In the world of industrial automation, whenever data exchange between multiple devices is required, RS485 is almost always the go-to communication standard. You'll often see a hard rule in manuals or wiring diagrams for field equipment: a 120-ohm termination resistor must be connected at both ends of the bus. Many engineer friends ask: Why 120 ohms? What happens if I leave it out? And sometimes, for short connections, it seems to work fine without it—what's up with that? Today, let’s set aside the complex communication protocols and look at this issue from the fundamental principles of electronics.
Signal Reflection: The Invisible Killer of Data Transmission
Waves and Reflections in Circuits
To understand termination resistors, you first have to think of the wires as a "transmission line." When a signal (voltage change) travels along this line, if it encounters an impedance mismatch, the signal will "reflect" just like ocean waves hitting a seawall. In high-speed or long-distance communication, if the signal reaches the end and isn't absorbed, it bounces back and overlaps with subsequent signals. On an oscilloscope, this overlap manifests as severe distortion of the signal waveform, causing the receiver to misinterpret "0"s and "1"s. This is why, in long-distance communication, failing to use a resistor leads to intermittent connection issues.
Why Exactly 120 Ohms?
This value wasn't picked out of thin air; it’s determined by the "characteristic impedance" of the twisted-pair cable. The standard shielded twisted-pair cables used in industrial settings typically have a physical characteristic impedance of roughly 100 to 120 ohms. The role of the termination resistor is to achieve "impedance matching," ensuring the receiver’s impedance matches the cable’s characteristic impedance. This allows the incoming electromagnetic energy to be fully absorbed, effectively eliminating reflections.
Is 120 Ohms an Ironclad Rule, or Can You Be Flexible?
Cabling Determines Impedance
In 2026's industrial landscape, we deal with a huge variety of cable specifications. If the cable you are using has a characteristic impedance other than 120 ohms, adding a 120-ohm resistor will actually cause an impedance mismatch. Therefore, while 120 ohms is the industry standard, if your wiring system is specialized, it is highly recommended to check the "characteristic impedance" parameters provided by the cable manufacturer and choose a matching resistor value for the best results.
When Can You Skip It?
Many engineers find that in a lab setting, communication still works even without resistors. This is usually because the transmission distance is very short (e.g., less than 10 meters) and the baud rate is low. In these cases, the energy of the reflected signal attenuates faster than the transmission speed, so its impact on waveform distortion is limited. However, in a factory environment, surrounded by heavy EMI sources like VFDs and high-power motors, noise gets superimposed onto your signal. Without a termination resistor to "stabilize" the signal edges, your bit error rate will skyrocket as equipment starts running.
Understanding Stable Communication Links from the Ground Up
In summary, the RS485 termination resistor is there to eliminate reflection effects within the transmission medium. Don't just view RS485 as simple digital communication; if you treat it as a physical transmission system, you must respect the fundamental circuit principle of "impedance matching." While many modern industrial devices have designed these resistors to be toggled via jumpers or DIP switches for convenience, understanding the principle behind them will allow you to get to the root of those pesky, sporadic communication failures.
During actual troubleshooting, if you encounter unstable communication, besides checking the ground and shielding, using an oscilloscope to observe whether there is abnormal ringing on the signal edges is often the best way to verify if your termination resistors are doing their job. Keeping your circuit clean and logically consistent is the key to ensuring the long-term stable operation of your automation system.
