Trying to setup a tachometer back to the breakout board (port 1, pin 10) to get a speed index for threading on a small lathe that I have just converted to CNC. I made my first cuts in manual mode to test it out. Would like to head towards threading tests. Tried using the machine's own digital tach I/O pins to route back to the breakout board. I measured +- 5 V off of the back of the digital tach module. That one triggers on 4 magnets equally spaced. No luck.

I then bought a hall sensor kit from Amazon that was 120V input so I could just wire it from the power supply (also 120) on the lathe. That works and the digital readout works on one magnet (one pulse per rev) so all good. Input circuit of the tach sensor is about 12v DC and when I measure the signal switched loop I get just about 7.5VDC. The tach is working properly and when the magnet passes in front of the tip of the sensor the light lights and the signal goes to zero VDC. So this is set up active low. It is a 3 wire sensor.

All that for this question. Is 7.5V going to be too much for the breakout board?

My understanding is that the breakout board is just seeing the change in switch state and then you assign that to the Index input in Config in Mach3.

I am stumped on this simple thing.

Yes, the BoB probably has 5v logic.  A simple potential divider with 4k7 to the 7.5v output and 10k to ground should fix it.

Is there any chance that 7.5 will blow the input circuit on the BOB?

Yes, that's why I suggested a potential divider!

Thanks. Got it.