Audi R8 2008 Service Training Manual page 32

Engine
Control Module Communications
The Engine Control Module J623 (ECM, master)
computes and controls the signals from the actuators
for cylinder bank .
Most sensors are connected to the ECM.
Both control modules are connected to the CAN data
bus; the slave control module is used as a receiver only.
The load signals required to compute and control
the signals for the actuators of cylinder bank 2 are
transmitted via private bus.
Engine Control Module
(ECM) J623
28
The slave control module acts as the misfire detector for
each of the eight cylinders. It also processes the signal
from the Engine Speed (RPM) Sensor G28.
Master and slave control modules are identical in design
and have the same part number. A voltage code in the
control module determines whether the control module
in the master or the slave.
If battery positive is applied to the encoding pin, the
control module is the master.
Engine Control Module
(ECM) 2 J624
Private Bus
CAN Data Bus