Editing Apple Desktop Bus (section)

== Technical Specifications ==

=== Physical Interface ===
{| class="wikitable" style="width:60%; text-align:center;"
|+'''ADB Connector Pinout (Mini-DIN 4)'''
! Pin !! Signal !! Description !! Specifications
|-
| 1 || ADB Data || Bidirectional serial data || Open collector, 5V pull-up
|-
| 2 || PSW || Power switch || Direct connection to power supply
|-
| 3 || +5V || Power supply || 500mA total, 100mA per device max
|-
| 4 || GND || Ground reference || Common ground
|}

The connector is physically identical to the 4-pin mini-DIN used for S-Video, leading to potential confusion. However, pins 1 and 2 are bridged in some S-Video cables, which can cause catastrophic damage if used with ADB devices.

=== Electrical Characteristics ===
* '''Bus topology:''' Multi-drop, daisy-chain capable
* '''Signal type:''' Open collector with 5V pull-up resistors
* '''Logic levels:''' TTL compatible (0V = low, 5V = high)
* '''Cable length:''' Maximum 5 meters total bus length
* '''Power distribution:''' 500mA at 5V total available
* '''Device power limit:''' 100mA per device maximum

=== Protocol Specifications ===

==== Bit Encoding ====
ADB uses self-clocking duty-cycle modulation where each bit period is 100μs:
* '''Logic 0:''' 65μs low, 35μs high
* '''Logic 1:''' 35μs low, 65μs high
* '''Start bit:''' Always logic 1
* '''Stop bit:''' Always logic 0

==== Transaction Structure ====
Each ADB transaction consists of:
# '''Attention signal:''' Host pulls bus low for 800μs
# '''Sync signal:''' Bus high for 70μs
# '''Command byte:''' 8 bits transmitted by host
# '''Stop bit:''' Logic 0
# '''Service Request window:''' 65-300μs (device can assert SRQ)
# '''Stop-to-start time (Tlt):''' 140-260μs bus high
# '''Data transfer:''' 2-8 bytes (if applicable)

==== Command Format ====
Commands are 8 bits structured as:
* '''Bits 7-4:''' Device address (0-15)
* '''Bits 3-2:''' Command type
* '''Bits 1-0:''' Register number

{| class="wikitable" style="width:70%; text-align:center;"
|+'''ADB Command Types'''
! Command !! Binary !! Description !! Action
|-
| SendReset || 00 || System reset || All devices reset to default state
|-
| Flush || 01 || Flush device || Clear device buffer for specified register
|-
| Reserved || 10 || Reserved || Not used
|-
| Reserved || 11 || Reserved || Not used
|-
| Listen || 10 || Write to device || Host sends data to device register
|-
| Talk || 11 || Read from device || Device sends register contents to host
|}

=== Device Registers ===
Each ADB device implements four registers:

{| class="wikitable" style="width:80%; text-align:center;"
|+'''Standard ADB Register Definitions'''
! Register !! Purpose !! Size !! Description
|-
| 0 || Device data || 2-8 bytes || Primary data (keyboard keys, mouse movement)
|-
| 1 || Device-specific || Variable || Extended features or configuration
|-
| 2 || Device-specific || Variable || Additional features (e.g., keyboard LEDs)
|-
| 3 || Status/Command || 2 bytes || Device ID, address, and handler ID
|}

==== Register 3 Format ====
Register 3 contains device identification and configuration:
* '''Bits 15-8:''' Exceptional event flags (device-specific)
* '''Bits 7-0:''' Device handler ID
* '''Bits 11-8:''' Device address (when read)
* '''Service Request Enable:''' Bit 13