9.2.1.1 Bus Loading, Length and Number of Nodes

A typical CAN application can have a maximum bus length of 40 meters and maximum stub length of 0.3 m. However, with careful design, users can have longer cables, longer stub lengths, and many more nodes to a bus. A high number of nodes require a transceiver with high input impedance such as the TCAN4420 transceiver.

Many CAN organizations and standards have scaled the use of CAN for applications outside the original ISO11898-2 standard. They made system level trade off decisions for data rate, cable length, and parasitic loading of the bus. Examples of these CAN systems level specifications are ARINC825, CANopen, DeviceNet, SAE J2284, SAE J1939, and NMEA 2000.

A CAN network system design is a series of tradeoffs. In ISO 11898-2 the driver differential output is specified with a 60-Ω bus load where the differential output must be greater than 1.5 V. The TCAN4420 is specified to meet the 1.5 V requirement across this load and is specified to meet 1.3-V differential output at 50-Ω bus load. The differential input resistance of this family of transceiver is a minimum of 20 kΩ. If 67 of these transceivers are in parallel on a bus, this is equivalent to an 300-Ω differential load in parallel with the 60 Ω bus termination which gives a total bus load of 50 Ω. Therefore, this family theoretically supports over 67 transceivers on a single bus segment with margin to the 0.9-V minimum differential input voltage requirement at each receiving node. However, for network design, margin must be given for signal loss across the system and cabling, parasitic loadings, timing, network imbalances, ground offsets and signal integrity thus a practical maximum number of nodes is much lower. Bus length may also be extended beyond 40 meters by careful system design and data rate tradeoffs. For example CANopen network design guidelines allow the network to be up to 1 km with changes in the termination resistance, cabling, less than 64 nodes on the bus, and significantly lowered data rate.

This flexibility in network design is one of its key strengths allowing for these system level network extensions and additional standards to build on the typical CAN bus length parameters. However, when using this flexibility the network system designer must take the responsibility of good network design to ensure robust network operation.