Apple Desktop Bus

The Apple Desktop Bus (ADB ) is a defined by Apple interface standard for the connection of external devices to a computer, which was used from 1986 to 1999. ADB is considered one of the forerunners of the Universal Serial Bus (USB).


It is a serial bus with low transfer rate (10 kbit / s ) to connect input devices such as keyboards, mice, joysticks, software dongles and graphics tablets. A maximum of 15 devices could be addressed on this bus, they are connected via a 4-pin mini- DIN connector.

Of the four lines of the bus two for a 5 V power supply can be used, a line is used for communication and the fourth to turn on the computer.

The devices and the computer to communicate over a single bi-directional line. Generally, the communication is initiated by the computer by the one of the devices are addressed and sends this data or retrieves data from it. Each unit may have four logical registers which can be accessed in this manner. Will send data a device, so it can trigger on the bus a service request at the end of transmission of each command, and then the computer polls all devices one by one until the service request no longer occurs.

Devices on the ADB are supplied through the interface with power, at least unless they need relatively minor benefits. The total budget for all devices on a ADB to 500 mA is available, but there is no management of the electricity consumption, so that there may be non-functional configurations if too many or be connected to powered devices.

The devices on the ADB are divided into 7 classes: keyboards, mice, dongles, Absolute devices (ie, devices that transmit absolute coordinates, such as graphics tablets ), data transfer (data transfer), miscellaneous, and a reserved class. Through the address at which the device identifies itself, the class is defined. Each device can also contain an 8 -bit identifier that was given by Apple to the manufacturer. Some of the equipment classes have fixed prescribed data formats in which the devices communicate. This made it possible to use devices without special drivers in many cases. This basic idea has been continued today with the device classes on the Universal Serial Bus.


The first computer with ADB was not an Apple Macintosh, but the Apple IIgs in 1986. Starting the Macintosh SE and II and all Mac models with the ADB were then fitted until this since 1997, first in the iMac, step by step through the Universal Serial Bus (USB) has been replaced. The blue and white Power Mac G3, introduced in 1999, had both ADB and USB, the first G4 had the ADB also provided in the chipset, but no more for female, but this was not provided on the motherboard and could be retrofitted.

For software developers had the option to use the ADB, for some time a certain importance, since it could be in older versions of MacOS operated with an ADB keyboard, the low-level debugger.

The ADB has also been used at the workstations of the company NeXT.

Similar connections

  • The cable / connector of the ADB are identical to the S-Video standard, but ADB signals have nothing in common with S-video signals; either function or signal type, level or direction of transmission. S- video components must therefore not be connected to ADB components.
  • Hewlett -Packard Interface Loop (HP -IL)