Gray code encoder question

Hi I have a question about a gray code absolute 8 bit encoder. The encoder is connected to 8 inputs on an Allen Bradley PLC. if you turn the encoder 1 full turn it will count from 0 up to 255 but then instead of returning to zero it starts counting backwards on the next turn until it gets to zero and then on the next turn starts counting up again. I have 2 machines which both seem to work like this. Will this be a special encoder or is this done in software?

This is probably not answering your question but typically, one would get a special encoder PLC module.

Since you said it "works". i"m curious. The PLC logic must be tracking the phase, right?

I'm not that familiar with allen bradley but the physical inputs seem to be programmed into a special gray code block (which i cant open) and then in other blocks and on the HMI i can see a decimal position for the encoder. The input card is just a standard input card.

You're probably seeing the negative numbers because you're looking at a SINT datatype tag (which is an 8-bit Signed integer) and the actual underlying bits do not represent an integer at all, but rather a Gray Code bit pattern.

The protected instruction is just a decoder for the Gray Code. You should be able to just leave it alone and use the integer position output value.

Google 'Gray Code' to better understand what you are seeing. The raw value from the encoder can be confusing. Any images of the code disks from online articles should give you a better feel for the situation.

And kudos for spelling it correctly; Dr. Frank Gray is one of the great heroes of the Bell Labs era.

look in the software as the position should be the variable, and when an overflow it should be checked if it is a psoitive one or a negative turn.
and listin to bernie and ken