ranger@ecsvax.UUCP (05/07/87)
Apple Desktop Bus: Description The Apple Desktop Bus (ADB) is a standard for input devices connecting to the Apple IIGS and other computers that follow the Apple Desktop Bus protocol. In the Apple IIGS, the Apple Desktop Bus is run by a microcontroller that accepts input from the devices connected to the ADB, and makes appropriate calls to the Apple IIGS tools. Because the Apple Desktop Bus is run by an intelligent microcontroller, a number of different types of input devices may be connected to the ADB simultaneously: the computer's keyboard and a mouse and perhaps a tablet, light pen, second keyboard, or joystick. Each device has a unique bus address, so that the ADB microcontroller may direct its commands to a particular piece of equipment. The ADB is limited to a maximum of 16 unique devices. On the Apple IIGS, the control function is performed by the M50740 Keyboard Microcontroller. It uses a superset of the 6502 instruction set, and contains 96 bytes of RAM and 3K bytes of ROM. When the microcontroller requests input from a device, it sends a signal to the input device to "talk". If no return input information action (key pressed, mouse movement, button clicked, etc.) has occurred, the microcontroller keeps waiting for the device to respond until a time-out occurs. The host may also instruct a device to "listen" to data being sent on the bus from the host. All devices on the Apple Desktop Bus must include the intelligence to respond to both talk and listen commands. The Apple Desktop Bus uses a 4-pin mini-DIN jack and a 4-wire cable, with serial interface signals. When appropriate, the input device will have two ADB jacks, so that devices may be daisy-chained from the host. The Apple Desktop Bus Mouse does not have a second connector, so it must be at the end of the chain. Apple Technical Communications