Editing Apple Desktop Bus (section)

=== Physical ===
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|date=February 2021}} 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 signal|self-clocking]] by sending a 0 as low for 65 microseconds and high for 35 μs, while sending a 1 uses the opposite timing.<ref name="macintosh-hardware">{{cite book|url=https://archive.org/details/apple-guide-macintosh-family-hardware/page/n357/mode/2up?view=theater|title="Guide to Macintosh Family Hardware, second edition"|page=317-320|isbn=0-201-52405-8|date=1990|author=Apple Computer, Inc}}</ref> It is a multi-drop [[open collector]] design with pull-ups to 5&nbsp;V and uses [[Carrier-sense multiple access with collision detection|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.<ref name="macintosh-hardware"/> 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]] [[Application-specific integrated circuit|ASIC]] as well as associated [[#Patents|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 [[PIC microcontrollers|Microchip]] PIC16CR54 microcontroller.

[[File:Apple Desktop Bus Microchip.jpg|thumb|ADB is implemented in an early Microchip PIC microcontroller in the Macintosh SE.]]