Electronic – Question about modem (analog/digital)

You don't need anything as complicated as a A/D converter. This can be done with digital logic signals easily enough. On one side, tie each line to a digital output via a 1 kΩ resistor. On the other side, tie each line high via separate 10 kΩ resistors, then each to its own digital input. Then it becomes a matter of driving only one output at a time low with the others high and checking for that pattern at the other end.

This would be a lot easier to do in a microcontroller directly than to communicate all those digital inputs and outputs back to a tablet, which then runs thru this very simple algorithm. A micro can run thru the whole test nearly instantaneously in human time, so it can be doing it continually and setting a pass/fail indicator after every test. The device would always be saying "fail" until a correct cable is plugged in, then it would pretty much instantly say "pass".

If you really needed information back to a tablet for logging or whatever, then you can use a micro with a serial port and external circuitry to provide a RS-232 connection via a DB-9 connector. Then you can use a off the shelf USB to serial adapter to communicate between the tablet and the micro.

Spehro's comment is basically the answer. Imagine a link which subtracts a random, varying noise value between 0 and 0.5V from the input. You measure 4.5V at one end, but that could mean either a 4.5V input with no noise or 5V minus noise. You can't tell, and have lost your signal entirely.

With a digital signal varying between 5V and 0V over the same link, you can declare everything over 2.5V to be a '1', and lose no signal.