11.1.1 Bus Loading, Length and Number of Nodes

The ISO 11898 standard specifies a data rate up to 1 Mbps, maximum CAN bus cable length of 40 m, maximum drop line (stub) length of 0.3 m and a maximum of 30 nodes. However, with careful network design, the system may have longer cables, longer stub lengths, and many more nodes to a bus. Many CAN organizations and standards have scaled the use of CAN for applications outside the original ISO 11898 standard. They have made system level trade-offs for data rate, cable length, and parasitic loading of the bus. Examples of some of these specifications are ARINC825, CANopen, CAN Kingdom, DeviceNet and NMEA200.

A high number of nodes requires a transceiver with high input impedance and wide common mode range such as the TCAN33x CAN family. ISO 11898-2 specifies the driver differential output with a 60-Ω load (two 120- Ω termination resistors in parallel) and the differential output must be greater than 1.5 V. The TCAN33x devices are specified to meet the 1.5-V requirement with a 50-Ω load across a common mode range of –12 V to 12 V through a 330-Ω coupling network. This network represents the bus loading of 120 TCAN33x transceivers based on their minimum differential input resistance of 40 kΩ.

For CAN network design, margin must be given for signal loss across the system and cabling, parasitic loadings, network imbalances, ground offsets and signal integrity, thus a practical maximum number of nodes may be lower. Bus length may also be extended beyond the original ISO 11898 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
1 km with changes in the termination resistance, cabling, number of nodes and data rate.

This flexibility in CAN network design is one of the key strengths of the various extensions and additional standards that have been built on the original ISO 11898 CAN standard.