Modbus RTU RS-485 guide: This document provides the essential technical rules for the correct design, wiring, and commissioning of industrial networks. Following these steps ensures high noise immunity and long-term hardware reliability.
1. Software Settings for Modbus RTU RS-485 guide
Configuring software parameters is the first critical step in building a stable network. Even a minor mismatch here can lead to a complete lack of communication or difficult-to-diagnose intermittent errors.
- Matching Parameters: All devices on the bus must share identical settings for Baud rate, Parity, and Stop bits.
Official Standard (modbus.org): 19200 bps, Even Parity, 1 Stop bit.
Note: If No Parity is used, the standard recommends 2 Stop bits to maintain frame length. - Unique Address (Slave ID): Every Slave must have a unique address (1–247). Never use the same ID for two devices.
Best Practice: Configure devices “one by one” before adding them to the common bus to avoid address conflicts. - Response Timeout: In this Modbus RTU RS-485 guide, we emphasize that the Master must wait enough time for the Slave to answer.
Rule of Thumb: Add a 10-15% safety margin above the manufacturer’s specified response time.
2. Physical Environment and Topology
Unlike simple serial interfaces, RS-485 is a line with a precisely specified impedance and requires specific discipline during connection.
- Cable Type (Twisted Pair): Use Twisted Pair (FTP is recommended). Do not use flat cables or separate wires.
- Linear Topology (Daisy Chain): Connect devices in a straight line (1 → 2 → 3).
Critical: Avoid “Star” patterns or long stubs. They cause signal reflections and data corruption. - Third Wire (GND): Connecting the grounds (GND/Common) of all devices with a third wire is mandatory.
- Shielding (Shield): Ground the shield at only one point (usually at the Master) to avoid ground loops. For more details, see our guide on wiring RS-485 in non-isolated systems.
3. Termination and Fail-safe Biasing
Termination and biasing ensure the signal remains clean during transmission and while the line is in an idle state.
- Termination: Place 120 Ω resistors at the two physical ends of the bus.
Critical: Never place termination resistors on intermediate devices. - Fail-safe Biasing: Keeps the line stable when no device is transmitting. Typically done at the Master.
Resistor Values: Use 560 Ω for +5V power and 1500 Ω (1.5 kΩ) for +12V power. - Quick Multimeter Diagnostics:
Power OFF: Resistance between A and B should be ~60 Ω.
Power ON (Idle): DC voltage between A and B should be between 0.2V and 0.5V.
4. Final Checklist
Before commissioning, verify these 7 points:
- [ ] Addressing: No duplicate addresses in the network?
- [ ] Parameters: Are Baud rate and Parity identical on all devices?
- [ ] Topology: Is the wiring a “Daisy Chain” without stubs longer than 30 cm?
- [ ] GND Wire: Is the third wire (GND/Common) connected to every device?
- [ ] Termination: Are there exactly two 120 Ω resistors (at the ends)?
- [ ] Biasing: Is the Idle voltage between A and B at least 200mV?
- [ ] Shielding: Is the shield grounded at only one point?