mesa 7i76 configuring a spindle encoder

I don't know the pin name (I don't have a 7i76). Do you have any pins that contain "enc" in the name?

can't see any

hm2_5i25.0.encoder.00.position

Clive s wrote: can't see any
hm2_5i25.0.encoder.00.position

That's probably the one. ....enc0 is one of the optional MPG channels, now I think about it.
Sorry for the confusion.
Does hm2_5i25.0.encoder.00.position change value as you rotate the spindle?

Andy Looking at hm2_5i25.0.encoder.00.position when triggering the opto from 5V to 0V the hal meter stays on 0 and does change

I have taken my encoder disk off so that I can make sure the ir light is blocked for testing

It sounds like you only have one channel? And you mentioned the index earlier?

The position won't change if you only drive the index pin.

Andy I have just thought. the 7i76 has IDX+ and IDX- I have not grounded the IDX- also A+ and A- etc Do you think I need to ground the (0v) IDX- A- etc

which hal pin should change just with IDX connected