CAN Bus Basics
Two wires, differential voltage and the messages that travel on them.
Loading…
What you will learn
- Describe CAN-H and CAN-L and their typical voltages
- Explain termination resistors and their purpose
- Interpret common CAN communication DTCs
Advertisement
CAN (Controller Area Network) is a two-wire, differential, multi-master bus. The dominant topology is a linear bus with a terminating resistor near each end — commonly around 120 Ω each, giving roughly 60 Ω measured across the pair when the bus is healthy and powered down. Exact values depend on the network design.
Because messages are shared, a fault on the bus or in one module can pull the whole network down. That is why a cluster of U-codes across unrelated systems should always make you reach for the bus, not the modules.
Loading…
| Test | Method | Training expectation |
|---|---|---|
| Bus resistance | Ohmmeter across CAN-H / CAN-L, ignition off | Roughly 60 Ω on a healthy two-terminator bus |
| CAN-H voltage | Scope or breakout, bus active | Around 2.5 V idle, rising toward 3.5 V dominant |
| CAN-L voltage | Scope or breakout, bus active | Around 2.5 V idle, falling toward 1.5 V dominant |
| Short to ground | Resistance to chassis | No unwanted continuity |
CAN voltage and resistance values are network-specific. These are common teaching figures for a 500 kbit/s high-speed CAN; verify against the manufacturer data before acting.
Loading…
Loading…
Key points
- CAN uses a twisted pair with differential signalling
- Terminating resistors are typically about 120 Ω each
- A shorted or open bus silences many modules at once
All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.
Advertisement