Introduction: When RS485 “Works” — Until It Suddenly Doesn’t
In many industrial projects, RS485 communication seems to work fine at the beginning.
But after some time, problems start appearing:
- Random data errors
- Intermittent disconnections
- Unstable Modbus communication
- Devices going offline without clear reason
In most cases, the root cause is surprisingly simple:
👉 Missing or incorrect 120Ω termination resistor
What Does the 120Ω Resistor Actually Do?
RS485 is a high-speed differential communication system.
The signal travels along twisted-pair cables at very high speed.
When the signal reaches the end of the cable and no termination resistor is present, it reflects back — similar to an echo.
This creates:
- Signal distortion
- Overlapping waveforms
- Data misinterpretation (0 becomes 1, or vice versa)
✔ Solution: Impedance Matching
A 120Ω resistor is added at both ends of the RS485 bus to match the cable impedance.
👉 This ensures:
- No signal reflection
- Clean waveform
- Stable communication
“It Works Without It” — But That’s Risky
Many engineers say:
“My RS485 works fine without termination.”
Yes — but only under certain conditions:
- Very short cable (e.g., 2–5 meters)
- Low baud rate (e.g., 9600)
- Minimal electrical interference
⚠️ The Problem in Real Industrial Environments
In actual projects:
- Cable length increases (5m → 50m or more)
- Baud rate increases (9600 → 115200)
- Electrical noise from VFDs, motors, and pumps
- More devices added to the bus
👉 Without termination:
- Error rate increases dramatically
- Communication becomes unreliable
- Troubleshooting becomes difficult
How to Properly Install the 120Ω Termination Resistor
1️⃣ Install Only at Both Physical Ends
For a standard RS485 daisy-chain topology:
Device A —— Device B —— Device C —— Device D
↑ ↑
120Ω 120Ω✔ Only the first and last device should have termination resistors
❌ Do NOT add resistors at every device
2️⃣ Check Built-in Termination
Many industrial devices (flowmeters, PLCs, transmitters) already include internal termination resistors.
- Often controlled by DIP switch or jumper
- Enable ONLY at the two ends
3️⃣ Use Proper Power Rating
Use at least:
👉 120Ω / 0.25W resistor
For long-distance or high-load systems, consider higher rating.
Common Mistakes That Cause RS485 Failure
❌ No termination resistor
❌ Too many termination resistors
❌ Star topology instead of daisy chain
❌ Poor grounding or shielding
❌ Long stub lines
👉 These issues often appear as “mysterious communication problems”
Practical Tip from Field Experience
In many real projects we’ve supported:
👉 More than 60% of RS485 communication issues are related to wiring and termination — not the device itself.
Before replacing equipment, always check:
✔ Termination
✔ Wiring structure
✔ Grounding
Need Help With RS485 or Modbus Communication?
If you are facing:
- Unstable RS485 communication
- Modbus data errors
- Flowmeter / transmitter not responding
- Difficult field troubleshooting
👉 We can support you with:
✔ RS485 wiring design
✔ Communication troubleshooting
✔ Compatible industrial instruments (flowmeters, transmitters with RS485 / Modbus)
✔ Pre-configured solutions for stable communication