9 PIN D-SUB FEMALE at the computer
9 PIN D-SUB MALE at the mouse cable
The DE-9 (commonly called DB-9) female receptacle on the rear panel of the pre-ADB Macintosh carries the mouse interface for all four models: Macintosh 128K (1984), 512K (1984), 512Ke (1986), and Plus (1986). Despite using a shell physically identical to the Mac’s RS-422 serial ports, the connector carries an entirely different signal set based on quadrature encoding. Motion is detected by two infrared beams per axis producing square waves 90° out of phase; the interrupt lines (X1, Y1) connect to the Zilog 8530 SCC and signal that movement has occurred, whilst the quadrature lines (X2, Y2) connect to the MOS 6522 VIA and determine direction. The Macintosh Plus was the last model to use this interface; the Mac SE and Mac II (1987) replaced it with the Apple Desktop Bus.
| Pin | Name | Dir | Description |
|---|---|---|---|
| 1 | CGND | - | Chassis ground |
| 2 | +5V | OUT | +5 VDC |
| 3 | CGND | - | Chassis ground |
| 4 | X2 | IN | Horizontal movement line (connected to VIA PB4 line) |
| 5 | X1 | IN | Horizontal movement line (connected to SCC DCDA-line) |
| 6 | n/c | - | Not connected |
| 7 | SW- | IN | Mouse button line (connected to VIA PB3) |
| 8 | Y2 | IN | Vertical movement line (connected to VIA PB5 line) |
| 9 | Y1 | IN | Vertical movement line (connected to SCC DCDB-line) |
Signals
- CGND — Chassis ground; present on both pin 1 and pin 3.
- +5V — Supplies power to the mouse’s infrared emitters and sensor circuitry.
- X2 — Horizontal quadrature line; 90° out of phase with X1, connected to the VIA data register B port B to determine the direction of horizontal movement.
- X1 — Horizontal interrupt line; connected to the SCC DCDA input to signal that horizontal motion has occurred.
- n/c — Not connected.
- SW- — Mouse button line; active when the single mouse button is pressed, connected to VIA port B bit 3 (PB3).
- Y2 — Vertical quadrature line; 90° out of phase with Y1, connected to the VIA data register B port B to determine the direction of vertical movement.
- Y1 — Vertical interrupt line; connected to the SCC DCDB input to signal that vertical motion has occurred.
Notes
- Direction is Computer relative Mouse.
- The connector shell is physically identical to the Macintosh RS-422 serial DE-9 ports but carries a completely different, incompatible signal set; plugging a serial device into this port will not function.
- Pins 1 and 3 are both ground; pin 6 is not connected, leaving five active signals: power, two motion-interrupt lines, two quadrature lines, and the button.
- This interface was present unchanged across all four pre-ADB Macintosh models (128K, 512K, 512Ke, Plus).