In the world of embedded systems and vehicle networking, the Controller Area Network (CAN) protocol remains one of the most reliable and robust communication methods for distributed devices. Designed originally by Bosch in the 1980s, CAN is widely used in automotive, industrial automation, medical equipment, and marine systems. One crucial, often overlooked component of a properly functioning CAN Bus is the termination resistor. Despite their small size and simple function, these resistors are indispensable for ensuring reliable communication across the network.
What is a Termination Resistor?
In essence, a termination resistor is a resistive load placed at each end of a CAN Bus to match the impedance of the cable. The standard value for this resistor is 120 ohms, and it serves two primary purposes:
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Prevent signal reflections
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Stabilize the network’s voltage levels
Signal reflections occur when a transmitted signal reaches the end of the cable and is reflected back into the line. These reflections can interfere with incoming signals, causing data corruption or communication failure. Termination resistors absorb this energy, minimizing these disruptive reflections.
The Role of Termination Resistors in CAN Bus Topology
A typical CAN Bus consists of two wires: CAN High (CAN_H) and CAN Low (CAN_L). The network operates in a differential mode, where the logical state is determined by the voltage difference between the two lines.
To maintain signal integrity, each end of the main CAN Bus line must be terminated with a 120-ohm resistor connected between CAN_H and CAN_L. This creates an overall line impedance of 60 ohms, which matches the impedance of the twisted pair cable typically used in CAN networks.
Without proper termination:
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Electrical signals can reflect back along the line
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Bus voltage levels can become unstable
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Communication errors, such as bit stuffing and CRC failures, can occur
Common Applications of Termination Resistors
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Automotive Systems
In vehicles, ECUs (Electronic Control Units) are distributed across a CAN network. Termination resistors at the ends of the vehicle’s backbone bus ensure accurate data transmission between modules such as the engine control unit, ABS, and infotainment systems. -
Industrial Automation
In PLC-controlled systems and factory automation, CANopen networks require terminations at the master and furthest slave to maintain robustness in electrically noisy environments. -
Marine Electronics
The NMEA 2000 protocol, based on CAN, uses termination resistors at both ends of its backbone to guarantee reliable sensor and navigation data communication in marine environments. -
Medical Devices
Medical instruments like infusion pumps or diagnostic systems that communicate over CAN require proper termination to avoid failure in life-critical environments.
Practical Guidelines for Termination
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Use only two termination resistors per network. More than two can reduce signal levels; fewer than two leads to signal reflections.
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Locate them at the two physical ends of the CAN Bus line — not at intermediate devices.
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Value tolerance matters. Stick to 120-ohm resistors with 1% tolerance to ensure impedance matching.
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Don’t rely on automatic or software-based termination. Physical hardware resistors are always required in traditional CAN networks.
Consequences of Improper Termination
Failure to apply termination resistors properly can lead to:
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Intermittent data errors
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Total communication failure
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Increased electromagnetic interference (EMI)
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Node lockups or resets
During troubleshooting, improper termination is often the root cause of erratic behavior in a CAN network. Engineers should verify both the presence and the position of resistors before delving deeper into complex diagnostics.
Real-World Example: Diagnosing a Faulty CAN Bus
Consider an automotive technician troubleshooting a modern vehicle with intermittent communication faults between the engine control unit and the anti-lock braking system. Standard diagnostic tools show lost messages and bus errors. After ruling out software issues and connector problems, the technician inspects the physical layer and finds a missing termination resistor at the end of the bus. Installing a proper 120-ohm resistor resolves the issue immediately. This example highlights how even a minor oversight in termination can bring down an entire system.
Electrical Reflection: A Closer Look
To truly appreciate why termination resistors are needed, it’s important to understand electrical reflections in transmission lines. When a voltage signal encounters a change in impedance—such as at the end of an unterminated cable—it reflects back toward the source. These reflections can cause destructive interference, producing voltage spikes, oscillations, or false logic levels.
By terminating the line with a resistor equal to the characteristic impedance of the cable (typically 120 ohms), energy from the signal is absorbed rather than reflected. This keeps the signal clean, sharp, and unambiguous, ensuring the integrity of every CAN frame.
Termination vs. Biasing
While termination resistors are often discussed in CAN design, biasing resistors are another concept sometimes confused with termination. Biasing ensures the bus defaults to a known idle state when no devices are transmitting. Though not part of the standard CAN specification, some implementations—especially low-speed fault-tolerant CAN—incorporate biasing to prevent floating lines.
It’s important to distinguish these roles: termination controls reflections; biasing manages voltage levels in idle conditions.
Common Pitfalls in Implementation
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Terminating at the wrong location: A common mistake is placing termination at the first and last devices logically, rather than at the physical ends of the bus.
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Using incorrect resistor values: Swapping in 100-ohm or 150-ohm resistors “just to try it out” can lead to inconsistent results.
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Daisy-chaining terminations: Some engineers mistakenly connect multiple devices with internal termination resistors enabled. This results in excessive load, which can drag down the voltage differential, preventing correct data decoding.
Harsh Environment Considerations
In aerospace and military systems where temperature, vibration, and EMI are extreme, CAN Bus networks still thrive thanks to proper electrical design. Termination resistors in such systems may be ruggedized or sealed to ensure long-term reliability. In these mission-critical applications, even the placement of termination resistors is validated and tested as part of compliance procedures.
Termination in Modular and Configurable Systems
In systems where the CAN topology is modular—such as industrial controllers or development kits—termination must be flexible. Many devices include jumper-selectable or software-configurable termination, allowing adaptation without manual rewiring. However, flexibility comes with risk: if both ends are left unterminated, the system may become unstable without any obvious error messages.
To ensure reliability:
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Clearly label termination controls
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Document default settings
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Verify resistor placement during system commissioning
Preparing for High-Speed Protocols
As CAN evolves into CAN FD and CAN XL, the principles of termination grow even more critical. These faster versions introduce higher data rates, requiring:
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Tighter layout control
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Lower stub lengths
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Stronger signal integrity analysis
Tools like SPICE and IBIS simulation models can help engineers design termination schemes suitable for these advanced protocols.
Teaching Termination: A Hands-On Lesson
In educational labs, termination resistors provide an excellent way to teach the physical realities of network design. A simple setup with and without termination can visually demonstrate:
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Clear square waveforms (with proper termination)
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Ringing and signal degradation (without termination)
These experiments help students appreciate that network reliability isn’t just software-driven—it’s rooted in solid electrical design.
Conclusion
Though small and inexpensive, termination resistors are the unsung heroes of a stable and reliable CAN Bus. They play a fundamental role in ensuring that data travels from one node to another without distortion or error. Whether you’re designing a vehicle network, setting up industrial automation, or building a custom embedded system, always respect the physics of signal transmission — and don’t forget your termination.
References (External Links)
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Bosch CAN Specification 2.0
https://www.bosch-semiconductors.com/media/ip_modules/pdf_2/can/can2spec.pdf -
NXP CAN Termination Guidelines
https://www.nxp.com/docs/en/application-note/AN1798.pdf -
Texas Instruments: Understanding and Configuring Termination Networks for the CAN Bus
https://www.ti.com/lit/an/slla270/slla270.pdf -
Microchip: Introduction to the Controller Area Network (CAN)
https://www.microchip.com/en-us/application-notes/an713
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