Renishaw TP-20 Probe Connection

jackdavies wrote: Can the Index signal at TB3-13 be used as a 5v input?.

Probably. Though I don't think that you have mentioned which Mesa card you are using.

The circuit shown is "NPN", depending on the interface card you are using you might want to change to "PNP". (quotes because I am using the terms loosely to mean "switch to gnd" and "switch to +V").

I am using the 7i76e which switches to +V as far as I know, so NPN would be the right one.

7i76e is sinking inputs on GPIO.

FIELD I/O
The 7I76 has a 32 input, 16 output isolated field I/O system to support a wide range
of input and output devices. The isolated I/O is intended for low voltage DC control
systems (commonly 24VDC). Inputs are sinking type. That is they sense positive input
voltages relative to field ground. Outputs are sourcing (7I76) or sinking (7I76D) type.
Sourcing outputs supply field power to field ground referred loads. Sinking outputs ground
field voltage referred loads when on

The encoder inputs are differential, but I don't know about sourcing/sinking in that context.
Adding a differential line-driver to your circuit might be a win.

I think a transistor could work here as the grounds are shared, and then I could just input it to a standard input.

arduino.stackexchange.com/questions/508/...circuit-with-arduino

There already is a transistor, Q1.
But I think that it would be better if Q1 was between the positive field voltage and a GPIO input. If you changed to LM358 you could run the whole circuit at 24V and (with a few resistor changes) still only have 15mV at the probe contacts.

Will this circuit be at the probe end or the machine end?

Yes that would be perfect, would be best at the machine end. Actual probe is only 13mm dia so not much space to package other electronics haha.

Would you know how to modify the schematic to make that work by any chance?

Thanks a lot!