Modbus Function Codes Explained (FC01, FC03, FC06)

Modbus function codes define how data is read or written from a device.

Understanding function codes is critical for correct communication.

Common Modbus Function Codes

FC01 – Read Coils

Reads digital outputs (ON/OFF values).

FC03 – Read Holding Registers

Reads analog values such as temperature, voltage, and configuration.

FC06 – Write Single Register

Writes a value to a register.

Why Function Codes Matter

The same address can return different data depending on the function code used.

Example:

• FC03 reads holding registers

• FC01 reads coils

Using the wrong function code results in incorrect or failed communication.

Example

Address: 40001

• FC03 → returns register value

• FC01 → invalid or wrong data

Convert Addresses Correctly

Use the calculator to match address format with function codes:

👉 Modbus Address Calculator