HCSCI allows you extend each single input to x16 using a cheap extension boards (multiplexers 1x16). All multiplexers used for inputs are connected to 4 common address lines: S0,S1,S2,S3 (or pins #22,23,24,25). So, you need only 4 wires to connect all input multiplexers together and, of course you need to use 2 power lines - +5V and Comoon Ground for each board.
The SIG (can be labeled as "Z" on some boards) output of every input multiplexer is connected directly to the pin assigned in the configurator for this input extension.
Note: The "EN" signal of each input extension board should be always logical "0" (set to GND). Usualy the multiplexer breakout board has a pull-down resistor on this 'EN" pin, so it can be already set to logical "0" but you need to be sure it is, so just connect it to GND.
You can group a set of switches into one module/panel with multiplexer extension boards. Place several modules in different parts of your cockpit and connect them to the main board using only one pin for each multiplexer and the common 4 address wires that are laid across the cockpit to each multiplexer:
NOTE: the "golden tubes" on diagrams above are NOT the grounded shields, just some "binding" tube to make the whole wiring cleaner.