The conventional cabling type within a CAN network is the line topology but stub lines are restricted in maximum length and tree and star topologies are not available. If we find a way to eliminate these restrictions for the available topologies we can increase the system efficiency while also reducing the costs. The use of repeaters gives us the flexibility for optimizing the structure of our CAN network.
Flexible Topology
The restriction to line topology results from the high frequency behavior of the lines. To avoid reflections, which decrease the signals quality, a constant dissertation of the electrical impedance for the whole system is needed. Therefore, according to ISO 11898, segments with different topology than the line topology have to be decoupled from this high frequency behavior. This can be done by using protocol transparent repeaters and bridges. A really elegant solution is the use of protocol transparent repeaters due to the fact that this solution is realizable with acceptable effort and all benefits of the CAN protocol – data availability and consistency – can be kept while having a solution to the restrictions of the high frequency behavior.
Error suppression
Other benefits of using CAN repeaters is the reduction of interference by realizing segments with shorter line length and the easier localization of errors in wiring. If there is a short circuit between the signals CAN_high and CAN_low the communication is completely disabled for the whole network. A short circuit within a detached segment disables this segment but the remaining part of the network is working without any restrictions. Same situation if there is a short circuit between CAN_High and Ground. If the worst comes to the worst and additionally to the short circuit from CAN_low to Ground CAN_high is hot-wired to the supply voltage (you’ll get a permanent dominant level) a localization of the error will be possible with some additional hardware logic.
Sub-segments with optical signal transmission
Repeater technology adds additional benefits due to the possibility to galvanically decoupled sub-segments of the CAN network with sections connected through optical fibre links. The easiest way is to use a repeater with integrated optical fibre couplers. Advanced detachment can be done by submitting the CAN signal over optical fibres. With this solution we will be able to use CAN networks in areas with extremely high electrical interferences and the network is also save for high voltages, e.g. a lightning.
Usage of a repeater: An example
A manufacturing plant is connected with CAN. The backbone of the network is located directly in a cable channel at the production line. To connect the single stations a loop has to be inserted with conventional wiring. We assume a cable length of 15 m from the backbone to the furthest node. This length is easily reached when connecting nodes apart.
Lets assume that seven of these stations with a length of 20 m between two branch points on the backbone are installed. The resulting cable length can be calculated: 2 x 15 (outer stations to backbone) plus 6 x 20 m (between backbone branches plus 5 x 30 m (loop at inner stations). This totals to a length of 300 m with an approximate cable propagation delay of 1650 ns.
On the other hand, consider the same application, but with use of one CAN repeater at each branch point on the backbone. The backbone is now directly connected from the first of the last node in the network. The cable consists of 2 x 15 m (outer stations to backbone) and 6 x 20 m (between backbone branches) which results in a length of 150 m. Added are five branches with one repeater and 15 m of cable each. The total installed cable length is now 225 m. For timing considerations the maximum propagation delay between any two nodes has to be calculated. Using fast repeaters built with today’s standard transceivers the internal delay can be assumed to be equal of that of 25 m cable. In this case the longest delay in wiring is between the outer nodes on the outer branches connected with repeaters (critical path). The equivalent length is 2 x 15 m (outer stations of backbone) plus 2 x 25 m (equivalent repeater delay) plus 4 x 20 m (backbone) totalling to 160 m with a delay of 880 ns.
Comparing both installations the repeater solution with 880ns has only the half duration time of the normal installation (1650ns) The possible data rate is nearly doubled for the repeater solution.
Conclusion
By using a CAN repeater, we can reach essential technical benefits, especially for networks with long distances. By selecting a appropriate network topology a increase of the supported bit rate will increase the system performance as well. An additional benefit which is not conditioned by the selected network topology is the easier fault localization. Application areas for CAN repeater are extensive installations of engine- and plant constructors or in building automation.
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