GeneralManual

CAN bus wiring: termination, cable length and grounding

Updated

Wire a CAN bus so a logger records it reliably. Terminate both ends, keep the cable within the length your bit rate allows and deal with ground offsets. This applies to every Influx Technology logger and module on CAN.

How a CAN bus is built

CAN (Controller Area Network) is defined in ISO 11898. Every node on the bus has three parts that matter for wiring:

CAN controller
Takes data from the node's microcontroller, processes it and passes it to the transceiver. It does the same in reverse for received data.
CAN transceiver
A transmitter and receiver in one. It turns the controller's data into electrical signals on the bus lines, and bus signals back into data.
Termination
A resistor, normally 120 Ω, at each end of the bus. It stops signals reflecting back from the ends as echoes and keeps the bus at the right DC levels.

The bus is a single twisted pair, CAN-H and CAN-L, running from one end to the other. Each node joins it on a short branch line (a stub). Removing one node does not stop the others communicating.

A linear CAN bus with a terminator at each end CAN BUS · TOPOLOGY A linear CAN bus with a terminator at each end CAN-H CAN-L 120 Ω terminator 120 Ω terminator ECU ECU Logger Stub: keep it short Logger ground tied to the vehicle ground Terminate only the two physical ends of the bus. With the power off, CAN-H to CAN-L measures about 60 Ω.

New to CAN? See Understanding CAN bus: an easy guide.

Terminate both ends

ISO 11898 requires a nominal characteristic impedance of 120 Ω. Each end of the bus takes a 120 Ω resistor across CAN-H and CAN-L.

  1. Find the two ends. They are the two nodes, or connectors, furthest apart along the cable.
  2. Fit one 120 Ω resistor at each end. Many ECUs already contain one. If a logger or a test lead forms an end of the bus, add a terminator, such as INF4310. This 120 Ω terminator has a 9-way D-Sub.
  3. Measure it. With the power off, measure the resistance between CAN-H and CAN-L. About 60 Ω means both terminators are in place. About 120 Ω means one is missing.

Signal levels

The two lines carry the same data as mirror-image voltages. When a pulse on CAN-H rises from 2.5 V to 3.75 V, CAN-L falls from 2.5 V to 1.25 V. The receivers read the difference between the lines, which is why nodes still understand each other when their grounds differ.

CAN-H and CAN-L in recessive and dominant bits CAN BUS · SIGNAL LEVELS CAN-H and CAN-L in recessive and dominant bits 3.75 V 2.5 V 1.25 V CAN-H CAN-L 2.5 V both at 2.5 V 1 Recessive 0 Dominant 0 Dominant 1 Recessive 0 Dominant 1 Recessive Bit State A dominant bit (logic 0) overrides a recessive bit (logic 1) on the bus. Levels are nominal; real transceivers vary within the ISO 11898-2 limits.

If a logger shows errors, these articles help you find the cause.

Cable length and bit rate

The highest CAN bit rate in the standard is 1 Mbit/s. The faster the bus, the shorter it must be. Arbitration needs each bit to reach the furthest node and come back before the bit is sampled.

Maximum bus length by bit rate (rule of thumb)
Bit rate Maximum bus length
1 Mbit/s About 40 m
500 kbit/s About 100 m
250 kbit/s About 250 m
125 kbit/s About 500 m
Maximum bus length by bit rate CAN BUS · LENGTH Maximum bus length by bit rate 0 100 200 300 400 500 Bit rate 1 Mbit/s about 40 m 500 kbit/s about 100 m 250 kbit/s about 250 m 125 kbit/s about 500 m Maximum bus length (m) Rule of thumb for ISO 11898 high-speed CAN. The table in this guide holds the same figures.

Every node must use the same bit rate. For how the rate is set, see CAN bus baud rate and its calculation.

Grounding

Two cases need different treatment.

Between CAN nodes

CAN does not require a common ground between nodes, so nodes may sit at slightly different ground potentials. Each node then sees different single-ended voltages on the lines (the common-mode voltage), while the difference between CAN-H and CAN-L stays the same. Large ground offsets increase electromagnetic emissions. If your system is sensitive to emissions, remove the offsets.

Between a logger and its own sensors

Analogue and digital inputs are measured against the logger's own ground. Wire every sensor and input to the same ground as the logger's power supply, or to the analogue ground pins where the logger has them. An offset here goes straight into the reading.

Supply and isolation

Keep the logger's supply within its specified range. REXGEN loggers are protected internally against low-energy voltage events caused by noise on the supply wires.

On REXGEN loggers the CAN bus and USB are not isolated. Take care when you plug USB into a logger connected to a vehicle. Disconnect the logger's power supply while it is connected to a PC over USB. CAN isolators (INF4313) and USB isolators (INF4311) are sold separately.

For product pinouts and cables, see these guides.

Still stuck?

Our engineers answer support questions on every Influx Technology product.