Common “Difficulties” in the Field
One of the most troublesome issues during device debugging: RS-485 communication is unstable, with intermittent data transmission.
The program logic is fine, the cable connections are correct, and there are no address conflicts, yet communication timeouts, data corruption, and sporadic failures still occur. Changing cables, adjusting baud rates, and checking shielding have all been tried, but the problem persists.
Until the communication lines at both ends are checked—often revealing a neglected detail: Termination resistors are either not added or incorrectly connected.
This seemingly insignificant 120Ω resistor is actually the key to the stability of RS-485 communication. Many engineers treat it as an “optional accessory,” but in practical projects, it often determines the success or failure of the entire system.
Why is the 120Ω Termination Resistor So Critical?
The Core Issue: The “Ghost” of Signal Reflection
RS-485 is essentially a differential signal transmission protocol. When high-frequency digital signals propagate along the transmission line, if the impedance at the end of the line is not matched, the signals will reflect like light encountering a mirror. These reflected signals will superimpose with the original signals, causing:
- Signal Distortion: Originally clear square waves become sawtooth waves
- False Triggering: The receiver misinterprets noise as valid data
- Communication Timeout: Packet verification fails, requiring retransmission
- System Instability: Intermittent performance, making it difficult to locate issues
The “Golden Value” of 120Ω
Why specifically 120Ω? This is not a random choice:
Characteristic Impedance Matching Principle: The characteristic impedance of standard RS-485 twisted pair cables is approximately 120Ω. When the termination resistance equals the characteristic impedance, the signal reflection coefficient is zero, achieving perfect matching.
Formula: Reflection Coefficient ρ = (ZL – Z0) / (ZL + Z0)
- ZL: Termination Resistance (120Ω)
- Z0: Cable Characteristic Impedance (120Ω)
- When ZL = Z0, ρ = 0, no reflection
Practical Experience: Correct Application of Termination Resistors
1. Standard Connection: Add Resistors Only at Both Ends
[Master PLC]---120Ω---[Slave 1]---[Slave 2]---[Slave N]---120Ω
↑ ↑
Bus Start Point Bus End Point
Key Points:
- Add termination resistors only at thetwo ends of the physical bus
- Do not add resistors at intermediate nodes
- Each termination resistor should have a power rating of ≥1/4W, preferably 1/2W
2. Actual Wiring Example
Taking Siemens S7-1200 as an example:
| Terminal | Connection Description | Termination Resistor Connection |
|---|---|---|
| A+ | RS-485 Positive Signal Line | At both ends, connect a 120Ω resistor between A+ and B- |
| B- | RS-485 Negative Signal Line | Same as above |
| Ground | Signal Ground | Connect all device signal grounds |
3. Handling Multi-Branch Networks
If your system is star or tree structured (though not recommended), the handling method is:
Master Station ── Branch 1 ── Device A ── Device B (120Ω)
└── Branch 2 ── Device C ── Device D (120Ω)
Termination resistors must be added at the end of each branch, but this will reduce communication quality. The best solution is to switch to a daisy-chain bus structure.
4. On-Site Debugging Tips
Signal Quality Testing Methods:
- Use an oscilloscope to observe the signal waveform
- Normal waveform: Square wave edges are steep, with no ringing
- Abnormal waveform: Edges have bounce, ringing, or overshoot
Experience Data Table:
| Communication Distance | Recommended Cable | Termination Resistance | Maximum Baud Rate |
|---|---|---|---|
| <50 meters | RVVP 2×0.75 | 120Ω±5% | 187.5Kbps |
| 50-500 meters | RVVP 2×1.0 | 120Ω±5% | 93.75Kbps |
| 500-1000 meters | Dedicated RS-485 Cable | 120Ω±5% | 19.2Kbps |
5. Common Mistakes and Solutions
Mistake 1: Adding Termination Resistors to Every Device
- Consequence: Bus impedance is too low, signal amplitude decreases
- Solution: Keep resistors only at the two ends
Mistake 2: Using Non-Standard Resistance Values
- Common: Replacing with 100Ω or 150Ω
- Consequence: Impedance mismatch, still has reflections
- Solution: Strictly use 120Ω ±5% precision resistors
Mistake 3: Ignoring Resistor Power Rating
- Phenomenon: Resistors heat up, resistance value drifts
- Solution: Use 1/2W metal film resistors
Advanced Techniques: Handling Special Scenarios
1. Long-Distance Communication (>1km)
[PLC]---120Ω---[Repeater]---120Ω---[Repeater]---120Ω---[Device Group]
Each segment uses independent termination resistors, and repeaters provide signal amplification.
2. High-Interference Environments
In addition to standard termination resistors, also need to:
- Add common mode chokes
- Use shielded twisted pairs, with the shield grounded at a single point
- Add surge protectors
3. Mixed Device Networking
Experience when mixing devices from different manufacturers:
- Set the same baud rate and data format
- Ensure all devices support the RS-485 standard
- Measure the input impedance of each device with a multimeter
On-Site Troubleshooting Process
When encountering RS-485 communication issues, check in the following order:
- Check Termination Resistors: Use a multimeter to measure if there is a 120Ω resistor at both ends
- Measure Cable Connectivity: Ensure A+, B-, and GND are correctly connected
- Check Device Addresses: Ensure there are no duplicate addresses
- Verify Communication Parameters: Baud rate, data bits, stop bits, parity bits
- Signal Quality Analysis: Use an oscilloscope to observe if the waveform is normal
Value Summary
Ten years of on-site experience tells me: Details determine success or failure, and fundamentals determine height.
The 120Ω termination resistor may seem trivial, but it carries the stability of the entire RS-485 network. In industrial settings, a small resistor can be the dividing line between stable operation and frequent failures.
Remember this phrase: Don’t let a $2 resistor ruin a $200,000 project.
In automation projects, we do not pursue “good enough,” but rather “fail-safe.” Every seemingly simple technical detail contains profound engineering principles. Only by truly understanding and strictly adhering to these standards can we stand firm in the fiercely competitive non-standard automation industry.