputs

Hi all,

Fairly new to PLC programming so I have what is hopefully a simple question.

Should you/would you map raw encoder inputs to internal tags to use in your program as you would with digital/analogue signals? And if so/or not, why?


Thanks in advance,
LB

If by raw inputs you mean channel A and B or Pulse and direction wired into normal digital inputs, then you shouldn't do it because:

- It's going to be harder to code. You'll have to check for flanks to feed a counter, etc...
- You will limit the bandwidth of the encoder to the cycle time of the PLC, or you'll miss pulses.
- You may be able to program interrupts on the CPU, but that's not a nice approach on PLC's, considering that there is dedicated hardware for it.

So the best way is really to use a counter card and interface it with the standard way for whatever brand of CPU you have.

This

If I am correct assuming your meaning, it's handy to be able to see the pulses for troubleshooting purposes. As for control, the inputs typically are too well filtered to be useful in moderate speed applications. But for troubleshooting, being able to see the inputs without having to get out a voltmeter and opening the cabinet can be useful.

Yes, my apologies, I meant Channel A & B inputs. I am using a CompactLogix L30ER CPU and I have a High Speed Counter card so I will use the inputs directly from the card in my program. Thanks for the responses!

With CompactLogix HSC cards, tags are automatically made that show the status of the channels. For example, Local:X:I.InputStateA0 will show the status of A0.