In CAN bus systems, the linear topology is the most commonly used configuration. When a "T" branch connection is employed in this setup, the branch length must not exceed 0.3 meters, as specified by standards like ISO 11898-2. But what should be done if a longer branch is required? This article explores the limitations of linear topology, the impact of T-branches, and how to handle cases where longer branches are necessary.
CAN (Controller Area Network) is a robust fieldbus protocol widely used in industrial automation and automotive applications. It supports distributed control and real-time communication between multiple nodes. The main topologies used in CAN networks include linear, star, tree, and ring. Each has its own advantages and limitations, with linear topology being the most common due to its simplicity and cost-effectiveness.
In high-speed CAN implementations, ISO 11898-2 recommends a linear bus structure, where all nodes are connected in a single line. This configuration uses a shared bus as the transmission medium, with termination resistors at both ends to match impedance (typically 120Ω). The "hand-in-hand" connection is a typical example of this setup, ensuring minimal signal distortion and optimal performance.
However, in practical installations—especially in industrial environments or on locomotives—longer cables and frequent maintenance often require the use of terminal blocks. This leads to the use of a "T" type branch connection, which can introduce signal reflections and degrade communication quality.
The issue arises because the "T" branch creates an impedance mismatch, causing signal reflections at the junction. These reflections can lead to signal distortion, undershoots, and potential false triggering. To minimize these effects, the branch length should be kept as short as possible. According to ISO 11898-2, the maximum allowed branch length is 0.3 meters at a baud rate of 1 Mbps. Since this is the highest speed supported by CAN, the same limit applies to lower baud rates for stable operation.
If a longer branch is unavoidable, it's essential to analyze the signal integrity under different conditions. This involves measuring key parameters such as minimum and maximum voltage amplitudes, signal rise and fall times, and overall waveform quality. These metrics help determine the acceptable range for branch length without compromising communication reliability.
Without specialized tools, assessing signal quality manually can be time-consuming and error-prone. Fortunately, tools like CANScope offer built-in signal quality analysis features. With just one click, the tool evaluates waveforms from each node, scores them, and displays the results in a histogram. This visual representation helps quickly identify problematic nodes and assess the physical layer performance of the entire network.
By understanding the limitations of T-branch connections and using proper tools for signal analysis, engineers can ensure reliable CAN communication even when longer branches are necessary. Whether designing a new system or troubleshooting an existing one, attention to topology and signal integrity remains crucial for optimal performance.
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