Modbus RTU Wiring Guide — RS-485 Network Design
Key Takeaway
How to wire a Modbus RTU RS-485 network — cable selection, daisy-chain topology, termination resistors, bias resistors, shielding, max distance, and common mistakes.
Quick Answer
Modbus RTU over RS-485 requires shielded twisted pair cable in daisy-chain topology with 120-ohm termination resistors at both bus ends. Maximum 32 devices per segment, up to 1200m at 9600 baud.
Cable Requirements
- Shielded twisted pair, 120-ohm impedance
- Capacitance below 50pF/m
- Belden 9841 or Alpha 5461 recommended
Topology
Daisy-chain (correct) — NOT star topology. Star connections cause signal integrity failures due to reflections and impedance mismatches.
Distance vs Baud Rate
| Baud Rate | Max Distance |
|---|---|
| 9600 | 1200m |
| 19200 | 600m |
| 38400 | 300m |
| 115200 | 100m |
Termination
120-ohm resistors across A/B at BOTH ends of the bus only. Not at intermediate nodes. Measure ~60 ohm A-to-B with power off to verify.
Bias Resistors
Pull-up on B (560-1000Ω to VCC), pull-down on A (560-1000Ω to GND). One set per bus segment, typically at the master.
Shield Grounding
Connect shield to earth ground at ONE end only to avoid ground loops.
Common Mistakes
- No termination → reflections, noise, intermittent errors
- Star topology → signal integrity failure
- Multiple bias sets → noise
- Shield grounded both ends → ground loop
See also Modbus troubleshooting and Bristol FB300 RS-485 wiring for similar field bus considerations.
Frequently Asked Questions
Yes. 120-ohm at both physical ends of the bus. Without termination, high baud rates are particularly susceptible to errors.
32 unit loads per segment standard. Extended with repeaters. Modbus addressing allows 247 unique slave addresses.
Shielded twisted pair, 120-ohm impedance, under 50pF/m capacitance. Belden 9841 is the industry standard. Avoid Cat5/Cat6.