CAN Bus Wiring Rules_En_V1

This chapter describes the rules to follow in order to ensure reliable CAN communication. These rules are valid and must be applied for any type of CAN bus connection, whether it is the inter-GENSYS communication bus or the J1939/CANopen CAN bus used to communicate with an electronic computer (ECU) and/or input/output modules.

Use shielded cables for CAN communications to protect them from electromagnetic disturbances.

The table below lists the connections of a standard DB9 CAN connector:

The cables used must correspond to the specifics of the CAN bus. Always use shielded twisted pairs. Wire the CAN network as a linear bus (no mesh, loop or star topology) as shown below:

Both ends of the CAN bus must incorporate a 120Ω termination resistor. The illustration below shows an example of a CAN bus linking three modules. Do not install a resistor on the middle module. Use the same cable quality on all the CAN network.

MAXIMUM LENGTH and COMMUNICATION SPEED

The maximum length of a CAN bus essentially depends on the communication speed, but also on the quality of the cables and connectors used. The table below gives the theoretical maximum length of a CAN bus according to the communication speed.

The table below indicates the communication speed of a CAN bus according to the protocol used:

CANBUS is a high speed network that requires high quality wiring to function properly. As such, it is susceptible to incorrect wiring. The majority of CANBUS communication problems are caused by poor wiring, incorrect termination, or using multiple frequencies on the same bus. Here are some tips for diagnosing CANBUS communication problems:

There must be exactly two (2) termination resistors of 120 ohms each at the physical ends of the CANBUS. These resistors can be external to the device, but some devices have termination resistors installed inside the device. The correct termination resistance can be checked by measuring the resistance between CANHigh and CAN-Low with a multimeter. To do this, it is essential that all devices on the network are not powered (it will not work if a device on the network is powered). It should read about 60 Ohms. There must be exactly 2 termination resistors (120 Ohms each) on the network. Always use termination resistors. Even if the network seems to work without terminating resistors in quiet environments

The two terminating resistors must be on the physical ends of the CANBUS (ends of the main box). If they are not at the physical ends of the network, then the effectiveness of the terminating filter resistors will be negated.

The network should have the shape of a tree. The main length of the CANBUS is like the trunk of the tree, and there may be small branches sticking out from the main trunk. The maximum trunk and branch length depends on the baud rate, but is generally limited to less than 2 feet (0.6 meters).

All CANBUS wires must be twisted pair cables, even short lengths (greater than 1 inch / 2 cm). Twisted pair cable is an essential part of how differential mode filtering works on CANBUS, and without it the signal can be easily distorted.

CAN-High and CAN-Low wires must be the same length. Differences in length between CAN-High and CAN-Low wires can cause distortion due to common mode filtering.

Make sure CAN-High and CAN-Low are connected in the correct order. If CAN-High and CAN-Low are backward on any device on the network, it will drop or severely distort traffic and make communication impossible.

Make sure CAN-High and CAN-Low are not shorted or shorted to ground or V+. Test to make sure there is no electrical conductivity between CAN-High and CAN-Low or CAN-Low (this can occur if one of the wires is shorted or is pinched or sliced by a box metallic).

All devices on the CANBUS network must run the same baud rate. If devices are running multiple different baud rates, this can choke all CANBUS traffic. If the baud rate of an Orion BMS product needs to be changed, disconnect the device from the network and update the baud rate with only the BMS product and CAN adapter connected. If the baud rate is changed on an Orion BMS product, the Orion BMS product must be restarted for the new baud rate to take effect.

All network devices must share the same Earth. Different ground potentials between devices will lead to the formation of common mode voltages on the CANBUS. These distort communication signals and can lead to damaged equipment and can also pose a safety hazard. If the same grounds cannot be used, an external CANBUS isolator may be required to provide galvanic isolation to disturb ground loops.

If the CANBUS cable used is a shielded cable, the shield should only be grounded at one end. If both ends of the CANBUS cable shield are grounded, ground loops may form and cause interference.

Make sure all CANBUS cables are securely connected. CANBUS wire joints must be soldered together or securely spliced. Junctions CANNOT simply be twisted together or held together with nuts or friction. Do not use terminal blocks, telephone cable joints or other signal processing blocks. These can distort CANBUS communication signals.

When removing the CANBUS wires, it can be easy to accidentally puncture the insulation of the CAN-High or CAN-Low wires. Inspect any joints or sections of wire where the CANBUS outer sleeve was removed to ensure that the CAN wires have not been inadvertently damaged.

Wherever possible, route CANBUS wires away from disturbed circuits. Do not twist CANBUS wires together with other wires.