From Line Topology to Star Repeater

The CAN repeater CRep S8C, like many products from EMS Dr. Thomas Wünsche, was developed based on a specific customer request. In this case, the company was engaged in the manufacture of machines for the production of plastic film. These machines, which can be quite large, use sensors and actuators networked via CAN at many different locations. Their control system is located at a central point in the control cabinet. The system can be wired in three fundamentally different ways.

Classic Line Topology

In the classic line topology, all CAN nodes are connected in a line with a single cable. Although this method is frequently used and is certainly justified for simple systems due to its low cost, it does have serious disadvantages. If a defect interrupts the line, or if a defective node sends a permanently dominant CAN signal, the entire system becomes inoperable. If system expansions become necessary at a later date, integration into the existing line is often no longer possible. In addition, following loop formation, the cable length easily exceeds the maximum permissible length for the selected baud rate. Many of these problems can be solved simply by using standard repeaters like CRep NM.

Star wiring with standard repeaters

Star wiring with standard repeaters is characterized by the fact that stub lines are connected to a main line using CAN repeaters. Multiple CAN nodes can be grouped together in the stub line segments. This provides an alternative to the difficulties of the classic line topology. The use of repeaters enables long stub lines, allowing a smaller overall system. It also facilitates the subsequent integration of new sensors and actuators. Last but not least, the system stability is increased, since if a segment is lost, the remaining system maintains communication. Furthermore, segmenting the system significantly speeds up troubleshooting. This generally leads to increased system availability. However, there are some challenges with this solution that must be considered. Firstly, the transit time of two repeaters must be taken into account when calculating the maximum cable length, which can lead to problems with the selected baud rate in large systems. Secondly, the costs of using many repeaters may exceed the specified limits.

The star wiring method with standard repeaters was already used by a customer of EMS Dr. Thomas Wünsche. The high costs per segment were the deciding factor in seeking an alternative solution. This was realized in the form of the CRep S8C. By combining and implementing the common main line in an FPGA component, it was possible to combine eight independent CAN segments in one device. This leads to the third option.

Star Wiring with a Star Repeater

In this method, a star repeater forms the node of the star topology. In addition to the advantages of star wiring with standard repeaters already described, wiring with a star repeater offers further improvements. The most obvious benefit is the significant reduction in costs per segment, as one star repeater performs the tasks of up to eight standard repeaters. By relocating the main line to the device, only one repeater transit time needs to be considered when calculating the maximum cable length. Another advantage is the significantly increased flexibility of the overall system. Up to three CRep S8Cs can be cascaded, creating a maximum of 24 CAN channels. The individual channels are accessible via 3-pin pluggable terminal blocks. A terminating resistor is already integrated into each channel. This allows individual sensors or actuators to be connected or disconnected without adapting the entire network.

Star wiring with a star repeater is suitable for a variety of reasons:

When many CAN nodes need to be networked at different locations within a system or machine.

When a line topology is not possible due to the expected large size.

When the network needs to be able to be flexibly expanded with additional nodes.

When system availability must be maintained even if parts of the system fail.

Examples of this can be found in a wide variety of industries, such as deep-sea mining, access control systems at campsites, or specialty machinery.