Resistance in the CAN bus

Twisted pair wire for digital signals.

Resistance is the opposition or hindrance to the flow of electrons in a circuit. Connectors, switches, and wires all offer some resistance. For example, twisted pair wire typically contains 120 ohms of impedance, and the CAN bus has a 120-ohm resistor at each end. These resistors are often located inside two control modules to normalize voltage and prevent reflections from corrupting the data on the bus.

A simple automotive CAN bus.

Two 120-ohm resistors provide 60 ohms of resistance. This is because two 120 ohm resistors installed parallel between CAN HI and CAN LO have an equivalent resistance of half the value of either resistor or 60 ohms. Adding a third module with another 120-ohm resistor would modify the resistance on the bus to 40 ohms. Four 120-ohm resistors would result in 30 ohms.

A simple automotive CAN bus.

All modules must be powered down or the vehicle's battery disconnected to measure resistance in the CAN bus circuit. The resistance between the CAN HI and CAN LO pins on the connector of the control module containing the resistor will measure 120 ohms. In addition, some vehicles have a gateway.

A simple automotive CAN bus with a gateway.

In the example above, the DLC and both networks connect to a gateway. Check the manual to locate these connections. Measurements on a CAN bus with a gateway must be measured in another place, like an easy-to-access control module or the CAN HI and CAN LO connection to the network on the gateway.

CAN HI and CAN LO waveforms.

The resistance between the CAN HI and CAN LO pins and ground on the DLC should be infinity or megaohms. When there is no action on either CAN line, the voltage is around 2.5 Vdc. If CAN HI shorts to CAN LO, the voltage will remain around 2.5 Vdc on both lines. When a module sends a message, the CAN HI line (Yellow) increases above 2.5 Vdc, and CAN LO (Green) decreases below 2.5 Vdc. When compared, high and low CAN bus waveforms mirror each other.