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Apple Desktop Bus: Difference between revisions

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| style        = Serial
| style        = Serial
| pinout_caption = ADB connector (female socket)
| pinout_caption = ADB connector (female socket)
}}
|pinout=[[File:Apple Desktop Bus Connector Pinout.png|150px]]}}
 
'''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.
'''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 ==
== History and Development ==
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=== ADB to USB Adapters ===
=== ADB to USB Adapters ===
* '''Griffin iMate::''' Original commercial adapter (discontinued)
* '''Griffin iMate:''' Original commercial adapter (discontinued)
* '''Wombat::''' Modern bidirectional ADB-USB converter
* '''Wombat:''' Modern bidirectional ADB-USB converter
* '''Belkin F5U118::''' Simple ADB to USB adapter
* '''Belkin F5U118:''' Simple ADB to USB adapter


=== USB to ADB Adapters ===
=== USB to ADB Adapters ===
* '''Wombat::''' Allows USB devices on ADB Macs
* '''Wombat:''' Allows USB devices on ADB Macs
* '''DIY solutions::''' Arduino-based converters
* '''DIY solutions:''' Arduino-based converters


=== Replacement Controllers ===
=== Replacement Controllers ===
* '''PIC16F87/88::''' Drop-in replacement for failed SE controllers
* '''PIC16F87/88:''' Drop-in replacement for failed SE controllers
* '''Custom ASICs::''' Modern reproductions for restoration
* '''Custom ASICs:''' Modern reproductions for restoration


== Legacy and Impact ==
== Legacy and Impact ==
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* Hardware copy-protection dongles
* Hardware copy-protection dongles
* Modems and telecommunications devices
* Modems and telecommunications devices
== Protocol Details ==
=== Complete Timing Specifications ===
The ADB protocol operates with precise timing requirements for all signal transitions:
{| class="wikitable" style="width:90%; text-align:center;"
|+'''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:
{| class="wikitable" style="width:80%; text-align:center;"
|+'''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:
{| class="wikitable" style="width:85%; text-align:center;"
|+'''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
|-
| colspan="4" style="background:#f0f0f0" |
|-
| $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
|-
| colspan="4" style="background:#f0f0f0" |
|-
| $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 ===
{| class="wikitable" style="width:75%; text-align:center;"
|+'''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):'''
{| class="wikitable" style="width:70%; text-align:center;"
! 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 ==
== Troubleshooting ==
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=== Diagnostic Tools ===
=== Diagnostic Tools ===
* '''ADB Parser::''' Classic Mac OS tool for bus analysis
* '''ADB Parser:''' Classic Mac OS tool for bus analysis
* '''TechTool::''' Hardware diagnostic suite
* '''TechTool:''' Hardware diagnostic suite
* '''MacsBug::''' Low-level debugging access to ADB
* '''MacsBug:''' Low-level debugging access to ADB
[[Category:Apple Desktop Bus]]
[[Category:Apple Desktop Bus]]
[[Category:Apple Vintage Computers]]
[[Category:Apple Vintage Computers]]
[[Category:Apple Vintage Accessories]]
[[Category:Apple Vintage Accessories]]
Retrieved from "https://wiki.retrotechcollection.com/Apple_Desktop_Bus"