DIY wired remote control unit design

A full microprocessor to run the keypad seems a bit much, especially if you don't have experience designing around them. There are I/O expanders that should be easier for you to work with; most have interrupts that make interfacing with another microprocessor a snap. I don't like recommending I2C for off-board, but 1m isn't too terribly long. Shift registers are another possibility. An exponential resistance ladder is probably not tenable with 16 switches.

There are plenty of examples of implementing the PS/2 protocol — both host side and device side — in the Arduino environment. My recommendation would be to use a simple Arduino board at each end of the connection and the PS/2 keyboard protocol to connect them.

Wire the switches to a 16-bit D to A converter (or use a resistors). Then connect to an analog input port that has at least 16-bit resolutions. If your analog inputs have only 8-bit resolution, then wire 8 switches to each.

You could use two 74HC151 to sense 16 switches using a total of 7 wires.

1. Vdd
2. GND
3.4.5. address
6. address (via OE and inverter) or data 1 
7. data 0 

Just scan the switches fast enough to detect changes. No point going really fast- you will need debouncing for mechanical switches anyway, so maybe scan at 100~150usec per switch, or about 500Hz for the array. I would use a periodic ISR and incorporate debouncing into the ISR (adjusting the scan rate to suit the bounce specifications of the switches).

Each switch will need a pullup resistor (could be part of an array). No diodes are required with this approach.

An even simpler approach is to use a PCF8575 I2C port expander. That requires only 4 wires, and the chip does not necessarily require pullup resistors for each input (and hobbyist modules are available). There are at least 3 manufacturers of the chip, so it's not single-sourced. Again you would poll the chip at < 1kHz to find the switch states and debounce.

If you can fit two x 1m Cat-5e cables;

That gives you 8 pairs and a ground wire shield for 16 signals and 2 shields that can be 0V and/or 5V and switch closure to ground or 5V.

You may use hardwire or use RJ-45 plug/jacks.

1. Typical standard Cat‑5e patch cords are about 5.0–5.5 mm OD.
2. Slim Cat‑5e cables with 28 AWG are around 3.5–3.8 mm OD, e.g. C2G and Tripp‑Lite/Eaton “slim” Cat‑5e lines.
3. “Ultra‑flat” Cat‑5e exists at about 5.0 mm × 1.2 mm if a flat profile helps more than a small round OD.

Few wires and lots of switches, means you can make a long word of the switch status (0 for open, 1 for closed...) and squirt that word every millisecond or ten over your cable. There's serial communication chips in most old PCs that facilitated that, and some of those 'serial port' functions are still built-ins on modern controller ICs.

If you need speed, fast updating can be handled by the old AMD taxi chips Am7968/Am7969 which might still be available. You could alternate between two 9-bit halves of your 16 bit switch state, allowing the 'extra' bit to choose which of two output latches to update.

Better, DS92LV16 is a serializer/deserializer (SERDES) that's still available, it's kinda overkill because it can be clocked at gigabit rates... but it kills the problem.