120 Ohm Resistor: Color Code, CAN Bus & RS-485 Termination Guide

The 120 ohm resistor might seem like just another value in your parts bin, but it serves a very specific and critical purpose that sets it apart from its neighbors. If you’ve worked with CAN bus networks in automotive or industrial applications, or set up RS-485 communication lines, you’ve encountered this resistor in its starring role: line termination.

I’ve debugged countless communication issues over the years where the root cause turned out to be missing or incorrect termination. Understanding when and how to use a 120 ohm resistor for termination can save you hours of troubleshooting frustration.

What is a 120 Ohm Resistor?

A 120 ohm resistor provides exactly 120 ohms of electrical resistance. While it works perfectly fine for general circuit applications like any other resistor value, its claim to fame lies in matching the characteristic impedance of standard twisted-pair communication cables.

Most twisted-pair cables used for differential signaling protocols like CAN bus and RS-485 have a characteristic impedance close to 120 ohms. This isn’t coincidence; it’s physics. When you place a 120 ohm resistor at the end of such a transmission line, it absorbs signal energy instead of reflecting it back, ensuring clean data transmission.

Key Specifications of 120 Ohm Resistors

Parameter	Typical Value	Notes
Resistance	120 Ω	E24 standard value
Common Tolerances	±5%, ±1%	Gold or Brown tolerance band
Power Ratings	1/8W, 1/4W, 1/2W	1/4W most common for termination
Temperature Coefficient	50-100 ppm/K	Metal film preferred for stability
Package Types	Through-hole, SMD	Both widely available

120 Ohm Resistor Color Code

Identifying a 120 ohm resistor by its color bands is straightforward. The brown-red-brown pattern creates a distinctive look that’s easy to recognize once you know what to look for.

4-Band 120 Ohm Resistor Color Code

The standard 4-band configuration:

Band Position	Color	Value	Meaning
1st Band	Brown	1	First significant digit
2nd Band	Red	2	Second significant digit
3rd Band	Brown	×10	Multiplier
4th Band	Gold	±5%	Tolerance

Reading it: Brown-Red-Brown-Gold gives you 12 × 10 = 120Ω with ±5% tolerance.

That gold tolerance band means your actual resistance falls between 114Ω and 126Ω. For termination applications, this tolerance is perfectly acceptable.

5-Band 120 Ohm Resistor Color Code

For precision applications requiring tighter tolerance:

Band Position	Color	Value	Meaning
1st Band	Brown	1	First significant digit
2nd Band	Red	2	Second significant digit
3rd Band	Black	0	Third significant digit
4th Band	Black	×1	Multiplier
5th Band	Brown	±1%	Tolerance

The 5-band code Brown-Red-Black-Black-Brown represents 120 × 1 = 120Ω with ±1% tolerance.

6-Band 120 Ohm Resistor Color Code

When temperature stability matters for precision termination:

Band Position	Color	Value	Meaning
1st Band	Brown	1	First significant digit
2nd Band	Red	2	Second significant digit
3rd Band	Black	0	Third significant digit
4th Band	Black	×1	Multiplier
5th Band	Brown	±1%	Tolerance
6th Band	Brown	100 ppm/K	Temperature coefficient

Resistor Color Code Reference Chart

Color	Digit	Multiplier	Tolerance
Black	0	×1	–
Brown	1	×10	±1%
Red	2	×100	±2%
Orange	3	×1,000	–
Yellow	4	×10,000	–
Green	5	×100,000	±0.5%
Blue	6	×1,000,000	±0.25%
Violet	7	–	±0.1%
Gray	8	–	±0.05%
White	9	–	–
Gold	–	×0.1	±5%
Silver	–	×0.01	±10%

SMD 120 Ohm Resistor Markings

Surface mount 120 ohm resistors use numerical codes:

3-Digit SMD Code

A 120 ohm resistor in 3-digit format is marked 121.

Breaking it down:

• First two digits (12) = significant figures
• Third digit (1) = number of zeros to add = one zero

Result: 12 + one zero = 120Ω

4-Digit SMD Code

Precision SMD resistors use 1200 for 120 ohms:

• First three digits (120) = significant figures
• Fourth digit (0) = multiplier = ×1

Result: 120 × 1 = 120Ω

EIA-96 Code

For 1% tolerance SMDs, the EIA-96 system marks 120Ω as 09A:

• 09 = code for 121 (closest E96 value)
• A = multiplier of ×1

SMD Marking System	Code for 120Ω	Typical Tolerance
3-Digit	121	±5%
4-Digit	1200	±1%
EIA-96	09A	±1%

CAN Bus Termination with 120 Ohm Resistors

This is where the 120 ohm resistor truly earns its keep. Controller Area Network (CAN) is a robust communication protocol used extensively in automotive systems, industrial automation, and robotics.

Why CAN Bus Needs Termination

CAN uses differential signaling over twisted-pair cables. When high-frequency signals reach the end of an unterminated cable, they reflect back like waves bouncing off a wall. These reflections interfere with incoming data, causing communication errors, corrupted messages, and system instability.

The 120 ohm resistor placed at each end of the CAN bus absorbs this signal energy, preventing reflections and ensuring clean waveforms.

Proper CAN Bus Termination Setup

Component	Requirement	Notes
Termination resistors	2 × 120Ω	One at each end of the bus
Placement	Between CAN_H and CAN_L	At physical endpoints only
Resulting impedance	60Ω total	Two 120Ω in parallel
Cable type	Twisted pair	Characteristic impedance ~120Ω

How to Verify CAN Bus Termination

Here’s a diagnostic technique I use regularly:

1. Power down all CAN nodes
2. Set your multimeter to resistance mode
3. Measure between CAN_H and CAN_L pins
4. Check the reading:

Measured Resistance	Interpretation
~60Ω	Correct termination (2 × 120Ω in parallel)
~120Ω	Only one termination resistor present
Very high (MΩ)	No termination resistors
<60Ω	Too many termination resistors

If you measure 120Ω, you need to add another 120 ohm resistor at the opposite end of the bus. If you measure significantly less than 60Ω, some nodes may have internal termination enabled that needs to be disabled.

CAN Bus Termination in Practice

Many modern CAN devices include built-in termination that can be enabled via DIP switches or software configuration. Before adding external 120 ohm resistors, check your device documentation. Common devices with selectable termination include motor controllers, ECUs, and CAN interfaces like those from Kvaser, PEAK, and Vector.

Scenario	Action
Two end nodes with built-in termination	Enable termination on both
End nodes without built-in termination	Add external 120Ω resistors
Node in middle of bus	Never enable termination
Short test setup (<1m cable)	One termination often sufficient

RS-485 Termination with 120 Ohm Resistors

RS-485 is another differential signaling protocol that benefits from proper termination using 120 ohm resistors. It’s widely used in industrial control systems, building automation, and long-distance serial communication.

Why RS-485 Requires Termination

Like CAN, RS-485 transmits data over twisted-pair cables with a characteristic impedance around 120 ohms. Without termination, signal reflections cause:

• Data corruption and communication errors
• Increased bit error rates
• Unstable system behavior
• False triggering during idle states

The 120 ohm resistor placed at each end of the RS-485 bus matches the cable impedance and absorbs signal energy.

RS-485 Termination Configurations

Configuration	Termination Location	Resistor Count
Half-duplex (2-wire)	Both ends of A/B pair	2 × 120Ω
Full-duplex (4-wire)	Receiver side of master and furthest slave	2 × 120Ω per direction
Point-to-point	Far end from transmitter	1 × 120Ω minimum

When Termination May Be Optional

For short cable runs at low data rates, you might get away without termination. However, I always recommend installing 120 ohm resistors as a best practice:

Condition	Termination Recommendation
Cable < 10m, baud < 9600	Optional but recommended
Cable > 10m or baud > 9600	Required
Cable > 100m	Absolutely required
Noisy industrial environment	Always required

Bias Resistors vs. Termination Resistors

Don’t confuse termination with biasing. RS-485 networks often need both:

• Termination resistors (120Ω): Prevent signal reflections at cable ends
• Bias resistors (560Ω-1kΩ): Hold the bus at a known state when idle

Some integrated RS-485 transceivers include failsafe biasing, reducing the need for external bias resistors. Termination, however, remains a physical cable property requirement.

Common Termination Problems and Solutions

Problem: Intermittent Communication Errors

Symptoms: Random CRC errors, dropped messages, devices going offline intermittently

Likely cause: Missing or incorrect termination

Solution: Verify 60Ω across differential pair with bus powered down. Add or adjust 120 ohm resistors as needed.

Problem: Communication Works at Low Speed But Fails at High Speed

Symptoms: System works at 125 kbps but fails at 500 kbps or 1 Mbps

Likely cause: Signal reflections becoming significant at higher frequencies

Solution: Ensure proper 120 ohm termination at both ends. Check for stub connections or improper star topology.

Problem: Only One Direction of Communication Works

Symptoms: Can transmit but not receive, or vice versa

Likely cause: Termination resistor shorting one of the differential lines

Solution: Verify resistor is connected between CAN_H and CAN_L (or A and B), not to ground.

Selecting the Right 120 Ohm Resistor for Termination

Power Rating

For most termination applications, 1/4W resistors are sufficient. The power dissipation depends on the differential voltage:

Protocol	Typical Differential Voltage	Power in 120Ω
CAN bus	2-3V	~50mW max
RS-485	2-5V	~200mW max

A 1/4W (250mW) 120 ohm resistor handles these levels comfortably.

Resistor Type

Type	Tolerance	Best For
Carbon film	±5%	General termination
Metal film	±1%	Precision applications
Wirewound	±5%	High-power termination

Metal film resistors offer better temperature stability, which can matter in automotive or industrial environments with wide temperature swings.

Package Considerations

Application	Recommended Package
Prototyping	Through-hole axial
Production PCB	SMD 0603 or 0805
External terminator adapter	D-sub with built-in resistor

Useful Resources and Tools

Online Calculators

• DigiKey Resistor Color Code Calculator – Decode any color band pattern
• DigiKey SMD Resistor Code Calculator – Convert SMD markings

Technical Documentation

• CSS Electronics CAN Bus Termination Guide – Practical termination advice
• Kvaser CAN Termination Whitepaper – Deep dive into CAN signal integrity
• Texas Instruments RS-485 App Note – Comprehensive RS-485 termination guidance

Component Suppliers

• DigiKey – Wide selection of precision resistors
• Mouser – Comprehensive parametric search
• Phoenix Contact – Industrial termination solutions

Frequently Asked Questions

What color code is a 120 ohm resistor?

A 4-band 120 ohm resistor has the color code Brown-Red-Brown-Gold. Brown represents 1, Red represents 2, the second Brown is a ×10 multiplier, and Gold indicates ±5% tolerance. Multiply 12 by 10 to get 120 ohms. For 5-band precision resistors, the code is Brown-Red-Black-Black-Brown with ±1% tolerance.

Why is 120 ohms used for CAN bus termination?

The 120 ohm value matches the characteristic impedance of standard twisted-pair cables used in CAN networks as defined by ISO 11898-2. When a resistor matches the cable impedance, it absorbs signal energy at the line end instead of reflecting it. Two 120 ohm resistors, one at each end of the bus, create a 60 ohm parallel load that ensures proper signal levels and prevents data-corrupting reflections.

How do I test if my CAN bus is properly terminated?

Power down all CAN nodes, then measure resistance between CAN_H and CAN_L with a multimeter. Proper termination shows approximately 60 ohms (two 120 ohm resistors in parallel). A reading of 120 ohms indicates only one termination resistor is present. Very high resistance means no termination exists. Values significantly below 60 ohms suggest too many termination resistors are active.

Can I use a different resistor value instead of 120 ohms for termination?

While 120 ohms is the standard, small variations (110-130 ohms) typically work acceptably. The value should match your cable’s characteristic impedance. Using significantly different values causes impedance mismatch, leading to partial reflections. For reliable communication, especially at high speeds or long distances, stick with 120 ohm resistors as specified by CAN and RS-485 standards.

What is the SMD code for a 120 ohm resistor?

For standard tolerance SMD resistors, a 120 ohm value is marked 121 using the 3-digit code (12 with one zero added). For precision 1% resistors using the 4-digit code, look for 1200. The EIA-96 code for approximately 120Ω is 09A. Always verify with a multimeter if you’re uncertain, as proper termination resistance is critical for communication reliability.

Summary

The 120 ohm resistor serves a specialized but critical role in differential communication systems. While you can certainly use it for general circuit applications, its true value lies in terminating CAN bus and RS-485 networks where proper impedance matching prevents signal reflections and ensures reliable data transmission.

Remember the key diagnostic: measure 60 ohms across your differential pair when the system is powered down. If you don’t see that value, you likely have a termination problem that a pair of 120 ohm resistors can solve.

Keep a supply of these resistors in your toolkit, both through-hole for prototyping and SMD for production boards. When communication problems arise, proper termination should be one of your first checks.