Subaru 602 - Page 10

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June 2013
Electrical Systems Operation & Diagnosis Module (602)
The CAN wires are twisted together along the entire length of the LAN. This protects the integrity
of the CAN signal if an unintentional signal is created in the circuit from an overlying circuit that
induces a magnetic field into the CAN wires. This magnetic field will place noise or an unwanted
voltage signal into the CAN signal.
The polarity of the unwanted signal will usually be constant. Superimposed over the CAN signal
the noise will cancel itself out as the control units only view the potential difference between the
two wires. The more twists per meter a wire has the more it reduces noise.
Noise (ARTWORK)
CAN communications are designed to operate at different speed levels dependant on the operating
speeds of the components on different parts of the LAN. The power train side of the LAN operates
at 500 Kbps (500,000 bytes per second). The body side of the LAN operates at a slower rate of
125 Kbps (125,000 bytes per second). The individual sides of the LAN are referred to as bus. Any
references to the high speed bus are referencing to the power train side of the LAN and the low
speed bus is referring to the body side of the LAN. Communications between the high speed and
low speed bus sides of the LAN are not possible unless the data to be shared is adjusted to the
same rate of speed. This adjusting process is referred to as passing through the gateway. The
gateway function is accomplished by the Body Integrated Unit (BIU). Additional functions of the BIU
control and monitor body related electrical circuits such as the rear defogger and door switches.
Components of the high speed bus include the Engine ECM, Transmission Control Module,
ABS/VDC ECM and the Steering Angle Sensor (Vehicles with VDC). The number of control
modules on the CAN continue to increase. The most recent additions are the Keyless Access and
EyeSight™ Control Modules.
Components of the low speed bus include the Combination Meter and A/C Control Unit (Auto A/C).
The twisted wire connecting all components of the CAN to the BIU must maintain its twisted shape
to help with keeping out electrical interference. Repairs to the wire are allowed but the length of
the twisted wire cannot differ more than 3.94 inches (8 cm).
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