Apple Desktop Bus

Apple Desktop Bus
ADB connector (female socket)
Type	Computer peripheral interface
Designer	Steve Wozniak
Design date	1985-1986
Manufacturer	Apple Computer
Production	1986-1999
Superseded	USB
External	Yes
Hot-pluggable	Technically capable but not recommended
Cable	4-conductor shielded
Pins	4
Connector	Mini-DIN 4 (same as S-Video)
Electrical	Single data line, open collector
Maximum devices	16
Protocol	Serial, self-clocking
Data signal	Bidirectional serial
Data rate	10-125 kbit/s
Style	Serial

Apple Desktop Bus (ADB) is a proprietary bit-serial peripheral bus connecting low-speed input devices to computers. Designed by Steve Wozniak and introduced on the Apple IIGS in September 1986, ADB was created to provide a simple, low-cost method for connecting keyboards, mice, and other input devices in a daisy-chain configuration without requiring hubs. The technology was quickly adopted for the Macintosh line beginning with the Macintosh SE and Macintosh II in March 1987, and remained Apple's standard peripheral interface until its replacement by USB in 1998-1999.

History and Development

Origins

During the early development of the Macintosh in the 1980s, Apple engineers had selected the sophisticated Zilog 8530 for serial communications. However, this solution was expensive and complex for simple input devices. Steve Wozniak, working on the Apple IIGS project in 1985-1986, developed ADB as a simpler, more cost-effective alternative that could support multiple devices on a single bus.

The design philosophy emphasized simplicity and low cost while maintaining reliability. Wozniak's solution used a single data line with self-clocking signals, eliminating the need for separate clock lines and reducing component count. The protocol was sophisticated enough to support device enumeration, collision detection, and service requests, yet simple enough to implement with minimal hardware.

Implementation Timeline

• September 1986: First implementation in Apple IIGS
• March 1987: Adopted by Macintosh SE and Macintosh II
• 1987-1998: Standard on all desktop Macintosh computers
• 1991-1998: Used in PowerBook line
• 1989-1996: Implemented in some NeXT computer models
• 1998: Phased out with introduction of iMac G3 (USB-based)
• 1999: Last external ADB port on Power Macintosh G3 (Blue and White)
• 2000-2005: Internal ADB protocol continued in some PowerBooks for built-in keyboards/trackpads

Technical Specifications

Physical Interface

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:

1. Attention signal: Host pulls bus low for 800μs
2. Sync signal: Bus high for 70μs
3. Command byte: 8 bits transmitted by host
4. Stop bit: Logic 0
5. Service Request window: 65-300μs (device can assert SRQ)
6. Stop-to-start time (Tlt): 140-260μs bus high
7. 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

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:

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

Device Addressing and Enumeration

Default Addresses

ADB Default Device Addresses

Address	Device Type	Notes
$0	Reserved	SendReset command
$1	Security dongles	Copy protection devices
$2	Keyboards	All keyboards default to this
$3	Mice/Pointing devices	Mice, trackballs, trackpads
$4	Graphics tablets	Absolute positioning devices
$5	Reserved	Vendor-specific
$6	Reserved	Vendor-specific
$7	Reserved	Vendor-specific
$8-$E	Relocated devices	Collision resolution addresses
$F	Reserved	Global polling address

Enumeration Process

The ADB Manager performs device enumeration at startup:

1. Reset: SendReset command clears all devices
2. Discovery: Poll each default address with Talk Register 3
3. Collision Detection: If multiple devices respond:
   • Devices detecting collision stop transmitting
   • Non-colliding device completes transmission
4. Address Resolution:
   • Send Listen Register 3 with new address ($8-$E) and handler ID $FE
   • Only non-colliding device moves to new address
   • Repeat until all devices have unique addresses
5. Verification:: Poll new addresses to confirm relocation
6. Handler Installation:: Install appropriate device drivers

Service Requests

Devices can request service by:

1. Pulling bus low during stop bit of any transaction (65-300μs)
2. Host detects extended stop bit as Service Request (SRQ)
3. Host polls all devices' Register 0 sequentially
4. Device with data becomes new active device
5. Active device is polled continuously until another SRQ

Device Handler IDs

Device handlers define how the host interprets device data. Standard handler IDs include:

Common ADB Handler IDs

Handler ID	Device Type	Description
$00	Self-test fail	Device failed internal diagnostics
$01	Standard mouse	100 CPI relative positioning
$02	Standard keyboard	Original keyboard protocol
$03	Extended keyboard	Supports extended key codes
$04	ISO keyboard	International layout support
$05-$07	Reserved keyboards	Vendor-specific
$08-$0F	Reserved mice	Vendor-specific
$10-$1F	Graphics tablets	Absolute positioning devices
$20-$2F	Other devices	Joysticks, special controllers
$30-$FD	Vendor-specific	Custom implementations
$FE	Change address	Special collision resolution
$FF	Self-test pass	Device diagnostics successful

Hardware Implementation

Host Controllers

Different Apple systems used various ADB controller implementations:

ADB Controller Implementations

System	Controller Type	Implementation Details
Apple IIGS	Custom ASIC	Integrated with Mega II chip
Macintosh SE	PIC16CR54	Microchip microcontroller (Apple-branded)
Macintosh II/IIx	VIA + transceiver	6522 VIA with discrete transceiver
Macintosh IIfx	IOP processor	Dedicated I/O processor
Macintosh Portable	Power Manager	Integrated with PMU chip
PowerBook series	PMU	Power Management Unit integration
Later Macs	Cuda/Egret	Combined ADB and power management

VIA Implementation

On most 68k Macintoshes, ADB was implemented using:

• VIA (Versatile Interface Adapter): 6522 chip for CPU interface
• Transceiver IC: Handles bus electrical interface
• State lines (ST0, ST1): Define transaction states

Transaction states:

• State 0: Even byte from application
• State 1:: Odd byte from application
• State 2:: Even byte to application
• State 3:: Odd byte to application

Standard Devices

Keyboards

ADB keyboards implemented two protocols:

Standard Keyboard Protocol:

• Register 0: Two key events (2 bytes)
• Each byte: Key code (7 bits) + up/down flag (1 bit)
• Rollover: 2-key guaranteed

Extended Keyboard Protocol:

• Distinguishes left/right modifiers
• Handler ID change from $02 to $03
• Additional key codes for extended keys
• Used by Apple Extended Keyboard and later models

Mice and Pointing Devices

Standard Mouse Protocol:

• Register 0: Button state and movement deltas (2 bytes)
• Byte 0: Button (bit 7) + X delta (bits 6-0)
• Byte 1: Button (bit 7) + Y delta (bits 6-0)
• Resolution: 100 counts per inch typical
• Movement range: -64 to +63 per packet

Extended Mouse Protocol:

• Supports multiple buttons
• Higher precision movement data
• Additional registers for extended features
• Handler ID varies by capability

Graphics Tablets

Graphics tablets could operate in two modes:

• Relative mode:: Emulates standard mouse
• Absolute mode:: Reports absolute position
• Required vendor-specific drivers
• No standard protocol defined by Apple

Limitations and Issues

Hot-Swapping

While ADB protocol supports hot-swapping, most implementations were unsafe:

• Problem:: No current limiting on ADB ports
• Risk:: Hot-swapping could blow motherboard fuse (typically 1.1A polyfuse)
• Exceptions:: PowerBook G3 "Wall Street" had protected ports
• Repair:: Required fuse replacement or expensive motherboard swap

Collision Issues

• Multiple identical devices caused address collisions
• Resolution process could fail with >3 identical devices
• Some devices had non-random address generation

Performance Limitations

• Bandwidth:: Limited to 125 kbit/s theoretical (10-50 kbit/s typical)
• Latency:: Polling-based system introduced input lag
• Device limit:: 16 devices maximum, practically fewer
• Cable length:: 5 meters total maximum

Power Limitations

• Total power:: 500mA at 5V for entire bus
• Per device:: 100mA maximum
• PowerBooks:: Reduced to 200mA total on battery

Software Support

ADB Manager

The ADB Manager in Mac OS handled:

• Device enumeration and address assignment
• Polling active devices
• Service request handling
• Device handler installation and management
• Register read/write operations

Key ADB Manager routines:

• ADBOp: Perform ADB transaction
• ADBReset:: Reset bus and enumerate devices
• SetADBInfo:: Install device handler
• GetADBInfo:: Query device information
• CountADBs:: Return number of devices

System Software Compatibility

• Apple IIGS:: GS/OS with built-in ADB support
• Mac OS:: System 3.2 through Mac OS 9.2.2
• A/UX:: Full ADB support in Unix environment
• NeXT:: Limited support on some models
• BeOS:: Third-party drivers available
• Linux:: Kernel support for PowerPC Macs

Modern Usage and Adapters

ADB to USB Adapters

• Griffin iMate:: Original commercial adapter (discontinued)
• Wombat:: Modern bidirectional ADB-USB converter
• Belkin F5U118:: Simple ADB to USB adapter

USB to ADB Adapters

• Wombat:: Allows USB devices on ADB Macs
• DIY solutions:: Arduino-based converters

Replacement Controllers

• PIC16F87/88:: Drop-in replacement for failed SE controllers
• Custom ASICs:: Modern reproductions for restoration

Legacy and Impact

ADB represented several important innovations:

• First successful hot-pluggable peripheral bus (though implementation issues limited this)
• Automatic device enumeration and configuration predating USB
• Low-cost implementation enabling affordable peripherals
• Power distribution through the peripheral bus
• Collision detection and resolution without user intervention

The protocol influenced later designs:

• USB adopted similar enumeration concepts
• NeXT used ADB on some models
• Protocol concepts influenced FireWire development
• Demonstrated viability of serial peripheral buses

Technical Documentation

Patents

Key Apple patents related to ADB:

• 4,875,158 - Method for requesting service by a device
• 4,910,655 - Apparatus for transferring signals and data
• 4,912,627 - Method for storing information
• 4,918,598 - Method for selectively activating and deactivating devices

Developer Resources

• Inside Macintosh: Devices - Complete ADB programming guide
• Technical Note HW01 - "ADB - The Untold Story: Space Aliens Ate My Mouse"
• Guide to Macintosh Family Hardware - Hardware specifications

Common ADB Devices

Apple Devices

• Apple Keyboard (M0116)
• Apple Extended Keyboard (M0115)
• Apple Extended Keyboard II (M3501)
• Apple Adjustable Keyboard (M1242)
• AppleDesign Keyboard (M2980)
• Apple Desktop Bus Mouse (A9M0331/G5431/M0142)
• Apple Desktop Bus Mouse II (M2706)
• Graphics tablets (various models)

Third-Party Devices

• Kensington TurboMouse trackballs
• Gravis GamePad and MouseStick
• Wacom ArtZ graphics tablets
• CalComp DrawingBoard tablets
• CoStar LabelWriter printers
• Hardware copy-protection dongles
• Modems and telecommunications devices

Troubleshooting

Common Problems

ADB Troubleshooting Guide

Problem	Possible Cause	Solution
No devices recognized	Blown ADB fuse	Replace fuse or motherboard repair
Intermittent device	Damaged cable	Check continuity, replace if needed
Device not found	Address collision	Disconnect other devices, restart
Erratic behavior	Bus termination issue	Check cable length (<5m total)
Power issues	Overloaded bus	Remove devices, check power draw
Random disconnects	Failing transceiver	Replace ADB controller

Diagnostic Tools

• ADB Parser:: Classic Mac OS tool for bus analysis
• TechTool:: Hardware diagnostic suite
• MacsBug:: Low-level debugging access to ADB