4 PIN MiniDin Connector on Apple Computer
| Pin | Name | Description |
|---|---|---|
| 1 | ADB | Apple Desktop Bus |
| 2 | PSW | Power Switch |
| 3 | +5V | Power +5V |
| 4 | GND | Ground |
Apple Desktop Bus (ADB) is a proprietary low-speed bidirectional serial bus developed by Apple Computer for connecting input devices such as keyboards, mice, and graphics tablets. Introduced on the Apple IIGS in 1986, it was subsequently adopted across all Macintosh models from the SE and Plus era through the late 1990s. ADB uses a single open-drain data line operating at approximately 10 kbps, with a theoretical maximum of around 125 kbps, and supports daisy-chaining of up to 16 devices on a single bus. The bus protocol uses a host-polled model with four commands (Talk, Listen, SendReset, Flush) and identifies devices via a 4-bit address and an 8-bit handler ID.
Notes
- Power +5V is 500mA, each peripheral should use 100mA max
- The ADB data line is open-drain (active low) with a pull-up resistor on the host side; all devices share a single bidirectional wire.
- The Macintosh Portable cannot supply sufficient power for most ADB devices; only low-power-consumption peripherals should be connected to its ADB port.
- Up to 16 devices may be daisy-chained on a single ADB bus via the pass-through port found on most ADB keyboards and mice.
- Apple held patents on ADB; third-party manufacturers were required to obtain a licence to produce ADB-compatible devices.
Signals
- ADB (Apple Desktop Bus): Macintosh bidirectional serial data line, single open-drain wire carrying all bus traffic; supports Talk, Listen, SendReset, and Flush commands. Each device has up to four registers and is identified by a 4-bit bus address and an 8-bit handler ID.
- PSW (Power Switch): Switch signal used to start up, reset, or interrupt the computer from an ADB keyboard or other input device.
References
- About the Apple Desktop Bus — Inside Macintosh: Devices, Chapter 5 (Apple Computer, Inc.).
- Guide to the Macintosh Family Hardware, Second Edition — Apple Computer, Inc. / Addison-Wesley, 1990 (Tables 8-1, 8-2, and 8-14)
- Apple IIGS Hardware Reference — Apple Computer, Inc. / Addison-Wesley, 1989