Can you explain common issues with CAN transceivers and how to troubleshoot them?

Many field engineers have encountered this frustrating scenario: the board works flawlessly in the laboratory, but once deployed on the industrial site, the bus frequently enters the Bus-Off state, or nodes randomly drop out with sporadic packet loss. Replacing the host MCU proves useless, and reflashing the firmware yields no results.

In reality, over 80% of CAN communication anomalies do not stem from software bugs, but rather from issues at the Physical Layer and within the CAN Transceiver. This guide dives into the underlying hardware logic to dissect common failure points of CAN transceivers and provides a practical, step-by-step field troubleshooting workflow.

Root Cause Analysis of Common CAN Transceiver Failures

The core function of a CAN transceiver is to convert the logic levels (TTL/CMOS) of the CAN controller into differential bus levels (CANH/CANL). Although differential signals offer high noise immunity, they are exceptionally sensitive to the electrical environment of the physical layer.

+-----------------+        +------------------+        +-----------------+
|  CAN Controller |  TTL   | CAN Transceiver  | Differential |   CAN Bus      |
|  (MCU/DSP)      | <----> | (e.g. TJA1050/   | <=========>  | (CANH / CANL)   |
|                 | Tx/Rx  |  SIT1050)        |   CANH/CANL  |                 |
+-----------------+        +------------------+        +-----------------+

1. Missing Termination Resistors or Impedance Mismatch Causing Signal Reflection

The CAN bus requires a 120-Ohm termination resistor at each of the two extreme ends of the transmission line to match the characteristic impedance of the cable.

• Symptom: When observing the CANH/CANL waveforms on an oscilloscope, significant overshoot, undershoot, and high-frequency ringing appear at the rising and falling edges.

• Physical Mechanism: When high-frequency signals encounter an impedance discontinuity at the end of a transmission line, the high-frequency energy cannot be absorbed and reflects back, creating a wave that overlays the original signal. When the amplitude of this reflected wave exceeds the sample point threshold, the CAN controller interprets it as a Bit Error, triggering a spike in error frames.

2. Excessive Common-Mode Voltage and Ground Potential Difference (GND Offset)

When wiring across different workshops, long distances, or high-power environments, reference grounds (GND) between different nodes often develop potential variations.

• Symptom: Communication functions normally over short distances, but halts instantly when node distances are extended or when high-power motors start up.

• Physical Mechanism: The common-mode input voltage range for standard non-isolated CAN transceivers is typically restricted between -12V and +12V. If the ground potential difference between Node A and Node B reaches several volts or tens of volts, combined with ambient electromagnetic induction, the common-mode voltage on CANH and CANL relative to GND will exceed the transceiver's tolerance limits. This causes the internal differential comparator to saturate and lock up, or even suffer catastrophic electrical overstress (EOS) breakdown.

3. Excessive Bus Capacitance Degrading Signal Edge Steepness

• Symptom: Communication is stable at low baud rates (such as 125 kbps), but increasing the speed to 500 kbps or 1 Mbps results in nodes failing completely to transmit or receive data.

• Physical Mechanism: Excessive nodes, overly long main cables/stubs, and external ESD protection devices add parasitic capacitance to the bus. The charging and discharging effects of this capacitance increase the RC time constant, smoothing out the rising and falling signal edges. At high baud rates, the dominant level fails to fully establish before transitioning back to the recessive state, causing the CAN controller to read erroneous signal levels at the sampling point.

4. Power Supply Ripple and Transient Overvoltage Interference

Switching power supplies, frequency inverters, and relay operations on industrial sites inject high-frequency voltage spikes into 5V/3.3V power rails or couple into CAN lines via spatial radiation.

• Symptom: The transceiver IC overheats abnormally, or the TXD line locks permanently into a dominant state (Dominant Timeout error).

• Physical Mechanism: Unstable power supplies cause the transceiver's internal reference source to drift, altering the differential output amplitude. Meanwhile, overvoltage surges can breach the IC's internal ESD protection structures.

Step-by-Step Field Troubleshooting Workflow

When troubleshooting CAN bus faults, always adhere to the principle of "Static First, Dynamic Second; Physical Layer First, Link Layer Second."

Step 1: Static Resistance Test (Checking Termination Matching and Wiring Continuity)

Disconnect system power and measure the resistance across the bus ends using a multimeter in resistance mode:

Step 2: Dynamic Voltage & Ground Potential Test (Checking Power and GND Offset)

Power on the system, switch the multimeter to DC voltage mode, and measure bus-to-ground voltages:

1. Recessive State Check:

   • Measure CANH to GND and CANL to GND while the bus is idle.

   • Standard Value: Both readings should be around 2.0V to 2.5V, with a differential voltage of $V_{diff} = CANH - CANL \approx 0V$.

2. Inter-Node Ground Potential Measurement:

   • Measure the potential difference between GND of Node A and GND of Node B using both AC and DC voltage modes.

   • Standard Value: DC voltage difference should be below 0.8V. If the difference exceeds 2V, galvanic isolation or equipotential bonding is required.

Step 3: Oscilloscope Waveform Analysis (Checking Signal Integrity)

Connect oscilloscope probes to CANH and CANL, and enable the Math function to calculate the differential waveform ($CANH - CANL$):

• Dominant State: Differential voltage $V_{diff}$ should range between 1.5V and 3.0V (typical value: 2.0V).

• Recessive State: Differential voltage $V_{diff}$ should range between -0.5V and 0.9V (typical value: 0V).

• Key Observations:

  • Edge Steepness: Is the rising edge during a bit period overly slurred? (If slurred, reduce the number of bus nodes or remove excessive filtering capacitors).

  • Overshoot & Ringing: Does the amplitude exceed 0.5V? (If exceeded, inspect termination resistors or branch stub matching).

Dominant                      Recessive
CANH (~3.5V) ---------\
                       \------------------- (~2.5V)
CANL (~1.5V) ---------/------------------- (~2.5V)
                     /
Vdiff (CANH-CANL) = 2.0V               Vdiff = 0V

Efficient Solutions for Industrial CAN Bus Challenges

In real-world engineering, building a discrete "MCU + Isolator + CAN Transceiver + TVS Protection" circuit from scratch for every project—especially when facing harsh factory conditions, legacy retrofit projects (e.g., TTL or RS485 interfaces), or multi-floor long-distance communication—results in prolonged R&D cycles and steep compliance hurdles for EMC certification.

To resolve these industrial challenges, Ebyte offers proven modular and device-level solutions:

For Board-Level Embedded Development: E810-TTL-CAN01 Isolated Transceiver Module

For engineers designing control boards who face level-shifting complexities, space constraints, or severe common-mode interference, the Ebyte E810-TTL-CAN01 provides a highly integrated solution.

• Built-in Galvanic Isolation: Integrates high-reliability signal and power isolation up to 3000V DC, blocking ground potential differences and industrial surges at the source to protect the host MCU.

• Compact Integration: Featuring a small footprint modular package, it integrates CAN control and micro-transceiver circuitry, supporting direct 3.3V/5V TTL level interfacing with MCU UART ports or CAN controllers.

• Industrial Protection & Compatibility: Supports bus speeds up to 1 Mbps, complies with CAN 2.0A/B specifications, and includes built-in port ESD protection and impedance matching to simplify PCB layout.

For Device-Level Field Integration: E810-DTU Series Industrial Converters

When field wiring is complete and legacy RS232/RS485 devices need to join a CAN network, or when long-distance repeater isolation is needed, the Ebyte E810-DTU Series CAN Protocol Converters are recommended.

• Transparent Transmission & Protocol Conversion: Supports multiple operational modes including "RS485/232 to CAN" and "Modbus RTU to CAN", allowing rapid networking without modifying host software code.

• DIN-Rail Industrial Design: Built with standard DIN-rail mounting and a rugged metal housing providing superior electromagnetic immunity (EMI), operating stably across broad temperature ranges from -40 degrees Celsius to +85 degrees Celsius.

• Hardware-Grade Reliability: Features built-in 120-Ohm termination resistors (switch-selectable) and up to 2KV lightning surge protection to eliminate node dropouts in demanding industrial applications.

Field Deployment Golden Rules

To prevent recurring fault cycles, strictly adhere to these three golden rules during system design and field installation:

1. Enforce Daisy-Chain Topology; Avoid Star Configurations

The physical topology of a CAN bus must follow a linear Daisy-Chain structure, with 120-Ohm termination resistors placed at both extreme ends of the main trunk. Keep branch stub lengths (Stub Length) as short as possible:

• At 1 Mbps, stub lengths must be under 0.3 meters.

• At 500 kbps, stub lengths must be under 1 meter.

• Star or tree topologies are strictly prohibited, as they induce severe impedance mismatches and signal reflections.

2. Standardize Shielding and Single-Point Grounding

Use Shielded Twisted-Pair (STP) cabling, ensuring CANH and CANL are routed on the same twisted pair.

• Shield Grounding Rule: The cable shield must be grounded at one single point only (typically at the bus gateway or main control panel). Grounding both ends creates ground loop currents due to inter-node potential differences, which couples noise back onto the CAN signal lines.

3. Galvanic Isolation is mandatory for Industrial Environments

Whenever communication distance exceeds 50 meters, or when nodes cross different power distribution boxes or power systems, galvanic isolation at the physical layer is mandatory. Utilizing modules with integrated power and signal isolation like the E810-TTL-CAN01 is the most cost-effective way to prevent common-mode overvoltage damage and elevate total Mean Time Between Failures (MTBF).

Frequently Asked Questions

Q1: Why does my CAN node enter the Bus-Off state when a high-power motor starts in the factory?

A: This is typically caused by a ground potential difference (GND offset) or high-voltage common-mode transient spikes induced by motor switching. When the ground potential between nodes exceeds the transceiver's common-mode input range (typically -12V to +12V), the differential comparator gets saturated, causing bit errors that rapidly increment the CAN controller's Error Passive and Tx Error Counters until it triggers a Bus-Off.

To resolve this:

• Measure the DC/AC voltage between node grounds using a multimeter; if it exceeds 0.8V, install a galvanically isolated CAN transceiver (such as the E810-TTL-CAN01 with 3000V DC isolation).

• Verify that the shielded twisted-pair cable is grounded at a single point to prevent ground loops.

Q2: How can I distinguish between a CAN termination resistor failure and a CANH/CANL physical short circuit?

A: You can diagnose this by measuring the static resistance across CANH and CANL with the power turned off:

• 60 Ohms: The physical layer termination is correct (two 120-Ohm resistors in parallel).

• 120 Ohms: One termination resistor is missing or a bus line is disconnected.

• Near 0 Ohms: CANH and CANL are shorted together or shorted internally through a damaged ESD protection diode.

• Greater than 120 Ohms or Open Loop: High impedance, indicating total termination loss or broken wiring.

Q3: Why does CAN communication work reliably at 125 kbps but suffer heavy frame losses or failure at 500 kbps or 1 Mbps?

A: High baud rates are extremely sensitive to bus capacitive load and signal reflections. At higher speeds, excessive parasitic capacitance (from long stub lines, too many nodes, or high-capacitance TVS diodes) increases the RC time constant, causing signal edge slurring (slow rise/fall times). As a result, the CAN controller reads incorrect bit values at the sampling point.

To fix this:

• Shorten stub line lengths to less than 0.3 meters for 1 Mbps networks.

• Ensure the bus uses a strict daisy-chain (line) topology rather than star or tree configurations.

• Inspect CANH/CANL waveforms on an oscilloscope to check for ringing or slow edge transitions.

Q4: Can I connect a 3.3V MCU directly to a 5V CAN transceiver like the TJA1050 or SIT1050?

A: Not directly without level shifting or using a compatible transceiver. While many 5V CAN transceivers accept 3.3V TTL levels on the TXD pin, their RXD output pin usually outputs 5V TTL logic, which can overvoltage and damage a non-5V tolerant 3.3V MCU I/O pin.

Using an integrated protocol converter module like the E810-TTL-CAN01 resolves this hardware incompatibility natively, as it supports wide-voltage I/O interfacing (3.3V/5V auto-matching) while delivering built-in power and signal isolation.