Protocol decoder:Iebus

iebus

Name	IEBus
Description	A multi-drop multi-master differential CAN-like bus
Status	100%
License	GPLv3+
Source code	decoders/
Input	logic
Output	iebus
Probes	—
Optional probes	?
Options	—

The iebus protocol decoder handles decoding of the IEBus, a lower-level layer of the AVC-LAN protocol which is used in multimedia control applications in Toyota and other brand cars. It's connected with Python output to avclan which is a higher-level decoder handling the decoding of frame contents, identifying devices on the bus based on their addresses and functions and so on.

Hardware

The protocol capture can be performed with a comparator circuit connected to 4 resistors:

Photos:

None

Protocol

The scope captures below show an IEBus operating in Mode 2 which means that the time for each bit is 39µs. The example scope trace below provides an overview of the voltages and timings that with which the bus operates:

Each frame begins with a start bit:

which is followed by a number of data bits where each bit can be synchronized by a raising edge, a synchronization period and a data period. This is highlighed in the trace below:

In practice the decoder implementation waits for the sync period to expire and then samples the bus to probe which bit is being transmitted. An alternative approach can be also used where a decoder can measure the pulse length between the rising synchronization edge and the falling edge before the next bit is being transmitted. The below traces provide a comparison between what happens on the bus when 0 and 1 are being transmitted:

Resources

The protocol itself has been documented pretty extensively on Wikipedia as well as other more niche pages:

• Wikipedia
• Softservice
• eLinux
• pop.fsck.pl
• Interfacebus