9.1.2 Bus Loading, Length and Number of Nodes

A typical CAN application can have a maximum bus length of 40 m 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 this transceiver family.

Many CAN organizations and standards have scaled the use of CAN for applications outside the original ISO 11898-2:2016 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 NMEA200.

A CAN system design is a series of tradeoffs. In ISO 11898-2:2016 the driver differential output is specified with a bus load that can range from 50 Ω to 65 Ω where the differential output must be greater than 1.5 V. The TCAN4550 is specified to meet the 1.5 V requirement with a across this load range and is specified to meet 1.4 V differential output at 45 Ω bus load. The differential input resistance of this family of transceiver is a minimum of 30kΩ. If 167 of these transceivers are in parallel on a bus, this is equivalent to an 180 Ω differential load in parallel with the 60 Ω from termination gives a total bus load of 45 Ω. Therefore, this family theoretically supports over 167 transceivers on a single bus segment with margin to the 1.2 V minimum differential input voltage requirement at each receiving node. However for CAN 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 the original ISO 11898-2:2016 standard of 40 m by careful system design and data rate tradeoffs. For example CANopen network design guidelines allow the network to be up to 1km with changes in the termination resistance, cabling, less than 64 nodes and significantly lowered data rate.

This flexibility in CAN network design is one of its key strengths allowing for these system level network extensions and additional standards to build on the original ISO 11898-2 CAN standard. However, when using this flexibility the CAN network system designer must take the responsibility of good network design to ensure robust network operation.