RS485 Modbus Transmitter Wiring & Networking Tutorial

RS485 Modbus RTU is the most common industrial transmitter interface. Correct wiring and networking are prerequisites for stable communication. Key points:

1. Bus Wiring

RS485 uses differential signals A (+) / B (-). Use twisted-pair shielded cable (e.g. STP 120Ω) with the shield grounded at one end. Adopt daisy-chain topology and avoid star branches.

2. Termination Resistors

Place a 120Ω termination resistor at each end of the bus to match characteristic impedance and suppress reflections. Use repeaters beyond 32 devices or 1200m cable length.

3. Address & Baud Rate

Assign a unique address (1-247) per device; common baud rates are 9600/19200/38400bps. Duplicate addresses corrupt data.

4. Troubleshooting Checklist

• No response: check A/B polarity, address, missing termination
• Intermittent errors: check shield grounding, cable quality, co-routing with power lines
• Multi-device conflicts: check duplicate addresses, mismatched baud rates

For full register definitions, download the Modbus Register Map.