9.2.2.1 Bus Loading, Length and Number of Nodes

The ISO 11898-2 Standard specifies 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 large number of nodes requires transceivers with high input impedance such as the ISO1042 transceivers.

Many CAN organizations and standards have scaled the use of CAN for applications outside the original ISO 11898-2 Standard. These organizations and standards 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, DeviceNet, and NMEA2000.

The ISO1042 device is specified to meet the 1.5-V requirement with a 50-Ω load, incorporating the worst case including parallel transceivers. The differential input resistance of the ISO1042 device is a minimum of 30 kΩ. If 100 ISO1042 transceivers are in parallel on a bus, this requirement is equivalent to a 300-Ω differential load worst case. That transceiver load of 300 Ω in parallel with the 60 Ω gives an equivalent loading of 50 Ω. Therefore, the ISO1042 device theoretically supports up to 100 transceivers on a single bus segment. However, 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, therefore a practical maximum number of nodes is typically much 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, less than 64 nodes, and a significantly lowered 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 11898-2 CAN standard. Using this flexibility requires the responsibility of good network design and balancing these tradeoffs.