Quote Originally Posted by MikeG View Post
I thought about the need for resistors, caps and opto to reduce the stray currents - it would be over kill for a small hobby cnc machine like what I have got.
Knock yourself out with whatever works for you.

But here's some real-life experience:

I got one of our software apprentices to knock up an interface module using a Uno - idea being to support a variety of input devices (typically switches), output these encoded via serial to a Pi driving a TFT display and sending the input switch states over ethernet to a control system. The generic nature of this meant that, depending on switch configurations, one or more inputs could be disconnected / open-circuit. Unused, unconnected inputs could be driven either high or low through the physically adjacent pins that were connected. Solution to this was to use the internal pull-ups but a floating input was highly susceptible to parasitic drives - whether from capacitive coupling, or possibly through contaminants(grease, sweat?) on the board.

Another one, recently, despite my views on opto-isolators, another user on here was experiencing sporadic limit switch triggers on a 6040 machine, which we resolved by using a commercially available opto-isolator board on the inputs to the BoB. Prior to that, without the spindle operating, but under axis drive (so just stepper motors) that user was experiencing sporadic limit switch trips within around 8 seconds of motion... had to use mach3's debounce setting values of more than 1000 (40mS for a 25kHz kernel) to get anything remotely steady operation before we got the opto-isolated inputs stable.

Of course, your results may vary.

80's?, damn, you pre-date me by a couple of years.