Apple Desktop Bus

~ the last words ~

ADB arrived on the Apple IIGS on 15 September 1986 and came to the Macintosh the following year with the Mac SE and the Mac II. It was a single-master bus: the computer polled, and nothing on the wire spoke until it was asked. Maximum theoretical throughput was 125 kbit/s and the number people actually measured was closer to 10.

That was enough. A keyboard generates a few bytes a second and a mouse not much more, so thirteen years of Apple input hardware ran over a wire slower than a fast modem. Sixteen addresses existed on paper and about five worked in practice. Apple supported three.

The iMac shipped on 15 August 1998 with two USB ports and no ADB socket. It had dropped SCSI and the Mac serial port in the same move. Griffin Technology had announced a USB-to-ADB adapter in July, before the machine it was for went on sale. The migration problem was visible to third parties a month ahead of the hardware that caused it.

~ the record ~

~ the round plug ~

Four pins in a mini-DIN shell: data, a power-on line, five volts, ground. The connector was physically identical to S-Video, so the cable went happily into the wrong socket. It was also close enough to the six-pin PS/2 plug the PC world used that the two still get mixed up in parts bins today.

~ a nominal fee and an assigned address ~

ADB was patented, and building a device for it meant taking an Apple license. Apple's own technical note says the fee was nominal in most cases, and that the licensing process is where a manufacturer was handed its default device address and handler ID. Apple therefore kept a register of everything that could be attached.

Thirteen years of third-party trackballs, graphics tablets, dongles and joysticks went through that process. That register is the difference between a connector and a bus. A connector is a shape; a bus has a list of who may speak on it.

~ what the fuse was for ~

Hot-plugging was electrically unsafe on nearly every machine that had the port. Connecting a device with the power on could blow a fuse that was soldered to the motherboard, which meant a repair rather than a replacement part. The bus guaranteed at least 500 mA across all devices and allowed each one 100 mA. Nothing in that budget protected it from a careless hand on a keyboard cable.

The keyboard plug I grew up with was the other one. A PC in Turkey in the nineties had a DIN-5 socket and then a PS/2 mini-DIN-6, round sockets that looked near enough to ADB to fool anybody who had not counted the pins. I never touched an ADB device. The machines that carried the port were rare in Turkey and priced out of reach on the occasions they did turn up, so the bus and I were kept apart by a price list rather than by anything technical.

~ the bus that outlived its connector ~

The PowerBook G3 Lombard of May 1999 removed the external ADB port and kept the protocol inside the case, where it still ran the built-in keyboard and the trackpad. PowerPC laptops carried it internally for years afterward, invisible, with no socket to name it by. The plug is what ended in 1999. The bus underneath ran on at roughly 10 kbit/s inside closed lids until Apple moved the trackpad to USB.

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