Watch this Video First
For in depth detail on the physical layer read this Application Report (SLLA270) from Texas Instruments.
CAN Bus Termination: There should be a 120 ohm termination resistor located at each end of the bus to prevent signal reflections. When you measure the resistance between CAN hi to CAN low on the wiring harness you should measure 60 ohms. This measurement should be conducted with power off. In some instances the termination resistor is located inside the Device and may be switched electronically. In these instances it cannot be detected by measuring resistance, if it is a display go into the menu and verify the termination resistor option is enabled.
If one of the termination resistors is missing communications may work temporarily but will be unreliable and eventually fail.
CAN Bus Voltage Levels: The CAN bus data Low and HI signals idle at 2.5VDC with respect to ground when there is no active data. When data is active CAN Hi switches between 2.5Vdc and 3.5Vdc and CAN Low switches between 2.5Vdc and 1.5Vdc. When these voltages are measured with a multimeter, only the average voltage is displayed. Typical voltage for CAN HI are 2.6Vdc-3Vdc and CAN Low are typically 2.4Vdc – 2Vdc. CAN Hi should be greater than 2.5Vdc and CAN Low less than 2.5Vdc. IMPORTANT: These measurements should be taken with power on and only one device on the network at a time. When multiple devices are connected on the network the CAN voltage measured is the average of all devices so you cannot determine if one of those is bad.
General Troubleshooting Procedure
This procedure is a general guideline; consult manufacturers wiring information to determine CAN connections and pin-outs on your equipment.
- Missing Termination Resistors
- CAN Hi and CAN Low wired backwards
- Damaged CAN Port due to lightning or welding
- Device Configuration Settings
Check Device Configuration Settings
If the device has configuration options via display menu, dip switch settings, jumper settings, or software download verify the following are correct.
- Baud Rate – J1939 use 250Kbps but some other networks use 500Kbps
- Device Source Address – verify each device on the network has a unique source address
- Verify your device is configured to receive data from the source address of the desired data source.
Verifying Network Termination Resistance
- With power off unplug connector from any device on the network and measure resistance between CAN Hi and CAN Low.
- Resistance should be 60 ohms if both termination resistors are present.
- If measurement is 120 ohms only one terminating resistor is present, two resistors are required (see note below).
- If 40 ohms is measured a third terminating resistor is installed and should be removed.
- If less than 40 ohms is measured there could be a short in the harness or a damaged CAN Port of one of the devices connected to the network.
NOTE: Some CAN devices have an internal termination resistor that is switched by software once the unit is powered up, consult manufactures literature to determine if this is the case.
Checking CAN Voltage
- Disconnect all devices from the network except for the Device you wish to test and turn power on.
- Measure voltage on any of disconnected plugs between CAN Hi and Gnd, should be between 2.5 – 3.0Vdc.
- Measure voltage between CAN Low and Gnd, should be between 2.5 to 2.0VDC.
- A low voltage of @1.4Vdc or less on either of these indicates a possible failure on the CAN Port of the device.
- If the voltages are exactly 2.50Vdc and do not change after several seconds it indicates the device connected is powered up but not broadcasting data.
Checking for Reversed Wiring
Perform the CAN Voltage Test from above and verify CAN Hi voltage is greater than CAN Low voltage. If not, the wires are reversed.
Checking Device CAN Port
If the CAN Voltage Test shows the voltage is low coming from a device you can verify the CAN Port is damaged by measuring resistance to ground, damage from lightning or welding usually causes a short to ground on one or both of the CAN lines.
- Unplug the connector from the device.
- Measure resistance on the connector pins of the device between CAN Hi and CAN Low , should be between 28k – 50k ohms.
- Measure resistance between CAN Hi and Gnd, should be Mega ohms or open.
- Measure resistance between CAN Low and Gnd, should be Mega ohms or open.
- If damage to an input has occurred measurement will typically be 10K ohm or less between CAN Hi/Low and ground