PC Hardware in a Nutshell, 3rd Edition

PS/2 Keyboard Interface

The PS/2 keyboard interface was introduced with the IBM PS/2 series in 1986, and is now used by all ATX and ATX-variant motherboards. PS/2 keyboards use the 6-pin mini-DIN connector (female at the PC), shown in Figure 19-2 and whose pinouts are described in Table 19-2. On the PC side, the PS/2 keyboard uses the same Intel 8042 or equivalent interface chip as the AT keyboard, which is also assigned IRQ1 and I/O base address 0060.

Figure 19-2. The PS/2 keyboard connector

Table 19-2. PS/2 keyboard interface signals and pinout

Pin	Signal name	Description
1	DATA	Keyboard data
2	RESERVED	No connection
3	GROUND	Signal ground
4	VCC	+5VDC
5	CLOCK	Keyboard clock
6	RESERVED	No connection
Shield	-	Ground

The AT and PS/2 keyboard interfaces use incompatible connectors, but are electrically and functionally identical. You can connect an AT keyboard to a PS/2 keyboard port or vice versa by building or buying an adapter that uses the pinouts listed in Table 19-3. Many new keyboards come with an adapter to allow their use with the other style connector. Such adapters can also be purchased at most computer stores.

Table 19-3. Pinouts for an AT-to-PS/2 or PS/2-to-AT adapter

PS/2 pin	AT pin	Description
1	2	Keyboard data
2	3	No connection
3	4	Signal ground
4	5	+5VDC
5	1	Keyboard clock
6	-	No connection