How to analyze CAN bus signals with a logic analyzer

What is the CAN bus and why does it fail in practice

The CAN bus, or Controller Area Network, is a robust communication protocol originally designed for the automotive industry and now widely adopted in industrial machinery. In practice, it allows different microcontrollers and sensors to exchange messages without requiring a central computer to coordinate traffic. Each message carries a priority identifier, ensuring safety-critical commands reach their destination without perceptible delays. However, when cables suffer physical damage or power supplies generate magnetic instability, the system begins dropping data packets intermittently.

To understand bus behavior, we must look at its differential physical layer, composed of two lines called CAN High and CAN Low. Instead of measuring voltage relative to a common ground, the network reads the potential difference between these two wires. When the differential voltage is close to two volts, we have a dominant bit, equivalent to a logical zero level. When the difference drops near zero volts, we have a recessive bit, equivalent to a logical one level. This symmetrical approach cancels most electrical noise induced by motors and frequency drives along factory cabling.

When the network exhibits sporadic locks, field engineers usually rely on inspection tools to uncover what is happening in the physical medium. In many cases, the fault lies not in the embedded software, but in subtle electrical issues that corrupt data frame transmission. As seen in field investigations and projects focused on embedded hardware design, precise diagnostics require directly visualizing the flow of electrical pulses traveling through the metallic conductors of the industrial installation.

How a logic analyzer works in network debugging

A logic analyzer is a benchtop instrument that captures multiple digital signals simultaneously over time, storing logic level transitions in a high-speed internal memory. Unlike a multimeter that only displays average values, or an analog oscilloscope focused on detailed waveform shapes, the logic analyzer focuses on the digital logic of the signal. It interprets whether a pin is high or low at precise instances in time, allowing entire serial communication protocols to be decoded in an automated and clean manner.

In practice, to inspect the CAN bus, you connect the logic analyzer input channels to the CAN High and CAN Low lines of the operating network. The instrument software reconstructs the captured electrical pulses, applies protocol rules, and displays a chronological table containing packet identifiers, transmitted data, and control bits. This turns an abstract set of voltage waves into understandable messages, facilitating the immediate identification of which device is sending corrupted data or causing collisions on the transmission medium.

Beyond decoding messages, the logic analyzer allows you to audit the temporal behavior of the network in milliseconds. You can verify whether the response time between a command sent by a PLC, an acronym for Programmable Logic Controller that automates industrial processes, and the actuator response is within expected limits. This fine visibility avoids guesswork during corrective maintenance, saving precious hours of machine downtime on automated production lines.

Identifying and fixing baud rate configuration issues

The baud rate defines the speed at which bits are transmitted and read on the bus, measured in kilobits per second or megabits per second. For communication to occur without errors, all devices connected to the same CAN network must be rigorously configured with the exact same speed. If a single node operates at a slightly divergent rate due to an uncalibrated crystal oscillator, it will misinterpret bit sampling instances, generating corrupted frames and triggering internal error counters.

When using the logic analyzer, you configure the decoder by entering the expected baud rate and sample point, which indicates the exact moment the bit should be read in the middle of its temporal cycle. If the tool starts displaying corrupted characters or red frame errors, it is a strong indication of speed mismatch among nodes. In complex industrial networks, applying sound design practices, similar to those used in deterministic industrial communication strategies, helps eliminate unwanted timing variations caused by propagation delays in cabling.

Another critical speed-related factor is cable segment length. Longer networks require lower transmission rates to allow signals to propagate across the entire physical extent without excessive attenuation. If someone alters the baud rate to increase data throughput without physically shortening the bus or improving cable quality, signal reflections at the ends will start corrupting bits, completely invalidating digital communication integrity.

Detecting electrical noise and interference on the physical layer

The industrial environment is full of severe electromagnetic noise generators, such as high-power motors, electromechanical relays, and power cables running parallel to data signals. When CAN cable shielding is improperly grounded or galvanic isolation is missing in transceivers, these noises induce spurious voltage spikes on differential lines. Although twisted pairs help cancel out much of this interference, high-amplitude noises can exceed circuit immunity margins and invert a bit's logical state during transmission.

Although logic analyzers primarily show digital signal interpretations, many advanced models or associated softwares allow viewing minor anomalies in edge transitions. Excessively sloped or ringing rising and falling edges reveal impedance matching problems. To ensure the bus operates free of destructive interference, checking for the presence of one-hundred-twenty-ohm termination resistors at both physical ends of the bus is fundamental, as they absorb signal energy and prevent standing waves from destroying transmitted data.

When intermittent faults persist even after reviewing termination, inspection must extend to the physical infrastructure of the installation. Verifying shield continuity, cable isolation against mechanical abrasion, and proper grounding of electrical panel enclosures solves the vast majority of noise issues. After all, no software analysis tool can save a bus whose physical integrity has been compromised by gross flaws in field cable assembly and routing. To explore these architectural concepts further in production, see our related technical analysis on How to calibrate industrial sensors via Modbus in the field.

Conclusion and preventive maintenance recommendations

Mastering the use of logic analyzers for CAN network inspection transforms industrial debugging from a guesswork-based process into an exact, evidence-based science. Understanding the relationship between the physical layer of differential signals, baud rate accuracy, and rigorous electrical noise mitigation enables engineers and technicians to maintain complex systems operating with maximum reliability and uptime.

As a practical recommendation for maintenance routines, establish periodic frame capture routines to monitor frame error rates before catastrophic machine failures occur. Keep the exact network topology documented, including cable lengths and termination values, facilitating future comparative analyses. By combining digital capture tools with good physical design practices, you ensure a resilient, secure bus prepared for modern industrial automation challenges.