9 PIN D-SUB MALE at the computer
9 PIN D-SUB FEMALE at the mouse/joy cable
The Atari ST family — comprising the ST, Mega ST, STE, Mega STE, and TT030 — uses a standard 9-pin DE-9 connector for mouse and joystick input. Two ports are present: Port 0 (the combined mouse/joystick port) and Port 1 (joystick-only). The ports are driven by the Intelligent Keyboard (IKBD) subsystem, an HD6301V1 8-bit microcomputer housed in the keyboard unit that decodes quadrature mouse signals, reads joystick switch states, and forwards data to the main CPU over a serial link at 7812.5 baud. The TT030 (1990) extends the design with support for a third mouse button whilst maintaining full pin compatibility with earlier ST and Mega STE hardware.
| Pin | Mouse | Joystick | Dir |
|---|---|---|---|
| 1 | XB | UP | IN |
| 2 | XA | DOWN | IN |
| 3 | YA | LEFT | IN |
| 4 | YB | RIGHT | IN |
| 5 | n/c | n/c | - |
| 6 | LEFTBUTTON | FIRE | IN |
| 7 | +5V | +5V | OUT |
| 8 | GND | GND | - |
| 9 | RIGHTBUTTON | res | IN |
Signals
- XB — Quadrature channel B for horizontal (X-axis) mouse movement; corresponds to joystick Up in digital mode.
- XA — Quadrature channel A for horizontal (X-axis) mouse movement; corresponds to joystick Down.
- YA — Quadrature channel A for vertical (Y-axis) mouse movement; corresponds to joystick Left.
- YB — Quadrature channel B for vertical (Y-axis) mouse movement; corresponds to joystick Right.
- n/c — Pin 5: not connected on both Port 0 and Port 1.
- LEFTBUTTON / FIRE — Left mouse button or joystick fire button (active low, pulled to GND when pressed).
- +5V — 5 V DC supply from the computer, shared by both ports.
- GND — Signal and power return.
- RIGHTBUTTON / res — Pin 9 has a dual role on Port 0: right mouse button when a mouse is attached, and the fire-button signal for Port 1 (joystick 1) when two joysticks are in use. Pin 9 is not connected on Port 1.
Notes
- Direction is Computer relative Device.
- Port 0 is the right-side DE-9 connector (mouse/joystick); Port 1 is the left-side connector (joystick-only).
- Mouse quadrature encoding: XA/XB encode horizontal motion, YA/YB encode vertical motion. The Atari mouse achieves a minimum resolution of 100 counts per inch with a maximum tracking velocity of 10 inches per second.
- The joystick type is four-direction switch-only (digital); no analogue signals are present.
- Pin 9 of Port 0 is shared: it carries the right mouse button signal and also the fire input for the joystick connected to Port 1.