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[Reply #15 on:]

Ahh ok, gotchya.  Thanks for the reply .

[Reply #16 on:]

@daSilva would you mind clarifying your thought here:

Edit2:
I just read your paragraph about "involvement". I see the danger that the configuration might become unmaintainable by permanent and uncontrolled changes

I think I have all the code worked out (indicators, relays, fault conditions, and computer interface).  I've been playing with a mock up design on a breadboard using an Arduino Nano and am about to start working on a Shield for the Seeeduinio Mega.

Those parts that were linked by daSilva look promising... however, all of the controls and indicators run on 24V.  It would be nice to protect my inputs and outputs from accidentally be crossed with 24V.

[Reply #17 on:]

Using this optoisolator: http://www.fairchildsemi.com/ds/MC/MCT62.pdf

that I had in my shop left over from a previous project, and a series 820 ohm 1W resistor, I was able to put up to 30V on the input pins and not smoke anything.  Circuit still works fine off 5V.

I think I'm going to use this for my input pins...