Editing Apple Desktop Bus (section)

== 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