Modbus Function Codes with Real-World Examples

Modbus function codes define what action a master requests from a slave device. These examples illustrate how I apply the most common codes when integrating PLCs, drives, and energy meters on the plant floor.

1. Read Coils (0x01)

• Use case: Polling motor start/stop commands or digital outputs.
• Tip: Group contiguous coils to minimize requests.

client.read_coils(address=0, count=8, unit=1)

2. Read Discrete Inputs (0x02)

• Use case: Monitoring safety circuits (light curtains, door sensors).
• Tip: Use separate polling intervals for safety-critical signals to avoid delays.

client.read_discrete_inputs(address=0, count=16, unit=1)

3. Read Holding Registers (0x03)

• Use case: Fetching process values, setpoints, counters.
• Tip: Apply scaling (divide/multiply) according to documentation.

result = client.read_holding_registers(40010, count=4, unit=3)
temperature = result.registers[0] / 10

4. Read Input Registers (0x04)

• Use case: Reading sensor data or energy meter values that are read-only.

client.read_input_registers(address=0, count=6, unit=5)

5. Write Single Coil (0x05)

• Use case: Start/stop commands, enabling outputs.
• Tip: Confirm the coil state in a follow-up read; some devices queue writes.

client.write_coil(address=4, value=True, unit=1)

6. Write Single Register (0x06)

• Use case: Adjust setpoints or modes (e.g., speed reference for a drive).
• Tip: Validate ranges before writing to avoid trips.

client.write_register(address=40020, value=1500, unit=2)

7. Write Multiple Coils (0x0F) and Registers (0x10)

• Use case: Downloading recipes, resetting counters, or switching multiple outputs simultaneously.
• Tip: Use this to guarantee all values change within the same transaction.

client.write_coils(address=0, values=[True, False, True], unit=1)
client.write_registers(address=40030, values=[120, 250, 370], unit=2)

8. Diagnostics (0x08)

• Use case: Testing communication links or resetting diagnostic counters.

client.diag_query(diag_code=0x0000, unit=1)  # return query data

9. Exception Codes to Watch

10. Safety Considerations

• Always validate writes against safe operating ranges.
• Implement permissions in the edge gateway; not every user should write to Modbus devices.
• Log all write operations with operator ID and timestamp.

Mastering function codes is key to reliable Modbus integration. Pair these commands with thoughtful error handling and security controls to keep your systems robust.