Troubleshooting a CAN (controller area network) network, whether in a vehicle, industrial system, or embedded device, requires understanding both physical and protocol-level challenges.
This Q&A collection draws from our CAN Bus Diagnostics: Practical Troubleshooting webinar. We're highlighting real-world questions from engineers and technicians, with straightforward answers to common issues in CAN 2.0 and CAN FD diagnostics.
Physical Layer: Wiring, Termination, and Signal Integrity
Q: What are the expected voltage levels on CAN-H and CAN-L?
A: For CAN-High, measure the voltage between CAN_H and ground. It should typically be between 2.5 and 3.5 volts DC.
For CAN-Low, measure between CAN_L and ground. It should range from about 1.5 to 2.5 volts DC. If these values are outside that range, check your termination and power supply.
Q: Is a DMM reliable for checking CAN voltages, or does bus load and sample rate affect readings?
A: A DMM (digital multimeter) gives an average voltage and is limited by its sample rate. On a lightly loaded bus, you’ll see voltages near 2.5V. On a busy network, you might see 2.7V on CAN_H and 2.3V on CAN_L, but you won’t catch transients or bit-level detail. An oscilloscope is better for that.
Q: How does the PCAN-MiniDiag FD measure termination resistance, and will it also measure resistance to ground?
A: The MiniDiag FD measures the termination resistance automatically as soon as it's connected to the bus by observing voltage and current behavior across the CAN lines. It does not explicitly measure resistance to ground for the purpose of detecting issues like lightning strike damage, so for that, a manual check using a multimeter or oscilloscope would still be recommended.
Q: Is there a way to test internal termination resistors that don’t show 120Ω when powered off?
A: Yes. Some devices have software-configurable termination that's only active when powered on. Check for 60Ω across the network with all devices powered, or use a tool like the PCAN-MiniDiag FD.
Q: Can grounding issues affect CAN communication?
A: Absolutely. A poor ground can cause noise, voltage shifts, or intermittent communication issues.
Q: How can I tell if my CAN node has a built-in transceiver?
A: Check the device documentation. If it connects directly to CAN_H and CAN_L, it includes a transceiver. If it only has TX and RX, you’ll need an external transceiver.
Q: How can I tell if a transceiver is damaged?
A: With power off, measure resistance between CAN_H or CAN_L and ground. If it's very low, you may have a short or damaged transceiver. If powered on and voltages are abnormal, that’s another red flag.
Q: Are stub lines or Y-cables okay to use?
A: Short stubs, generally under 30 cm, are acceptable. Y-cables can work but may introduce signal integrity issues if not designed properly.
Q: How long can a CAN bus cable be?
A: It depends on bit rate. At 1 Mbps, keep it under 40 meters. At 250 kbps, you can go up to 250 meters. For 125 kbps, up to 500 meters may work.
Bus Performance and Timing
Q: What is an acceptable level of CAN bus load?
A: A good rule of thumb is to keep the average bus load around 50%. Occasional peaks up to 70% are acceptable. However, higher sustained loads can increase the risk of latency and message collisions, especially in high-priority or time-sensitive systems.
Q: Can PCAN View show bus load?
A: Not directly. PCAN View lets you see how many messages are coming through, which can give you a rough idea of traffic. But it doesn’t calculate or display bus load percentage. For that, a tool like the PCAN-MiniDiag FD is a better choice.
Q: How can I find out if we're using CAN 2.0 or CAN FD?
A: Use a CAN diagnostic tool that can detect and display the bit rate and frame format. CAN FD frames have different structure and timing characteristics than CAN 2.0. Tools like the PCAN-MiniDiag FD or PCAN-USB Pro can help identify the protocol being used.
Q: What is a CAN sample point, and why does it matter?
A: The sample point determines when a bit is read on the bus. Timing mismatches here, especially in mixed systems, can lead to errors.
Q: Do all bit timing parameters need to match across devices?
A: No, but the bit rate and sample point should align. If these differ too much, communication issues will arise.
Q: Can using a Windows-based PC with a PCAN adapter introduce timing delays when sending messages?
A: Yes. Windows isn't real-time, so you may see small delays from the PC to the adapter. The adapter can transmit quickly (sub-millisecond), but delays depend on USB latency and bus load.
Error Detection and Acknowledgment
Q: What do “error active,” “error passive,” and “bus off” mean?
A: These reflect the health of a CAN node. “Error active” means normal operation. “Error passive” shows repeated errors but still communicates. “Bus off” means it’s stopped transmitting until reset.
Q: Why isn’t my node receiving acknowledgments (ACKs)?
A: Possible reasons include no other active nodes, incorrect bit timing, silent mode settings, or message filtering.
Q: Where can I look up how to interpret PCAN View error messages like 'stuff error' or 'bit error'?
A: The PEAK-System Forum is a good place to look. It includes user-reported error interpretations and troubleshooting tips.
Protocol-Level Behavior & Compatibility
Q: Will the CANedge1 work on a bus with 11 and 29 bit CAN IDs?
A: Yes, the CANedge1 supports both standard 11-bit and extended 29-bit CAN identifiers. It can log data from networks using either format or both simultaneously.
Q: Can I run multiple CAN protocols (like CANopen and J1939) on the same bus?
A: Yes, if they use different ID formats (11-bit vs. 29-bit) and the same bit rate. Just avoid ID collisions and filter properly.
Q: Can I mix CAN FD and classic CAN devices on the same network?
A: Only if you restrict traffic to classic CAN frames. CAN FD frames are not understood by classic CAN devices.
Q: What is the difference between ISO 14229 and J1939?
A: J1939 is an application-layer protocol for heavy-duty vehicles. ISO 14229 (UDS) is focused on diagnostics. They use different ID structures, addressing, and messaging patterns.
Tools & Diagnostic Methods
Q: What are the benefits of using a handheld diagnostic tool vs. a USB-to-CAN interface?
A: Handheld tools are fast, portable, and good for field use. USB-CAN interfaces offer deeper analysis and automation through PC-based tools.
Q: Is there a way to filter CAN messages in PCAN View?
A: PCAN View allows basic filtering by a range of CAN IDs but does not support bit-level filtering or filtering on specific data content. If you're looking for deeper filtering capabilities, such as setting up rules based on individual bits in the ID or specific data bytes, you'll need to use PCAN Explorer 6 or develop a custom application using PEAK’s APIs
Q: Does PCAN Explorer 6 allow filtering for CAN IDs by specific bits?
A: Yes, PCAN Explorer 6 supports advanced filtering. Unlike PCAN View, which filters only by a range of CAN IDs, Explorer 6 allows you to define filters based on specific bits within the CAN ID. This is especially useful for working with protocols that use structured ID formats, like J1939 or CANopen. For more information on using Explorer 6, check out our webinar: Getting Started with PCAN Explorer - Your First Step into Powerful CAN Bus Analysis
Q: Can I send CAN messages programmatically in response to specific received data?
A: Yes. PEAK offers a C/C++ API that allows you to write your own software to send and receive CAN messages through PCAN-USB interfaces. With this, you can configure your application to monitor for certain conditions or IDs and automatically respond with custom messages. It's useful for simulation, testing, or creating interactive diagnostic tools.
Q: What device would be best for remotely accessing CAN data on a customer's machine?
A: Use a CAN gateway device like the PCAN-Gateway FD or something with built-in networking (e.g., Ethernet, cellular). These allow remote diagnostics, logging, or real-time monitoring depending on setup.
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