Physical signaling
CAN High and CAN Low share a common recessive bias. A transmitter creates a dominant bit by driving the pair apart; receivers respond primarily to the voltage difference, which helps reject noise coupled equally into both wires.
HS-CAN / Communication network
HIGH-SPEED CAN / CAN FD
Powertrain, chassis, safety and gateway communication
NETWORK GUIDE01/14
TOPOLOGYTwo-wire differential bus with termination at the backbone ends
PHYSICALTwisted pair: CAN High and CAN Low
DATA RATETypically 500 kbit/s; CAN FD data phase may be faster
Correct theory and operation
HOW HS-CAN
COMMUNICATES.
Understand the normal architecture, signaling and message rules before deciding that a voltage or waveform is incorrect.
Bus access and arbitration
All nodes may begin transmitting when the bus is idle. The lower numerical message identifier wins non-destructive bit-by-bit arbitration because a dominant bit overwrites a recessive bit. Losing nodes stop transmitting and retry without corrupting the winning frame.
Frames and fault control
A classical CAN frame carries an identifier, control information, up to eight data bytes, a CRC, acknowledgement and end-of-frame bits. CAN FD permits a longer payload and may switch to a faster bit rate during the data phase. Nodes monitor bit, stuffing, CRC, format and acknowledgement errors and can enter error-passive or bus-off states.
Topology and termination
A linear backbone with short stubs is normally terminated at both physical ends. The two end resistors absorb edge energy and control reflections; they are not simply module resistors to be expected everywhere. Gateways route selected messages between separate CAN networks.
Wake, sleep and normal behavior
Network-management messages and local wake inputs determine when modules communicate or sleep. Normal traffic contains many interleaved identifiers from different modules, with both lines moving in complementary directions and returning to the recessive state between dominant bits.
Common mistakes
AVOID THE
FALSE CLUE.
- Testing with the connector unplugged when the fault only appears under load.
- Using a generic voltage value instead of the vehicle-specific specification.
- Skipping power, ground and reference checks before condemning the component.
- Moving the harness or clearing codes before capturing the original evidence.
- Replacing a part without repeating the test.
Reading the signal
Good waveform / normal communication
On the identified high-speed CAN segment, synchronized CAN-H and CAN-L show complementary dominant transitions and return to the expected recessive bias. Examine the differential signal as well as each line.
Faulty waveform / fault indications
A stuck line, collapsed differential signal, severe ringing or unequal transitions can indicate wiring, termination or transceiver problems. A healthy-looking physical waveform does not prove that message content is correct; use topology, decoded traffic and DTC evidence together.
COMPLETE TEST SEQUENCE
- Scan every available module and record which modules communicate before clearing codes.
- Use the topology to find shared branches, gateways and termination locations.
- With the network powered down and asleep, measure resistance only where the service procedure permits.
- Key on and check each line for shorts to power or ground before connecting a scope.
- Scope CAN H and CAN L together near the diagnostic connector, then move toward the suspected branch.
- Isolate branches or modules only in a controlled order while watching communication return.
Confirm the network, topology, awake state and test conditions from vehicle-specific information.