Modbus RTU over RS-485: wiring and termination that actually works
Use a daisy-chain bus, keep A/B polarity consistent, terminate the two physical ends of the trunk, avoid star wiring where possible, and make every serial setting match exactly before blaming the protocol.
Quick answer: Wire Modbus RTU RS-485 as a single daisy-chained trunk, not a star. Keep A/B (D+/D−) polarity consistent at every node, fit roughly 120 Ω termination only at the two physical ends, use twisted-pair cable, follow the manufacturer’s reference/ground guidance and make baud rate, parity, stop bits and slave IDs match.
How should a Modbus RS-485 network be wired?
For multi-drop Modbus RTU, the most reliable physical layout is a single trunk that passes from one node to the next. Keep branch stubs short. Long star branches create reflections and can make a network work at low baud rates but fail intermittently at higher speeds or with more devices online.
120 Ω 120 Ω
Why is RS-485 A/B polarity confusing?
One vendor's A may correspond to another vendor's B because naming conventions vary. Prefer terminal descriptions such as D+/D− or the manufacturer's wiring diagram when available. If every setting is correct and there is no response, reversing the differential pair at one end is a common diagnostic step.
Never assume wire color alone proves polarity; verify the terminals at both ends.
Where do termination and bias resistors go?
A typical RS-485 trunk uses approximately 120 Ω termination at the two physical ends of the cable, not at every device. Too many termination resistors load the bus; no termination on a long/high-speed network can increase reflections.
Bias/failsafe resistors hold the bus in a defined idle state when no driver is active. Many modern interfaces include internal biasing, so do not add external bias blindly. Check the master/gateway documentation first.
Shield and reference conductor
Use twisted pair cable intended for RS-485. For shielded cable, follow the project's grounding philosophy; in industrial installations the shield is commonly bonded in a controlled manner to avoid circulating ground currents. Where equipment provides a signal reference/common terminal, follow the manufacturer guidance—large common-mode voltage between isolated or non-isolated devices can cause unstable communication or damage interfaces.
Which serial settings must match?
| Parameter | Example | What happens if wrong |
|---|---|---|
| Baud rate | 9600 / 19200 | Usually no valid response |
| Parity | None / Even / Odd | CRC/framing errors or silence |
| Data bits | 8 | Framing mismatch |
| Stop bits | 1 or 2 | Intermittent or no response |
| Slave ID | 1–247 | Wrong device never answers |
Start with one device directly connected to the master. Once stable, add devices one by one. This quickly separates a protocol/configuration problem from a bus-loading or wiring problem.
Fast field checklist
- Test with one slave only.
- Confirm COM port, baud, parity, stop bits and slave ID.
- Verify A/B wiring end-to-end.
- Check termination at the two ends only.
- Inspect grounding/reference and shield practice.
- Reduce baud rate temporarily if the cable is long or noisy.
- Use a communication log to see whether requests are sent and whether any bytes return.
If the physical layer looks correct but the device still does not answer, continue with the Modbus timeout troubleshooting guide.
Modbus Organization reference
For protocol-level definitions, use the official MODBUS over Serial Line Specification and Implementation Guide V1.02.
Frequently asked questions
Where should the 120 Ω termination resistors go on a Modbus RS-485 bus?
At the two physical ends of the trunk cable only, typically at the master and at the last device. Do not terminate every device, because extra resistors load the bus. Short, low-speed links may work without termination, but long or fast networks need it.
What happens if A and B are swapped on RS-485?
The device usually does not answer at all, because the differential signal is inverted. Vendors label A/B inconsistently, so if every setting is correct but nothing responds, swapping the pair at one end is a safe, common diagnostic step.
How many devices can be on one Modbus RTU RS-485 bus?
Modbus addressing allows up to 247 slave IDs, but the electrical limit depends on transceiver unit loads: a standard RS-485 segment supports 32 unit loads, and fractional-load transceivers allow more. Use repeaters for larger networks.
Use ModbusBB while you troubleshoot
Connect, scan, read/write, inspect raw TX/RX traffic, trend data or simulate a device from the same Windows application.