PC Hardware in a Nutshell, 3rd Edition

AT Keyboard Interface

The AT keyboard interface was introduced with the IBM PC/AT in 1984, and is still used by the few AT and BAT motherboards still being produced. AT keyboards use the 5-pin DIN connector (female at the PC), shown in Figure 19-1 and whose pinouts are described in Table 19-1. On the PC side, the AT keyboard uses an Intel 8042 or equivalent interface chip, which is assigned IRQ1 and I/O base address 0060.

Figure 19-1. The AT keyboard connector

Table 19-1. AT keyboard interface signals and pinout

Pin	Signal name	Description
1	CLOCK	Keyboard clock; open collector CLK, CTS
2	DATA	Keyboard data; open collector RxD/TxD, RTS
3	RESERVED	Reset (usually not connected)
4	GROUND	Signal ground
5	VCC	+5VDC

The pin descriptions are self-explanatory, other than Pin 3. The 83-key IBM PC/XT keyboard and some 84-key IBM PC/AT keyboards used an early keyboard protocol that did not include a software reset command. For these keyboards, the PC uses Pin 3 to send a hardware reset to the keyboard. All systems and keyboards made in the last 15 years use a keyboard protocol that includes a software reset command, and nearly all recent keyboards leave Pin 3 unconnected.