PC Hardware in a Nutshell, Second Edition

Keyboard Interfaces

Keyboard interfaces are well standardized, and have been for years. This means that, with the exception of antique PC and PC/XT keyboards, you can plug any keyboard into any PC to which it can physically connect and expect it to work. The following sections detail the three keyboard interfaces currently in use.

AT Keyboard Interface

The AT keyboard interface was introduced with the IBM PC/AT in 1984, and is still used by current production AT and BAT motherboards. 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 fifteen years ...