What Is Modbus Function Code: Simple Explanation and Example

A Modbus function code is a number that tells a Modbus device what action to perform, like reading or writing data. It acts like a command in a conversation between devices in a SCADA system to control or monitor equipment.

How It Works

Think of a Modbus function code as a simple instruction in a conversation between two devices. One device asks the other to do something specific, like "read the temperature" or "turn on a motor." The function code is the number that represents this instruction.

When a Modbus master device sends a request, it includes a function code to tell the slave device what to do. The slave then performs the action and sends back the result. This system is like ordering food at a restaurant: the function code is the menu item number you say to the waiter, who then brings you the dish.

Example

This example shows how to use a Modbus function code to read holding registers from a device using Python and the pymodbus library.

python

from pymodbus.client.sync import ModbusTcpClient

client = ModbusTcpClient('192.168.1.10', port=502)
client.connect()

# Function code 3: Read Holding Registers
result = client.read_holding_registers(address=0, count=2, unit=1)

if not result.isError():
    print('Register values:', result.registers)
else:
    print('Error reading registers')

client.close()

Output

Register values: [123, 456]

When to Use

Use Modbus function codes whenever you need to communicate with industrial devices like sensors, meters, or controllers in a SCADA system. They let you read data (like temperature or pressure), write settings (like turning a valve on or off), or check device status.

For example, a factory might use function codes to monitor machine health or control motors remotely. This makes automation and monitoring simple and reliable.

Key Points

• Function codes are numeric commands in Modbus communication.
• Each code corresponds to a specific action like reading or writing data.
• They enable clear, simple instructions between master and slave devices.
• Common codes include 1 (read coils), 3 (read holding registers), and 6 (write single register).

Key Takeaways

Modbus function codes tell devices what action to perform in communication.

They are essential for reading and writing data in SCADA systems.

Each function code corresponds to a specific command like reading registers or coils.

Using function codes enables reliable control and monitoring of industrial equipment.