Why CAN bus branches are most reliable within 0.3 meters?
Linear topology is the most common in the CAN bus wiring specification. If the “T” type branch connection in the linear topology is used, the length of the branch cannot be greater than 0.3m according to the regulations. What should I do if a longer branch is required?
1. CAN topology classification
CAN (Controller Area Network) belongs to the category of industrial field bus, which is a multi-master asynchronous serial communication network that effectively supports distributed control and real-time control. The topological structure of CAN network mainly includes linear topology, star topology, tree topology and ring topology. The characteristics of these topologies are shown in Figure 1:
Figure 1 CAN topology features
2. Linear topology wiring method
There is a high-speed CAN physical layer specification in IOS-11898-2, in which it is recommended that the CAN network adopts a linear topology in the form of a bus. As shown in Figure 2, the linear topology CAN network uses a single channel (bus) as the transmission medium, and all stations pass through The corresponding hardware interface is connected to a common bus. The impedance matching of the linear topology is relatively simple, and only needs to be connected to the two ends of the trunk with appropriate termination resistors (usually 120Ω within 2km).
Figure 2 Linear topology
The linear topology is the most common in the CAN bus wiring specification. In the linear topology, the “hand in hand” connection is the most commonly used, as shown in Figure 3.
Figure 3 “Hand in hand” connection
However, in the vast majority of industrial sites and rail locomotives, due to the large number of overall cables, terminal blocks are required to facilitate maintenance. Therefore, a “T” type branch connection is used, as shown in Figure 4.
Figure 4 “T” connection
3. “T” connection branch constraint
In the T-type wiring method, there is a discontinuity of impedance caused by the branch length and the accumulation of the branch length, so the phenomenon of signal “reflection” occurs at the joint. The amount of reflected signal is determined by the change in transient impedance, and the larger the change, the more severe the reflection. A negative phase reflection occurs at the branch, causing signal level undershoot, which may exceed the noise tolerance and cause false triggering. In order to avoid this situation, it is hoped that the reflected wave will return to the source end as soon as possible, that is, the branch line should be as short as possible.
As shown in Figure 5, it is stipulated in IOS-11898-2 that the branch length should not be greater than 0.3m at 1M baud rate, and 1M baud rate is the highest baud rate of CAN, so at other baud rates, if the branch length is also Following the 0.3m specification, it can run stably.
Figure 5 “T” type network topology parameters
Fourth, how to determine the branch length
The branch length specification in IOS 11898-2 is under the condition of 1M baud rate. In some occasions, it may not be possible to achieve very short branches. According to different baud rates, the branch length specification can be adjusted appropriately. How many branch lengths can be achieved at different baud rates? It is necessary to analyze the signal quality of the node for evaluation, and measure the signal quality of the node under different branch lengths to find the appropriate branch length range.
As shown in Figure 6, to evaluate the signal quality of a node, it is necessary to measure the minimum voltage amplitude, maximum voltage amplitude, signal amplitude, waveform rising edge time, waveform falling edge time, and signal time of the node CAN differential signal for comprehensive scoring. The specific parameters The indicators are specified in ISO 11898-2.
Figure 6 Signal quality parameters
Signal quality evaluation is obviously a troublesome thing without professional tools. If you want to quickly evaluate the signal quality of nodes, you can use CANScope’s signal quality analysis plug-in for one-click analysis. The analysis plug-in analyzes the waveforms sent by each CAN node, automatically makes a comprehensive score, and then visually displays the signal quality of each CAN frame ID through a bar chart (as shown in Figure 7), so as to obtain the signal of each node. Quality, which quantitatively evaluates the physical layer quality of the node.
Figure 7 Signal quality histogram
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