English: A 16 key keypad can be connected to the host electronics through 16 separate connecting leads, plus a ground lead (see Figure A). Pressing a key completes a circuit to "ground" which is detected by an electronic controller. Any number or combination of keys can be pressed simultaneously without any problems.

These 17 leads can be reduced to just 8 by using x/y multiplexing. For a 16 key keypad a 4 x 4 x/y array is usually used (Figure B). 4 I/O lines are outputs and the other 4 are inputs. A circuit is completed between an output and an input when a key is depressed, each individual keypress creating a unique signal that is accurately interpreted by the host electronics.

X/Y multiplexed Keypads are usually operated one key at a time, however, two simultaneous keys can be pressed, if necessary, without any problems. However, care has to be taken when pressing three keys as "key ghosting" can occur with certain combinations.

Figure C shows that the 8 leads could be used to detect 28 separate keys if "tri-state multiplexing" is used instead of x/y multiplexing. This method is perfectly satisfactory when one key is pressed at a time, but issues can occur with some combinations if two keys are pressed simultaneously.

Tri-state multiplexing enables (n-1)(n/2) keys to be detected with just n I/O lines. If a diode is used, in series with each key, then the number of unique keys detectable is doubled .