Robustness and reliability in CAN-based networks
In the early 90’s, when CAN-based networks were first used in industrial automation, the robustness and the reliability were key issues. This has not changed until now. In applications in which robust and reliable communication are required, CAN-based solutions are a good option. The residual failure detection capability is high. The CAN protocol built-in failure detection mechanisms allow detection of up to five randomly distributed bit-failures or a burst of up to 15 bit-failures. In case of failure, all nodes in the network are informed about the detected error. All nodes discard this message, so that data consistency is still guaranteed. The faulty message is re-transmitted automatically. There is no handshake procedure necessary. This makes the failure recovery very fast; under normal conditions, the retransmission is caused after 23 bit-times, even in case of local failure. The recovering time for global failure is 17 bit-times. When there is a permanent failure in one node, it goes bus-off, in order to not disturb the communication of the others.
Besides reliability, in industrial applications robustness is required. In particular, the EMC performance is important. The usual differential voltage on the CAN high-speed bus-lines compliant with ISO 11898-2 provided is quite immune against electrical disturbances. Nevertheless, the most critical part is the design of the CAN physical layer, the network topology, and the optional protection circuitry. DeviceNet, one of the CAN-based higher-layer protocols provides in its specification, strict rules for the network design. CANopen, another CAN-based higher-layer protocols, gives just a few recommendations: bit-timing settings including sample-points, network length at given bit-rates, maximum length for single and all not terminated stubs. All the CAN controllers and transceivers are available in industrial temperature ranges, but also in extended (automotive) temperature ranges. This means, CAN-based networks are also suitable for outdoor applications and other challenging environments. CANopen is for example used in subsea applications as well as in the outer space in satellites.
All CAN-based networks need to be terminated at both ends when using bus-line topologies. If you do not use the appropriate termination resistors (e.g. not matching the impedance of the bus-lines) communication may be corrupted. In the past, users have often forgotten the termination resistors. Therefore, many device manufacturers have integrated termination resistors into their products, which need to be disconnected when they are not installed at the end of the bus-line. This also leads to a mismatch with the network impedance and causes reflections, which could corrupt the communication.
When respecting the design specifications and recommendations provided, CAN-based networks are reliable and robust commercial communication systems. For more detailed information visit the CAN in Automation (CiA) web-site (www.can-cia.org).
– Edited by Brittany Merchut, Project Manager, CFE Media, bmerchut(at)cfemedia.com