Kirk Wallace wrote: > Just in case, my guess is that the internal resistor was sized for > current limiting 5 Volts, so the current would go up by around 120% > without an external resistor, which is okay? > The 5 V isolated source is IN the UPC (and USC) board. It cannot be bypassed the way it is wired. You create a one by closing a circuit from the individual signal to the isolated ground. So, each digital input is a sourcing terminal that is grounded to complete the circuit. So, you never supply current to the input, you sink current from it to the ground terminal. (Harder to explain this in words that to draw a picture.) The internal resistor is 1 K Ohms, so you get about 3 mA from the 5 V source, through the opto-coupler's LED, the resistor and then to ground. If you were to apply 12 V to the input, that would reverse bias the opto-coupler's LED with about 7 V, which is definitely above the ratings. I know that adding a diode drop in the circuit will not prevent it from detecting the switch closure.
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