Pin 1 -- col 1 4 7 *
Pin 2 -- col 2 5 8 0
Pin 3 -- col 3 6 9 #
Pin 4 -- col A B C D

Pin 5 -- row 1 2 3 A
Pin 6 -- row 4 5 6 B
Pin 7 -- row 7 8 9 C
Pin 8 -- row * 0 # D

(a) Each switch shorts one row to column
(b) Each pin should be connected to one bit of an i/o port.