Conventional installation of CAN networks allows the line topology as the only possible wiring topology Stub lines, if they are used, are very restricted in length. Tree or star structures are not possible in general. This fact leads to unnecessary long cables and hence wasted money. In addition, later changes can be realized hardly without drawbacks regarding the maximum speed. While line structured systems are still easier to install than ring structures, a tree or star topology would in many cases better fit the application requirements.
Freeing restrictions of usable topology
For this reason it is advantageous to free CAN installations from the restrictions of line topology. An elegant solution can be realized by the use of protocol transparent CAN repeaters. These conserve the benefits of the CAN protocol while breaking the restrictions of high frequency line design.
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Use Cases in CAN networks
X Example without repeater
An example: In a production line, individual stations are connected via CAN. The backbone of the network is installed in a cabling channel running beside the production line. To connect each station in conventional wring technology a loop is inserted in the backbone which leads to the connected CAN nodes. The length of the cable from the backbone to the outermost CAN node is assumed to be 15 m, a length that is easily reached, if distributed nodes are attached. Because of the structural requirements in a conventional net the cable is pulled back to the cabling channel. With this design each loop adds a cable length of 30 m.
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.
✓ Example with repeater
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 to 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 40 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 40 m (equivalent repeater delay) plus 4 x 20 m (backbone) totalling to 190 m with a delay of 1045 ns.
Cost savings
The installed cable length is 300 m in the standard CAN network compared to 225 m in the repeater solution. Assuming costs of 15$/m for installed cable (the reader may insert own values, which should consider cable and installation cost) result in savings of cabling costs of $1,125. From this savings the cost of 5 repeaters (assuming a per unit cost of $120) has to be subtracted: the resulting saving is $525 (subject to cost assumptions being made).
The propagation delay of the wiring is approximately 1650 ns and using repeaters about 1045 ns. Due to the fact that the delay is only almost two-thirds the maximum possible data rate is increased remarkable.
Conclusion
Don’t forget the restrictions. Repeaters used in simple line topology decrease the maximum bit rate due to their internal propagation delay. Therefore benefits, both in cost and speed, can only be obtained from a modified wiring structure. As a general result it may be said that the profit of repeater use is high in large networks with geometrical layout that deviates from a line. Besides large production or transportation systems this structure can be found in storage systems or in building automation systems. So you might also be interested in which ways a CAN/CAN gateway in comparison to a CAN repeater can make your network a better CAN network.
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