Apple Desktop Bus
The Apple Desktop Bus ( ADB ) is an interface standard defined by Apple for connecting external devices to a computer, which was used from 1986 to 1999. ADB is considered to be one of the forerunners of the Universal Serial Bus (USB).
description
It is a serial bus with a low transfer rate (10 kbit / s) for connecting input devices such as B. 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 on the bus, two are used for a 5 V power supply, one line is used for communication and the fourth is used to turn on the computer.
The devices and the computer communicate over a single bidirectional line. Communication is generally triggered by the computer by addressing one of the devices and sending it data or requesting data from it. Each device can have four logical registers that can be accessed in this way. If a device wants to send data, it can trigger a service request at the end of the transmission of each command on the bus , whereupon the computer queries all devices one after the other until the service request no longer occurs.
Devices on the ADB are supplied with power through the interface, at least if they require relatively little power. A total of up to 500 mA is available for all devices on one ADB, but there is no management of the power consumption, so that configurations can not work if too many or too power-hungry devices are connected.
The devices on the ADB are divided into 7 classes: Keyboards, Mice, Dongles, Absolute Devices (i.e. devices that transmit absolute coordinates such as graphics tablets), Data Transfer, Miscellaneous, and a reserved class. The class is determined by the address at which the device registers. Each device can also contain an 8-bit identifier that was given to the manufacturer by Apple. Some of the device classes have prescribed data formats in which the devices communicate. This made it possible in many cases to use devices without special drivers. This basic idea has been continued today with the device classes on the Universal Serial Bus .
Historical
The first computer with ADB was not an Apple Macintosh , but the Apple IIgs from 1986. From the Macintosh SE and II onwards, all Mac models were also equipped with the ADB, until this was gradually replaced by the Universal Serial Bus from 1997, first in the iMac (USB) has been replaced. The blue and white Power Mac G3, introduced in 1999, had both ADB and USB, the first G4 still had the ADB in the chipset, but no longer a socket for it, but this was still provided on the motherboard and could be retrofitted.
For software developers, the option of using the ADB was still of certain importance for some time, since the low-level debugger could only be operated with an ADB keyboard in older versions of MacOS.
The ADB has also been at the workstations of the company NeXT used.
Similar connections
- The cables and plugs of the ADB are identical to the S-Video standard, but ADB signals have nothing in common with S-Video signals; neither function nor signal type, level or direction of transmission. S-Video devices must therefore not be connected to ADB devices.
