How to Build a CANopen Network: Bit Rate, Length and Termination
Topology, length by bit rate, 120 Ω termination and the 60 Ω check, node ID, NMT, PDO/SDO and the CiA 402 state machine.
This content supports preliminary sizing and architecture decisions; it does not replace the selected product manual or machine risk assessment.
1. Topology: a line, not a star
The CAN line runs from device to device like a chain, with short stubs. A star connection causes reflections and error frames.
2. Bit rate and bus length
The higher the bit rate, the shorter the permitted bus. The table below gives CiA's recommended values; every node must run at the same bit rate.
3. Termination and the 60 Ω check
A 120 Ω resistor goes between CAN_H and CAN_L at both physical ends. With power off, ~60 Ω between CAN_H and CAN_L is correct, ~120 Ω means one resistor, ~40 Ω one too many.
R_bus = 120 Ω ∥ 120 Ω = 60 Ω
4. Node ID, NMT and heartbeat
Every node needs a unique node ID. PDOs only flow once the node is put into Operational with NMT; heartbeat shows which node drops out.
5. PDO, SDO and SYNC
Cyclic process data (target and actual position, controlword and statusword) goes in PDOs; parameter access in SDOs. Synchronous PDOs are triggered by the SYNC message.
6. The CiA 402 state machine
The drive is enabled with the controlword (0x6040) through Shutdown, Switch On and Enable Operation; the statusword (0x6041) confirms each step. The mode of operation is selected with 0x6060.
0x6040: 0x0006 → 0x0007 → 0x000F
| Bit rate | Recommended max length | Note |
|---|---|---|
| 1 Mbit/s | ~25 m | Short, fast lines |
| 500 kbit/s | ~100 m | Common inside a machine |
| 250 kbit/s | ~250 m | Long machines |
| 125 kbit/s | ~500 m | Plant-level lines |
Frequently asked questions
Should every device have a terminator?
No. Only at the two physical ends of the line, never in between.
The drive talks but will not enable. Why?
The node may not be Operational in NMT, or the CiA 402 controlword sequence is incomplete; also check the hardware enable / STO input.
Technical sources
Sources are publications from standards owners and manufacturers.