I upgraded one of my CNC machines with a VFD controlled, 0.8kW brushless spindle and seems that has caused damage to the control board from stray currents from the spindle.
The tool input is always on and when I measure the resistance between the IN5 and GND it shows about 700 Ohms, which is different from other working inputs (limit switch inputs) that go through a different buffer chip, IN1 also is always on so I guess the buffer chip for those inputs is damaged.
It's an old board so I don't think it would be worth sending it for repair, and I can replace SMD chips myself anyway, a 74HC14D is the correct part, yes?

The stray current problem will still remain, since it's inevitable to have stray current when using an EMI filter like they one I have in the VFD so if I want to use the tool sensor I'll have to figure out a way to do it without damaging the board again, or my other board a newish Mk3 9 axis, and luckily I didn't try out the new spindle with that one.

I suppose I should use some sort of isolation circuit to protect the Mk2/4 inputs, any suggestions on the best way to do that?

My idea would be to use an optocoupler powered by the power supply in the VFD control box and have the Mk2/4 IN5 on the other side, as in this schematic.



Does it look correct?