Sixteen keys, eight wires

A key is a switch

Each key is two contacts that touch while you press it. On the soft keypad they are printed on two layers of plastic film; on the heavy-duty keypad they sit under separate raised keys. Either way, pressed means connected and released means not.

Sixteen switches could have a wire each, plus a shared ground. That is seventeen wires, and nobody builds a keypad that way.

Rows and columns

A matrix keypad puts every switch where a row wire crosses a column wire. Four rows and four columns cross in sixteen places, which is one crossing per key. Pressing a key connects its row wire to its column wire and nothing else.

Press 6 and row 2 is joined to column 3. No other key joins that pair, so the pair names the key. Eight wires carry all sixteen keys.

The eight come out of the keypad in a row: along the soft keypad's ribbon tail, and as eight pins on the heavy-duty keypad's bottom edge.

What a board has to do

The keypad cannot say which key is down. The board has to find out, by pulling one set of wires to 0 V and watching which wire on the other side follows it down through the pressed key.

Chapter three shows how the adapter does that in two reads. First, the problem the adapter exists to solve: eight wires is still a lot of pins.