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In CAN networks the underlying transport protocol sets structural limits to the system in standard applications. CAN currently manages a 1 Mbit/s transmission rate, given that the overall network cable length does not exceed 25 meters. If longer lines are necessary, the transmission rate must be reduced – the longer the distance, the higher the speed loss. However, with suitable topology components, high bandwidths can be realized even in large networks. Specific benefits of various solutions will be outlined in the following.
Figure 1: With suitable topology components, high bandwidths
are possible even in extensive CAN networks
CAN networks can be extended and made more flexible by means of various components. For example, repeaters enable star and tree structures instead of a simple daisy-chaining of bus nodes. Bridges and gateways, on the other hand, are mainly used for the physical extension of existing linear connections. With suitable components CAN networks can even be enabled for wireless communication.
CAN repeaters primarily serve the physical connection of two or more segments of a CAN bus system. Additionally, they allow for the implementation of tree and star topologies and long stubs. Repeaters and star couplers do not in general influence the real-time behavior of a system. An application scenario: three pitch controllers in a wind turbine shall communicate with the Master controller via CAN. The standard line topology of CAN is not equal to the task. However, a CAN repeater enables star connections to the individual wind turbine blades. It also establishes galvanic isolation and thereby improves lightning protection. In case of unexpected failures in the network, faulty segments can be taken off the network by means of an integrated monitoring function in order to maintain reliable communication between the other network participants. As soon as the failure has been repaired, the restored segment is reconnected without interruptions. CAN systems linked via a repeater represent autonomous electrical segments with optimum signal termination – thus, topologies can be realized which would be impossible with a simple linear bus topology for the danger of electrical reflections.
Figure 2: CAN repeaters enable star topologies in wind turbines
The functional range of repeaters includes coupling different physical CAN layers, e.g., translating between high-speed and low-speed CAN or connecting copper cables and optical fibers. Furthermore, they improve EMC and dispersion behavior of CAN systems. For instance, galvanic isolation integrated in the IXXAT CAN repeaters for up to 4 kV prevents the spreading of interferences through the network. By repeating the signals, the devices also filter errors caused by electromagnetic interferences or cable quality.
In contrast to CAN repeaters which are not principally meant for the extension of line topologies, CAN bridges and CAN gateways directly support the increase of the maximum network size. CAN bridges can connect networks using different bitrates or protocols. They are based on the store, (modify) and forward principle, receiving CAN messages from one network part and sending them via the other. Conversion and filter algorithms may be employed, enabling, e.g., protocol conversion between network parts.
Figure 3: By means of bridges, cable lengths in CAN networks can be extended
Among other things, the integrated filter function allows for messages to be filtered before being converted from one network to the other in order to keep the bus load in the particular networks as low as possible. Bus arbitration of system subsections takes place absolutely independently, which enables the higher maximum network size mentioned at the outset. Building automation is an area where CAN bridges are employed particularly often, namely to connect distributed subnetworks. In buildings it is particularly important that installations can be flexibly adapted to ensure that CAN communication works smoothly with typical line topologies with limited stub lengths. Radio transmission may be implemented in applications where communication by wire is difficult, such as rotary tables. In this case, IXXAT CANblue from HMS, which enables CAN data communication via Bluetooth, can be used for coupling. Data transmission occurs on layer 2 and is transparent. Therefore, this solution can be used with various CAN-based protocols from CANopen or DeviceNet to customer-specific variants. If several CANblue units are employed, the devices can be coupled dynamically.
Figure 4: IXXAT CAN@net II/Generic from HMS for coupling CAN systems via Ethernet