RJ11 FEMALE CONNECTOR at the Computer.
RJ11 MALE CONNECTOR at the Keyboard.
The pre-ADB Macintosh keyboard interface uses a 4-conductor modular jack (4P4C), colloquially referred to as RJ-11, mounted on the front panel of the computer. It was fitted to all four pre-ADB models — the Macintosh 128K (1984), 512K (1984), 512Ke (1986), and Plus (1986) — and was superseded by the Apple Desktop Bus (ADB) with the introduction of the Mac SE and Mac II in March 1987. Communication is synchronous serial: the clock is generated by an Intel 8021 microprocessor inside the keyboard, never by the host, and data is latched into the host’s VIA shift register on the rising edge of the clock. The coiled cable carries +5 V and ground on the outer pins (1 and 4) and the signalling pair on the inner pins (2 and 3).
| Pin | Name | Dir | Description |
|---|---|---|---|
| 1 | CGND | - | Chassis ground |
| 2 | KBD1 | ? | Keyboard clock |
| 3 | KBD2 | ? | Keyboard data |
| 4 | +5V | OUT | +5 VDC |
Notes
- Direction is Computer relative Keyboard.
- The connector is a 4P4C modular jack (all four positions loaded), not the 6P4C telephone line jack also commonly called RJ-11.
- The coiled cable reverses polarity at one end, as is standard for telephone handset cords. A standard telephone handset cable is therefore incompatible and risks shorting +5 V to ground, potentially destroying the keyboard controller.
- The clock line (pin 2, KBDCLK) is driven exclusively by the Intel 8021 MCU inside the keyboard. Keyboard-to-host transmissions use 8 × 330 µs cycles (160 µs low / 170 µs high); host-to-keyboard transmissions use 8 × 400 µs cycles (180 µs low / 220 µs high).
- The data line (pin 3, KBDDATA) is bidirectional; data is latched into the host VIA shift register on the rising edge of the keyboard clock.
- All four pre-ADB models (128K, 512K, 512Ke, Plus) share identical connector type and pin assignments.
Signals
- CGND — Chassis ground; return path for both power and signal.
- KBD1 — Keyboard clock (KBDCLK); generated by the Intel 8021 MCU inside the keyboard, not by the host Mac.
- KBD2 — Keyboard data (KBDDATA); bidirectional synchronous serial data line shared for host-to-keyboard and keyboard-to-host transmissions.
- +5V — +5 VDC supply from the host computer to power the keyboard’s internal microprocessor.