How to Read Modbus Register: Simple Guide with Example

To read a Modbus register, use the Read Holding Registers function (function code 0x03) by sending a request with the device address, starting register address, and number of registers to read. The device responds with the register values in a data frame you can decode.

Syntax

The basic syntax to read a Modbus register involves sending a request frame with these parts:

• Device Address: The ID of the Modbus device you want to read from.
• Function Code: Use 0x03 for reading holding registers.
• Starting Register Address: The first register number to read.
• Number of Registers: How many registers you want to read.
• CRC Checksum: Error-checking code to ensure data integrity.

The device replies with the requested register values in a similar frame.

plaintext

Request Frame Format:
[Device Address][Function Code][Starting Register High][Starting Register Low][Quantity High][Quantity Low][CRC Low][CRC High]

Example

This example shows how to read 2 registers starting at address 100 from a device with address 1 using Python and the pymodbus library.

python

from pymodbus.client.sync import ModbusTcpClient

# Connect to Modbus TCP device at IP 192.168.1.10, port 502
client = ModbusTcpClient('192.168.1.10', port=502)
client.connect()

# Read 2 holding registers starting at address 100
result = client.read_holding_registers(100, 2, unit=1)

if not result.isError():
    print('Register values:', result.registers)
else:
    print('Failed to read registers')

client.close()

Output

Register values: [1234, 5678]

Common Pitfalls

• Wrong Register Address: Modbus register addresses often start at 0 or 1 depending on the device; check your device manual.
• Incorrect Function Code: Use 0x03 for holding registers, not input registers or coils.
• Unit ID Mismatch: The device address (unit ID) must match the target device.
• Connection Issues: Ensure network or serial connection is stable and configured correctly.
• Ignoring Response Errors: Always check if the response contains errors before using data.

python

Wrong way:
result = client.read_holding_registers(101, 2, unit=1)  # Off by one address

Right way:
result = client.read_holding_registers(100, 2, unit=1)  # Correct address

Quick Reference

Parameter	Description	Example
Device Address	ID of the Modbus device	1
Function Code	Operation to perform	0x03 (Read Holding Registers)
Starting Register	First register to read	100
Quantity	Number of registers to read	2
CRC	Error check code	Calculated automatically

Key Takeaways

Use function code 0x03 to read holding registers from a Modbus device.

Always verify the starting register address matches your device's documentation.

Check the response for errors before using the register data.

Ensure the device address (unit ID) is correct in your request.

Use libraries like pymodbus for easier Modbus communication in code.