SLLS983J June 2009 – September 2019 ISO1050
The ISO11898 Standard specifies a maximum bus length of 40 m and maximum stub length of 0.3 m with a maximum of 30 nodes. However, with careful design, users can have longer cables, longer stub lengths, and many more nodes to a bus. A high number of nodes requires a transceiver with high input impedance such as the ISO1050.
Many CAN organizations and standards have scaled the use of CAN for applications outside the original ISO11898 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 CAN network design is a series of tradeoffs, but these devices operate over wide –12-V to 12-V common-mode range. In ISO11898-2 the driver differential output is specified with a 60-Ω load (the two 120-Ω termination resistors in parallel) and the differential output must be greater than 1.5 V. The ISO1050 is specified to meet the 1.5-V requirement with a 60-Ω load, and additionally specified with a differential output of 1.4 V with a 45-Ω load. The differential input resistance of the ISO1050 is a minimum of 30 kΩ. If 167 ISO1050 transceivers are in parallel on a bus, this is equivalent to a 180-Ω differential load. That transceiver load of 180 Ω in parallel with the 60 Ω gives a total 45 Ω. Therefore, the ISO1050 theoretically supports over 167 transceivers on a single bus segment with margin to the 1.2-V minimum differential input at each node. 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 thus a practical maximum number of nodes is typically much lower. Bus length may also be extended beyond the original ISO11898 standard of 40 m by careful system design and data rate tradeoffs. For example, CAN open 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 the key strengths of the various extensions and additional standards that have been built on the original ISO11898 CAN standard. In using this flexibility comes the responsibility of good network design.