Serial Mouse Pinout 🆕 Quick

Early optical mice (with special reflective pads) used the same DE-9 pinout but required +12V on Pin 4 (instead of +5V). Feeding one of these +5V will cause it to fail silently.

The is a foundational blueprint of legacy computing hardware. Before the widespread adoption of PS/2 or USB standards , the optomechanical mouse communicated directly with a PC using an RS-232 serial interface. Because standard RS-232 serial ports do not feature a dedicated 5V or 12V power rail, the serial mouse uses an ingenious hardware hack: it siphons electrical power directly from the communication handshake lines.

The mouse sends data to the computer via . It does not use Pin 3 (TXD) except in rare bidirectional protocols.

This article provides a deep dive into the serial mouse pinout, wiring diagrams, voltage requirements, and troubleshooting tips for legacy hardware. serial mouse pinout

You cannot simply wire a serial mouse to a USB connector. USB uses 5V logic and differential data pairs (D+/D-); RS-232 uses ±12V single-ended signals. Doing this will destroy your USB port.

To understand why the pins are assigned this way, we must look at the functions:

| Pin | Signal | Direction (mouse → PC) | Description | |-----|------------|------------------------|--------------| | 1 | CD / N/C | – | Carrier detect – not used for mouse | | 2 | RXD | ← Mouse transmits → PC | (main data line) | | 3 | TXD | PC → mouse | Data to mouse (rarely used) | | 4 | DTR | PC → mouse | +5V to +12V power for mouse | | 5 | GND | – | Ground | | 6 | DSR | ← Mouse → PC | Usually unused | | 7 | RTS | PC → mouse | +5V to +12V power (alternative/secondary power) | | 8 | CTS | ← Mouse → PC | Reset / activity (some mice use for handshake) | | 9 | RI / N/C | – | Ring indicator – not used | Early optical mice (with special reflective pads) used

For further reading, research the "Microsoft Serial Mouse Protocol Specification" (1987) or the "Mouse Systems PC Mouse Protocol."

A serial mouse uses the standard RS-232 interface to communicate with a computer. While modern systems use USB or PS/2, legacy serial mice typically utilize a 9-pin (DE-9) or 25-pin (DB-25) D-subminiature connector. Common 9-Pin (DE-9) Pinout

Understanding the exact mapping of these pins is essential for vintage PC restoration, embedded hardware engineering, or building custom serial-to-USB adapters. The Standard DB9 Serial Mouse Pinout Before the widespread adoption of PS/2 or USB

Apple’s Macintosh (pre-USB) used a connector, not DE-9. Pinout is completely different:

The serial mouse spoke in "packets." Every time you nudged it, it screamed a at 1200 baud . It would tell the computer: "I moved left by 5 units, and my left button is pressed!" .

Early serial mice were not self-powered. They relied on the host PC’s serial port to provide power. RS-232 voltage levels are odd: "High" is +5V to +15V, and "Low" is -5V to -15V.