Why Every CAN Bus Needs a 120 Ohm Resistor

You need a 120 ohm resistor at both ends of your controller area network. These resistors keep your can bus systems running smoothly by matching the cable’s impedance and stopping unwanted echoes during signal transmission. ISO 11898-2, the main standard for high-speed can, requires exactly two 120 ohm resistors to support reliable data transmission. Without them, signals can reflect and cause errors. You use these resistors to make sure your can network stays stable and accurate.

ISO 11898-2 states: Only two 120 ohm resistors should be present at the ends of the bus, matching the cable’s impedance.

Key Takeaways

• Use two 120 ohm resistors, one at each physical end of the CAN bus, to match the cable’s impedance and prevent signal reflections.
• Proper termination with these resistors keeps CAN bus signals clean, reduces errors, and ensures reliable data transmission.
• Check the resistance between CAN_H and CAN_L with a multimeter; a reading near 60 ohms confirms correct termination.
• Missing or incorrect resistors cause communication errors, data loss, and unstable network operation.
• Regularly inspect wiring and resistor placement to maintain strong signal integrity and avoid costly CAN bus failures.

120 Ohm Resistor Role

Impedance Matching

When you set up a controller area network, you need to think about how signals travel through the wires. Signals on the CAN bus act like waves moving along a twisted-pair cable. This cable has a special property called characteristic impedance, which is usually about 120 ohms. If you leave the cable ends open, the signal hits the end and bounces back. These reflections can mess up your data.

You use a 120 ohm resistor at each end of the bus to match the cable’s impedance. This makes the cable look endless to the signal, so it does not reflect. The value of 120 ohms is not random. Engineers measure the cable’s impedance by sending a square wave through it and adjusting a resistor until the signal looks clean on an oscilloscope. When you see no ringing or spikes, you know you have the right value. ISO 11898-2, the main CAN standard, says you must use two 120 ohm resistors for proper termination.

Tip: If you measure between CAN_H and CAN_L with a multimeter, you should see about 60 ohms. This means you have two 120 ohm resistors in parallel, which is correct for CAN bus systems.

• Standard CAN bus cables use twisted pair wires with a characteristic impedance close to 120 ohms.
• You place termination resistors at each end to match this impedance and keep signals clean.
• Proper termination helps the bus return quickly to the recessive state and reduces distortion.
• Engineers use a square wave generator and an adjustable resistor to find the best impedance match.

Signal Reflection Prevention

Signal reflections can cause big problems in CAN communication. If you do not use the right termination, signals bounce back from the cable ends. These echoes can distort your messages and lead to errors. Laboratory tests show that using two 120 ohm resistors at the ends of the bus stops these reflections. You get clean waveforms without glitches or spikes.

When you add the correct termination resistors, you dampen reflections and improve signal integrity. If you skip this step or use the wrong value, you might see distorted signals, data loss, or even system failures. CAN bus faults from reflections can cause your vehicle or device to stop working or show warning lights.

Note: Complex network shapes, like star or ring topologies, create more reflection points. Always use a simple linear bus with proper termination to avoid communication errors.

• Signal reflections happen when there is an impedance mismatch, such as missing or extra resistors.
• Two 120 ohm resistors in parallel give about 60 ohms total resistance, which suppresses reflections.
• Oscilloscope tests confirm that proper termination keeps signals stable and prevents glitches.
• If you see the wrong resistance between CAN_H and CAN_L, check your termination resistors right away.

Communication Reliability

Signal Integrity

You want your controller area network to deliver clear and accurate data. Signal integrity is the key to making this happen. When you use 120 ohm resistors at both ends of your CAN bus, you help keep the signals clean and sharp. These resistors match the impedance of the twisted pair cable, which stops signals from bouncing back and causing trouble.

If you skip proper termination, you risk signal reflections. These reflections create noise, voltage spikes, and random glitches on the CAN_H and CAN_L lines. You might see your devices go offline or act strangely. Even a small mistake in termination can lead to distorted signals and poor signal integrity. At higher speeds, the risk grows because fast signals are more sensitive to reflections.

Tip: Always check for about 60 ohms between CAN_H and CAN_L when the system is off. This quick test helps you confirm that both 120 ohm resistors are in place and your signal integrity is protected.

A well-terminated CAN bus absorbs extra energy and keeps the voltage levels steady. This means your network can handle more devices and longer cables without losing signal integrity. You get fewer glitches, less noise, and a much stronger foundation for reliable communication.

Error Reduction

Proper termination does more than just keep signals clean. It also cuts down on communication errors. When you use two 120 ohm resistors, you prevent energy from bouncing back and forth on the bus. This stops ringing and distortion, which can cause bits to flip or messages to get lost.

Here’s what happens when you get termination right:

• You see fewer error frames and less random noise.
• Your CAN nodes stay online and respond quickly.
• The system can run at higher speeds without trouble.
• Communication errors drop, even as you add more devices.

If you miss a resistor or use the wrong value, you open the door to problems. Unterminated branches act like antennas, picking up noise and sending it back into the network. This leads to more errors and unreliable data. Even if your CAN bus seems to work at low speeds, missing termination is risky and not good engineering practice.

Note: Measuring resistance with a multimeter gives you a quick check, but for the best view of signal integrity and error rates, use an oscilloscope or CAN analyzer.

By focusing on proper termination, you make sure your CAN system delivers reliable communication with strong signal integrity and minimal communication errors.

Missing or Incorrect Termination

Common Issues

When you set up a CAN bus, missing or incorrect termination resistors can cause many problems. You might notice your network works for a short time, but then it starts to fail. Signals bounce back and create noise, which leads to can communication issues. Devices may suddenly stop talking to each other, or you might see warning lights and error codes on your dashboard.

Here are some common symptoms you might see:

• Communication errors and random network failures
• Sudden loss of data or devices going offline
• Error frames and bus-off conditions that stop data flow
• Noise and distorted signals on the CAN lines
• Voltage readings stuck at 2.50 VDC, showing devices are powered but not sending data
• Resistance between CAN High and CAN Low that is not close to 60 ohms

You should always check the resistance between CAN_H and CAN_L with a multimeter when the system is off. If you do not see about 60 ohms, your termination resistors may be missing or installed incorrectly. Incorrect wiring, such as swapping CAN-High and CAN-Low, can also destroy communication.

Data Transmission Risks

Improper termination puts your data transmission at risk. When you do not use the correct resistors at both ends of the bus, signal reflections distort the CAN waveforms. This distortion leads to more error frames and makes your network unreliable. You may see intermittent data loss or even complete failure between nodes.

Some risks you face with poor termination include:

• Increased error frames that force devices to resend messages
• Voltage level changes on CAN High and CAN Low, which weaken signal integrity
• Data corruption and loss of important messages
• Reduced reliability in automotive and industrial systems

Diagnostic tools like CAN bus analyzers and oscilloscopes help you spot these problems. They show you when waveforms look wrong or when error frames increase. Always place two 120-ohm termination resistors at the physical ends of your CAN bus to protect your data transmission and keep your network stable.

120 Ohm Resistor Placement

End-of-Bus Installation

You need to place a 120 ohm resistor at each end of your CAN bus. These resistors, called terminal resistors, connect directly between the CAN_H and CAN_L lines. This setup matches the cable’s impedance and stops signal reflections. When you install the 120 ohm resistor at both ends, you help your network send data smoothly and avoid echoes that can cause errors.

To install the resistors, find the two physical ends of your CAN bus wiring. Place one 120 ohm resistor at each end, connecting it between the CAN_H and CAN_L wires. This placement is not just a suggestion. It is a requirement for proper can termination. If you use connectors, you can crimp the resistor legs inside a 2-pin connector shell and secure them to prevent movement. This keeps your installation strong and reliable.

Tip: Always double-check that you have only two 120 ohm resistors, one at each end. Extra resistors or missing ones can cause problems with your CAN bus.

Resistance Measurement

After you install the resistors, you should check your work. You can use a multimeter to measure the resistance between CAN_H and CAN_L. First, turn off all devices on the bus. Then, disconnect a connector and place the multimeter leads on the CAN_H and CAN_L pins. A correct reading should show about 60 ohms. This value comes from the two 120 ohm resistors in parallel.

If you see a value much higher or lower than 60 ohms, you may have a missing resistor or extra resistors somewhere on the bus. Sometimes, you might see a value like 43 ohms. This means you have more than two resistors, which can hurt your network’s performance. Adjust the termination until you get close to 60 ohms. This simple test helps you confirm proper termination and keeps your CAN bus running smoothly.

• Steps to verify correct termination:
  • Disconnect the CAN bus connector from the device.
  • Set your multimeter to measure resistance.
  • Place the leads on CAN_H and CAN_L.
  • Look for a reading near 60 ohms.
  • If the value is off, check your resistor placement.

Note: Using an oscilloscope can also help you spot signal reflections if you want to check your network’s health.

Troubleshooting Tips

Diagnosing Termination Problems

You can solve many CAN bus issues by checking the termination first. Start with a physical inspection. Look for damaged cables, loose connectors, or missing resistors. Tug gently on each wire to see if any connections feel weak. Sometimes, a simple tug-test reveals a bad crimp or a loose wire that causes trouble.

Use a multimeter to measure the resistance between CAN_H and CAN_L when the system is off. You should see about 60 ohms. If you get a much higher or lower value, check for missing or extra resistors. Oscilloscopes help you see the actual signal waveforms. Distorted or noisy signals often point to termination problems or interference. CAN bus analyzers and diagnostic software can also help. These tools show error frames and help you spot faulty nodes or wiring mistakes.

Tip: Isolate devices by connecting one at a time. This method helps you find which device or section of wiring causes the problem.

You should also check the wiring colors. Make sure green connects to green and yellow to yellow. Flick or shake the wiring gently to see if you can reproduce any intermittent faults. Always confirm that battery voltage is strong and breakers are in place.

Quick Fixes

You can fix many termination problems quickly with a few simple steps:

1. Place a 120-ohm resistor at each end of the CAN bus, connecting CAN_H and CAN_L.
2. Measure the resistance between CAN_H and CAN_L. Look for a value close to 60 ohms.
3. Inspect all wiring for solid connections. Use proper soldering or secure splices. Avoid simple twists or wire nuts.
4. Check for stripped wires or damaged insulation. These can cause interference and signal loss.
5. Test voltage, ground, and resistance levels to match manufacturer guidelines.
6. Shield or reroute cables if you notice electromagnetic interference.
7. Use terminated or nonterminated cables as needed for your setup.

Regular checks and careful installation keep your CAN bus reliable. Quick action when you spot a problem helps prevent bigger issues later.

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You ensure reliable performance in your CAN bus system by installing two 120 ohm resistors at the physical ends of the network. These resistors match the cable’s impedance, absorb noise, and prevent signal reflections. If you skip or misplace them, you risk communication errors and unstable operation. Always check for proper termination before troubleshooting. Following best practices keeps your data transmission strong and your network dependable, even in tough environments.

FAQ

What happens if you use more than two 120 ohm resistors on a CAN bus?

If you add extra resistors, you lower the total resistance. This can cause weak signals and more errors. Always use only two resistors—one at each end.

Can you use a different resistor value instead of 120 ohms?

You should not use a different value. The 120 ohm resistor matches the cable’s impedance. Using another value can cause reflections and data errors.

How do you know if your CAN bus has proper termination?

You can check with a multimeter. Turn off the system. Measure between CAN_H and CAN_L. You should see about 60 ohms. This means you have two 120 ohm resistors in place.

Where should you place the 120 ohm resistors?

• Place one resistor at each physical end of the CAN bus.
• Connect each resistor directly between CAN_H and CAN_L.
• Do not put resistors in the middle or at devices not at the ends.