Is it necessary to extend branches longer than 0.3m in a linear topology CAN bus wiring specification with a 'T' type branch connection?

I. CAN Topology Classification

CAN (Controller Area Network) is an industrial fieldbus that supports distributed and real-time control through a multi-master asynchronous serial communication network. The network topology can be linear, star, tree, or ring, as shown in Figure 1.

Fig. 1 Characteristics of CAN Topology

II. Linear Topology Wiring Mode

IOS-11898-2 recommends a linear topology for the CAN network, using a single channel as the transmission medium. All stations are connected to a common bus through the corresponding hardware interface, as shown in Figure 2. Impedance matching in linear topology is relatively simple. Only a suitable termination resistor is needed at the two ends of the trunk (usually 120Ω within 2km).

Fig. 2 Linear Topology

The 'hand-in-hand' connection is the most commonly used linear topology in CAN bus wiring specifications, as shown in Figure 3.

Figure 3 "Hand-in-Hand" Connections

However, in most industrial sites, rail locomotives require the use of terminal blocks due to the large number of cables. Therefore, the 'T' type branch connection, as shown in Figure 4, is used for easier maintenance.

Figure 4 "T" Connection

III. "T" Connection Branch Constraints

The T-wiring method may experience signal reflection at joints due to impedance discontinuities caused by branch lengths and their accumulation. The amount of reflected signal depends on the transient impedance change, with larger changes resulting in more serious reflections. Negative-phase reflections are generated at the branch, causing a signal level dip that may exceed the noise tolerance and cause false triggering. To prevent this, it is desirable for the reflected wave to return to the source as quickly as possible. Therefore, the branch should be kept as short as possible.

Figure 5 of IOS-11898-2 specifies that the branch length should not exceed 0.3m at 1M baud rate, which is the highest baud rate for CAN. Therefore, to ensure stable operation, the branch length should also adhere to the 0.3m specification at other baud rates.

Figure 5 "T" Network Topology Parameters

IV. How to Determine Branch Length

The branch length specified in IOS 11898-2 is based on 1M baud rate conditions. However, achieving a very short branch may not be possible on some occasions. The branch length specification can be adjusted appropriately depending on the baud rate.   To determine the achievable branch length under different baud rates, you need to analyze the signal quality of the node. Measure the signal quality of the node at different branch lengths to find the appropriate range.

Figure 6 shows that to assess the quality of the node signal, you need to measure the minimum and maximum voltage amplitude, signal amplitude, waveform rising and falling edge time, and signal time of the node CAN differential signal. The specific parameter indexes are detailed in ISO 11898-2.

Assessing signal quality can be a challenging task without professional tools. CANScope's signal quality analysis plug-in allows for quick and easy evaluation of a node's signal quality with just one click. The analysis plug-in examines the waveform emitted by each CAN node, scores it comprehensively, and visualizes the signal quality of each CAN frame ID through a histogram (as shown in Figure 7). This allows for obtaining the signal quality of each node and quantitatively evaluating the quality of the node's physical layer.

Fig. 7 Histogram of Signal Quality