(At the computer)
5 PIN DIN 180° (DIN41524) FEMALE at the computer.
The IBM PC AT (Model 5170, 1984) keyboard connector is a 5-pin DIN 180° circular connector conforming to DIN41524, mounted at the rear of the system unit. It carries a bidirectional serial keyboard interface operating at TTL levels over open-collector lines, with the keyboard supplying the clock signal. The connector is physically identical to that used on the earlier IBM PC XT, though the AT introduced a bidirectional protocol and designates pin 3 as Spare rather than the XT’s active-low keyboard reset line. The same connector and protocol were retained until IBM introduced the 6-pin mini-DIN with the Personal System/2 range in 1987.
| Pin | Name | Description | Technical |
|---|---|---|---|
| 1 | CLOCK | Clock | CLK/CTS, Open-collector |
| 2 | DATA | Data | RxD/TxD/RTS, Open-collector |
| 3 | n/c | Not connected | Reset on some very old keyboards. |
| 4 | GND | Ground | |
| 5 | VCC | +5 VDC |
Signals
- CLOCK — Bidirectional open-collector clock line; the keyboard generates the clock during transmission, and the host may inhibit communication by holding the line low.
- DATA — Bidirectional open-collector data line; keyboard scan codes are transmitted serially in 11-bit frames (start bit, 8 data bits, odd parity, stop bit).
- n/c (Spare) — Designated Spare by IBM on the AT connector; on the earlier PC XT this pin carried an active-low keyboard reset signal, but that function was not carried forward.
- GND — Signal and power return; 0 V reference for logic levels and supply.
- VCC — +5 Vdc keyboard supply at TTL levels; the keyboard draws power from the host system.
Notes
- Both CLOCK and DATA lines are open-collector and must be pulled to +5 Vdc; either the keyboard or the host may assert a low level.
- The AT bidirectional protocol allows the host to send commands to the keyboard (e.g. set LEDs, reset); the earlier XT protocol was one-directional (keyboard to host only).
- The 5-pin DIN 180° connector looks identical on XT and AT systems, but the protocols are not interchangeable; XT keyboards will not initialise correctly on an AT host without an adapter.
- Pin 3 is labelled Spare by IBM for the AT; its use as an active-low reset line is an XT-era characteristic.