Apple Desktop Bus

Apple Desktop Bus (ADB) is a proprietary bit-serial peripheral bus connecting low-speed devices to computers. It was introduced on the Apple IIGS in 1986 as a way to support low-cost devices like keyboards and mice, enabling them to be connected together in a daisy chain without the need for hubs or other devices. Apple Desktop Bus was quickly introduced on later Macintosh models, on later models of NeXT computers, and saw some other third-party use as well. Like the similar PS/2 connector used in many PC-compatibles at the time, Apple Desktop Bus was rapidly replaced by USB as that system became popular in the late 1990s; the last external Apple Desktop Bus port on an Apple product was in 1999, though it remained as an internal-only bus on some Mac models into the 2000s.

Early during the creation of the Macintosh computer, the engineering team had selected the fairly sophisticated Zilog 8530 to supply serial communications. This was initially done to allow multiple devices to be plugged into a single port, using simple communication protocols implemented inside the 8530 to allow them to send and receive data with the host computer. The idea was to allow multiple devices to be connected to a port, eliminating the need for many different types of ports or the use of expansion slots.

During development of this AppleBus system, computer networking became a vitally important feature of any computer system. With no card slots, the Macintosh was unable to easily add support for Ethernet or similar local area networking standards. Work on AppleBus was re-directed to networking purposes, and was released in 1985 as the AppleTalk system. This left the Mac with the original single-purpose mouse and keyboard ports, and no general-purpose system for low-speed devices to use.

The first system to use Apple Desktop Bus was the Apple IIGS of 1986. It was used on all Apple Macintosh machines starting with the Macintosh II and Macintosh SE. Apple Desktop Bus was also used on later models of NeXT computers. The vast majority of Apple Desktop Bus devices are for input, including trackballs, joysticks, graphics tablets and similar devices. Special-purpose uses included software protection dongles and even the TelePort modem.

The first Macintosh to move on from Apple Desktop Bus was the iMac in 1998, which uses USB in its place. The last Apple computer to have an Apple Desktop Bus port is the Power Macintosh G3 (Blue and White) in 1999. PowerPC-based PowerBooks and iBooks still used the Apple Desktop Bus protocol in the internal interface with the built-in keyboard and touchpad. Subsequent models use a USB-based trackpad.

In keeping with Apple's general philosophy of industrial design, Apple Desktop Bus was intended to be as simple to use as possible, ^{[citation needed]} while still being inexpensive to implement. A suitable connector was found in the 4-pin mini-DIN, which is also used for S-Video. In fact, many S-Video cables can be successfully used in place of their ADB counterparts. However, pins one and two on some (mainly lower-end) cables are bridged together, which can result in catastrophic damage to the system or ADB device. The connectors are small, widely available, and can only be inserted the "correct way". They do not lock into position, but even with a friction fit, they are firm enough for light duties like ADB.

ADB protocol requires only a single pin for data, labeled Apple Desktop Bus. The data signal is self-clocking by sending a 0 as low for 65 microseconds and high for 35 μs, while sending a 1 uses the opposite timing. It is a multi-drop open collector design with pull-ups to 5 V and uses collision detection to avoid device ID conflicts as well as clock stretching on the stop bit to indicate that a device requires a service request. Two of the other pins are used for +5 V power and ground. The +5 V pin guarantees at least 500 mA, and requires devices to use only 100 mA each. ADB also includes the PSW (Power Switch) pin which is attached directly to the power supply of the host computer. This is included to allow a keypress on the keyboard to start the machine without needing the ADB software to interpret the signal. In more modern designs, an auxiliary microcontroller is always kept running, so it is economical to use a power-up command over the standard USB channel.

The decoding transceiver ASIC as well as associated patents were controlled by Apple; this required vendors to work with Apple. In the Macintosh SE, the Apple Desktop Bus is implemented in an Apple-branded Microchip PIC16CR54 microcontroller.