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Superseded	USB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:

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

**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:

Reset: SendReset command clears all devices
Discovery: Poll each default address with Talk Register 3
Collision Detection: If multiple devices respond:
- Devices detecting collision stop transmitting
- Non-colliding device completes transmission
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
Verification:: Poll new addresses to confirm relocation
Handler Installation:: Install appropriate device drivers

Service Requests

Devices can request service by:

Pulling bus low during stop bit of any transaction (65-300μs)
Host detects extended stop bit as Service Request (SRQ)
Host polls all devices' Register 0 sequentially
Device with data becomes new active device
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

Protocol Details

Complete Timing Specifications

The ADB protocol operates with precise timing requirements for all signal transitions:

**ADB Protocol Timing Parameters**
Signal Type	Duration	Tolerance	Description
Reset pulse	3000 μs	Minimum	Host pulls bus low for 3 ms or more to reset all devices
Attention	800 μs	±3%	Host pulls bus low to initiate transaction
Sync	70 μs	±10%	Bus high between attention and command
Bit cell (total)	100 μs	±3%	Total time for one bit transmission
Logic 0 low time	65 μs	±3%	Low portion of logic 0 bit cell
Logic 0 high time	35 μs	±3%	High portion of logic 0 bit cell
Logic 1 low time	35 μs	±3%	Low portion of logic 1 bit cell
Logic 1 high time	65 μs	±3%	High portion of logic 1 bit cell
Stop bit	0	Always	Stop bit is always logic 0
Service Request	300 μs	Minimum	Device extends stop bit low to assert SRQ
Tlt (Stop-to-start)	140-260 μs	Range	Bus high between command and data
SRQ detection window	65-300 μs	Range	Period when devices can assert service request

Transaction State Machine

Each ADB transaction follows a structured sequence:

Idle State: Bus remains high, devices wait for attention signal
Attention Phase: Host pulls bus low for 800 μs
Sync Phase: Bus high for 70 μs
Command Transmission: 8 command bits + stop bit
Service Request Window: Devices can assert SRQ during stop bit low time
Tlt Period: Bus high for 140-260 μs
Data Transfer: Start bit + 2-8 data bytes + stop bit
Return to Idle: Transaction complete

Collision Detection Mechanism

The collision detection system enables automatic address resolution:

Collision Detection Process:

Device begins transmitting its data
Monitors bus state while transmitting
If transmitted '1' but bus reads '0', collision detected
Device immediately stops transmission
Sets internal collision flag
Remains silent for next transaction to its address

Collision Resolution Algorithm: When multiple devices share an address:

Host sends Talk Register 3 to shared address
All devices attempt to respond simultaneously
Devices detecting collision stop transmitting
Non-colliding device completes transmission
Host sends Listen Register 3 with new address ($8-$E) and handler ID $FE
Only non-colliding device moves to new address
Process repeats until no collisions remain

Service Request Mechanism

The Service Request (SRQ) signal allows devices to request host attention without being polled:

SRQ Assertion:

Device pulls bus low during stop bit of any command
Must maintain low for minimum 300 μs total
Can only assert if command was not addressed to requesting device
SRQ does not cancel the current transaction

Host Response to SRQ:

Detects extended stop bit duration
Completes current transaction
Polls Register 0 of all devices sequentially
Device with data becomes new active device
Continues polling active device until next SRQ

Extended Protocol Specifications

Extended Keyboard Protocol

The Extended Keyboard Protocol distinguishes between left and right modifier keys:

**Extended Keyboard Protocol (Handler ID $03)**
Register	Format	Description
0	2 bytes	Two key events: 7-bit keycode + 1-bit up/down flag each
1	Undefined	Reserved for vendor use
2	2 bytes	LED status and modifier key states
3	2 bytes	Device address and handler ID

Register 2 Extended Format:

Bit 0: Num Lock LED (0=off, 1=on)
Bit 1: Caps Lock LED
Bit 2: Scroll Lock LED
Bit 3: Compose LED
Bit 4: Kana LED
Bit 5: Power LED
Bit 6: Reserved
Bit 7: Delete key state

Extended Keycodes:

$32: Left Shift
$38: Right Shift
$37: Left Command
$3C: Right Command
$3A: Left Option
$3D: Right Option
$36: Left Control
$3E: Right Control

Extended Mouse Protocol

The Extended Mouse Protocol (Handler ID $04) supports multiple buttons and higher precision:

Standard Mouse Protocol (Handler ID $01):

Register 0: 2 bytes
Byte 0: Button state (bit 7) + X delta (bits 6-0, signed)
Byte 1: Button state (bit 7) + Y delta (bits 6-0, signed)
Resolution: 100±10 counts per inch
Movement range: -64 to +63 per packet

Enhanced Resolution (Handler ID $02):

Same format as standard protocol
Resolution increased to 200±10 counts per inch

Extended Protocol Features:

Support for up to 8 buttons
16-bit movement deltas
Configurable resolution up to 400 CPI
Scroll wheel support (vendor-specific)

Complete Handler ID Registry

Device Handler IDs identify specific device types and operational modes:

**Comprehensive ADB Handler ID Assignments**
Handler ID	Device Category	Description	Notes
$00	System	Self-test failed	Device diagnostics failure
$01	Keyboard	Apple Keyboard	Original keyboard protocol
$02	Keyboard	Apple Extended Keyboard	Standard extended keyboard
$03	Keyboard	Extended Protocol	Distinguishes left/right modifiers
$04	Keyboard	ISO Layout	International keyboard support
$05	Keyboard	Apple Adjustable Keyboard	Ergonomic split keyboard
$06-$07	Keyboard	Reserved	Future keyboard types
$08	Keyboard	JIS Layout	Japanese keyboard
$09-$0F	Keyboard	Vendor-specific	Third-party keyboards

$01	Mouse	Standard Mouse	100 CPI, single button
$02	Mouse	200 CPI Mouse	Enhanced resolution
$03	Mouse	Mouse Systems A3	Three-button mouse
$04	Mouse	Extended Mouse	Multi-button support
$05-$0F	Mouse	Vendor-specific	Third-party mice

$10-$1F	Tablet	Graphics tablets	Absolute positioning devices
$20	Joystick	Standard joystick	Game controllers
$21-$2F	Other	Special devices	Miscellaneous input
$30-$3F	Reserved	Apple reserved	Future Apple devices
$40-$FD	Vendor	Vendor-specific	Custom implementations
$FE	System	Change address	Collision resolution command
$FF	System	Self-test passed	Device diagnostics success

Advanced Implementation Details

Electrical Specifications

**ADB Electrical Characteristics**
Parameter	Specification	Notes
Bus voltage	5.0V ± 5%	Supplied by host
Logic high (VIH)	2.0V minimum	TTL compatible
Logic low (VIL)	0.8V maximum	TTL compatible
Output low (VOL)	0.4V maximum	At 4mA sink current
Pull-up resistance	470Ω - 1kΩ	External resistor required
Bus capacitance	200pF maximum	Per device
Rise time	1.0μs maximum	10% to 90%
Fall time	300ns maximum	90% to 10%
Current per device	100mA maximum	From +5V supply
Total bus current	500mA maximum	All devices combined

Reset and Initialization Sequence

Power-On Reset:

Host asserts reset (bus low) for minimum 3ms
Devices wait up to 1 second for initialization
Host begins enumeration at default addresses
Devices respond with Register 3 contents
Collision resolution if multiple devices present
Handler installation for recognized devices
Normal polling operations begin

Soft Reset (SendReset Command):

Command: Address $0, Command $0, Register $0
All devices reset to startup state
Devices clear pending actions
Return to default addresses
Preserve handler ID settings

Data Register Formats

Register 0 (Primary Data):

Always contains device's main data
Must have data available when asserting SRQ
2-8 bytes depending on device type
Updated only when new data available

Register 1 (Device-Specific):

Vendor-defined functionality
Configuration storage
Extended data for complex devices
Not standardized by Apple

Register 2 (Device-Specific):

Secondary configuration
LED control (keyboards)
Button mapping (mice)
Calibration data (tablets)

Register 3 (Status/Configuration):

Bits	Field	Description
15-8	Exceptional Event	Device-specific flags
15	Reserved	Always 0
14	Exception	Device exception occurred
13	SRQ Enable	Service requests enabled
12-8	Device Flags	Device-specific status
7-0	Handler ID	Current device handler

Error Handling and Recovery

Timeout Handling:

No response timeout: 260μs after Tlt
Incomplete data timeout: 130μs between bytes
Host marks device as non-responsive
Periodic retry attempts every 1 second

CRC and Data Integrity:

No built-in error detection
Devices responsible for data validation
Host can request retransmission via Talk command
Application-level checksums recommended for critical data

Bus Recovery:

Detect bus stuck low > 1ms
Issue reset pulse
Wait for bus to return high
Resume normal operations
Log error for diagnostic purposes

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