The bit monitoring feature of CAN (Controller Area Network) contributes not only to the enormous reliability of the network, but it is also responsible for its biggest drawback, namely the limited physical distance of the CAN bus.
Restricted distance is an issue for applications such as elevators, security systems and other building technologies.
The only way to extend the usable bus length without compromising the reliability is to lower the baud rate. In cases where baud rate is not the main focus, such as diagnostics and low-priority messaging, a CAN bus can be extended as far as the reach of the Internet.
While a distance of roughly 50m at 1Mbit/s seems to be restrictive, especially for building technologies, a baud rate of 1Mbit/s can nevertheless be considered overkill for such applications. Experience has shown that the vast majority of automation applications can work sufficiently with baud rates of 500kbit/s or even 250kbit/s - and that includes demanding motion control applications.
While CAN repeaters provide primarily electrical isolation and signal conditioning, they are not really suitable for extending the bus length.
It is possible to operate CAN with a simple twisted pair of wires, but the quality of the CAN bus cable can be a major factor in terms of maximum bus length. Poor cable quality will quickly reduce signal strength to below a receiver's threshold. The result will be signal errors and, consequently, increased bus traffic due to error frames and repeated messages.
CAN repeaters can be used to boost the signal strength and maintain standard bus lengths, but not extend them. The only extension is the one from a poor quality network to a properly functioning network. Ironically, CAN repeaters, due to their internal delay times in the range of milliseconds, will actually shorten the usable bus length in the range of several metres.
Some CAN vendors offer a variety of interfaces that support the communication between two separate CAN networks (eg the CAN-CBM-Bridge by ESD Electronics. Such devices make it possible to extend a CAN network by a factor of two, but they, too, have latency times due to reception, processing and re-transmission of CAN frames. Another disadvantage is that, for instance, both CAN networks cannot exchange error frames.
Such gateways, however, also post some advantages such as message filtering - to lower the bus traffic between the networks - and the use of different baud rates in the networks.
The ultimate way to extend the reach of a CAN application is the use of Ethernet gateways, which consequently even allow connection to the Internet. For instance, the Ethercan device by ESD Electronics, provides operation modes either to connect two separate CAN networks via Ethernet or to allow the monitoring of network activities through the Internet.
In the first mode, users can connect two separate CAN networks by Ethernet to maintain almost unlimited physical distances. Each network will need its own gateway, which, in turn, will contribute to higher latency times. The Ethercan also supports message filtering and can therefore decrease the number of messages between the networks.
The second mode, operation through the Internet, is supported by the Ethercan's internal web server. The Ethercan comes with an extensive PC software package with CAN analysing and monitoring features.
