How would I monitor I/O on multiple circuits at the same time?
Use eight comparators -- you can get 4/package using two LM339's -- and run the reference voltage (whatever you consider the level for a lead to be "active") to the + lead of each comparator.  The reference can be provided by a resistor divider, as shown in the schematic below (resistors R2 and R3).

Run each of the leads in your cable to the - leads of each comparator.  Connect the outputs to the cathode end of 8 LED's.  The anodes should be connected to +5 via resistors (shown as R1 in the schematic) chosen to limit the current to 6 mA (which is the minimum the LM339 can sink).

The reason for connecting the comparator "upside down" (reference to +, input to -) is so the comparator will ground the output (turn on the LED) when the input is > reference.

This scheme works whether the signals are analog or digital, as long as the reference voltage is chosen appropriately.



All eight channels can probably be built for two or three dollars.