Overview

Acting as a Modbus master unit, an ALPON X5 AI or ALPON X4 can:

• Poll data from one or more Modbus-compatible devices such as PLCs, sensors, or energy meters.
• Control actuators and other devices by writing to their registers.
• Bridge a local PLC network to any platform.

The sections below show two approaches: Modpoll, a lightweight command-line client for testing and polling, and Pymodbus, a Python library for building custom master applications. The procedure is identical on ALPON X4 and ALPON X5 AI.

Prerequisites

Method 1 · Modpoll for basic master functionality

Modpoll is a straightforward command-line tool for testing and polling Modbus-compatible devices.

1. 1

   Install Modpoll

   Download and extract Modpoll on the ALPON:

   bash · install modpoll

   wget https://www.modbusdriver.com/downloads/modpoll.tgz
   tar xzf modpoll.tgz
   cd modpoll/arm-linux-gnueabihf/

2. 2

   Run Modpoll

   Read data from a PLC device:

   bash · poll a device

   ./modpoll -m tcp -a 1 -r 100 -c 10 -t 4:float <DEVICE_IP>

   Parameter reference

   -m tcp Uses Modbus TCP mode.

   -a 1 Sets the slave address to 1.

   -r 100 Starts reading from register 100.

   -c 10 Reads 10 consecutive registers.

   -t 4:float Interprets the data as floating-point values.
3. 3

   Verify communication

   Confirm that the data returned from the device is accurate and matches the expected values.

Method 2 · Advanced configuration with Pymodbus

For advanced use cases, Pymodbus — a Python library — lets you build custom Modbus master applications.

1. 1

   Install dependencies

   Create a virtual environment and install Pymodbus:

   bash · virtualenv + pymodbus

   python3 -m venv venv
   source venv/bin/activate
   pip3 install pymodbus

2. 2

   Write a master application

   Save the following as modbus_master.py:

   python · modbus_master.py

   from pymodbus.client.sync import ModbusTcpClient
   
   # Configure Modbus TCP client
   
   client = ModbusTcpClient('<DEVICE_IP>', port=502)
   
   # Connect to the device
   if client.connect():
       # Read 10 holding registers starting from address 100
       response = client.read_holding_registers(100, 10, unit=1)
       if not response.isError():
           print("Data:", response.registers)
       else:
           print("Error:", response)
   
       # Write a value to register 200
       client.write_register(200, 42, unit=1)
       print("Value written successfully.")
   
       client.close()
   else:
       print("Connection failed.")

3. 3

   Run the script

   bash · run

   python3 modbus_master.py

   Why Pymodbus for production

   • Flexible, customizable workflows.
   • Supports both Modbus TCP and RTU.
   • Easy integration with databases and cloud platforms for advanced analytics.

Best practices

Network configuration

• Use a reliable Ethernet connection for Modbus TCP.
• Keep Modbus devices on the same subnet as the ALPON, or reachable over TCP/IP.
• Avoid overlapping the Sixfab default subnets (100.0.0.0/16 and 10.42.0.0/24) with your Modbus devices.

Security

• Use a firewall to restrict access to critical ports (e.g. 502 for Modbus TCP).
• Keep the ALPON updated with the latest patches.

Containerized deployment

• Deploy Modbus master applications in isolated containers using the ALPON's container environment.

Troubleshooting

tail -f /var/log/syslog