CAN wiring harness layout design principles

CAN Harness Requirements

The wiring harness layout design requires multiple on-board ECUs and offline tools in the network, as shown in Figure 2.

For CAN harness design, the following points are generally required:

1) The total length of the CAN bus is less than 40m;

2) ECU spacing < 20m;

3) The number of ECU nodes is less than 20;

4) The wiring harness uses shielded wire (high-speed CAN) or twisted pair wire (low-speed CAN);

1. 5) The wiring harness requires that the branch line must be as short as possible; the diagnostic node is directly connected to the bus, and the port length should be as short as possible, with a maximum length of 1 meter. The connection between the diagnostic instruments from the connector should also be as short as possible, with a maximum length of no more than 5 meters.

6) When laying out the wiring harness, try to keep the CAN line smooth and avoid folding. Make sure that when each branch line is connected to the main line, the connection point of CAN_H and CAN_L is kept at the same position of the main line. In principle, the node distance D between each node is not allowed to be equal in length, and should be kept as far away from the power line and signal line as possible, and parallel routing should be avoided as much as possible;

7) Do not lay out the CAN bus near the interference source. If it cannot be avoided, use shielded wires. When using shielded wires, the shielding layer must be grounded at either end (
or both ends). The requirements for the length of each trunk and branch line of the CAN line in the vehicle wiring harness are as shown in Table 1:

In order to avoid interference from cable power signals, the nodes of the CAN network should be as close to the trunk as possible. In practical applications, short branches should be used to connect to the bus as much as possible. In order to reduce standing waves, the ECUs in the CAN network should not be placed at equal distances and the tail of the cable should not all have the same length lead.

Terminal resistance

The terminal resistor can be placed adjacent to or in the two online ECUs with the longest distance. Non-terminal ECUs can be connected optionally. According to the standard requirements, the terminal resistor provided on the CAN bus is 60Ω, so the two farthest nodes on the vehicle CAN bus must each have a 120Ω terminal resistor (required between CAN_H and CAN_L).

The terminal resistance is required to be between 110Ω and 130Ω. The typical value is 120Ω.