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RevPi as Modbus TCP Slave

Introduction​

This tutorial explains how to configure a RevPi base module as a Modbus TCP Slave to communicate with a Modbus TCP Master. It includes step-by-step instructions for setting up the RevPi and querying data using the Modbus protocol. This tutorial is for industrial automation professionals, IoT developers, and anyone looking to use a RevPi module as a Modbus TCP Slave in a networked environment. Configuring the RevPi as a Modbus TCP Slave allows external Modbus TCP Masters to access and control the RevPi's data registers.

Prerequisites​

Hardware​

  • RevPi base module (eg. RevPi Connect 4)
  • Master device or software: For example, “qModMaster” running on a Windows PC.
  • Matching cables with RJ45 connectors
  • Power supply for RevPi Connect

Software​

  • A modern web browser (e.g., Google Chrome or Mozilla Firefox).
  • qModMaster: Downloadable from SourceForge for use in this example.

System Setup​

Ensure that:

  1. The RevPi base module and master device are located in the same network.
  2. IP addresses are properly configured, and the devices can communicate with each other.

Step 1: Hardware Setup​

  1. Connect the RevPi Connect to the master device using an RJ45 cable.
  2. Power on the RevPi Connect by connecting it to a suitable power supply.

Step 2: Configure Modbus TCP Slave in PiCtory​

▷ Start pictory.

Webstatus login

▷ Add Base Module and Add the virtual Modbus TCP Slave to your configuration.

  • Drag the base module from the Device Catalog onto the virtual DIN rail.

  • Open the folder Virtual Devices in the Device Catalog.

    • Drag Modbus TCP Slave to the base module on the virtual DIN rail.
      ➜ The Modbus TCP Slave will now appear in the configuration

▷ Configure Modbus TCP Slave

  • Select the Modbus TCP Slave in the configuration.

PiCtory config

  • Set the following parameters in the Value Editor:
    • TCP Port: '502' (default value according to the Modbus specification).
    • Max. Modbus TCP Connections: '10' (or other suitable value).

PiCtory config

▷ Save the configuration and restart the driver to apply the changes.

Step 3: Querying the CPU Temperature​

The CPU temperature of the RevPi Connect can be queried using the following command in the terminal:

/usr/bin/vcgencmd measure_temp

The output will display the temperature, for example:

temp = 48.7'C

To prepare the data for Modbus processing, convert the temperature to x10 °C format using this command:

/usr/bin/vcgencmd measure_temp | awk ' { print substr($0,6,2) substr($0,9,1) } '
487

➜ This outputs the temperature in a suitable format, e.g., 487 for 48.7°C.

Writing Data to a Modbus Register​

To write the converted temperature data into a Modbus register, use the following command:

piTest –w Output_1,$(/usr/bin/vcgencmd measure_temp | awk ' { print substr($0,6,2) substr($0,9,1) } ')

For continuous updates, run the process in a loop:

while true; do piTest -w Output_1,$(/usr/bin/vcgencmd measure_temp | awk ' { print substr($0,6,2) substr($0,9,1) } '); sleep 1; done &

Output:

Write value 492 dez (=01ec hex) to offset 11.

Write value 498 dez (=01f2 hex) to offset 11.

Write value 492 dez (=01ec hex) to offset 11.

[…]

Step 4: Querying Data Using qModMaster​

  1. Download and install qModMaster on a Windows PC.

    • Set the following parameters:
      • Modbus Mode: TCP
      • Unit ID: 1
      • Scan Rate (ms): 1000
      • Function Code: Read Input Registers (0x04)
      • Start Address: 0
      • Number of Registers: 1
      • Format: Decimal

    qmodmaster config

  2. Configure qModMaster

▷ Click on Menu > Options and select ModbusTCP.

▷ Enter the IP address of the RevPi Connect in the Slave IP field.

▷ Set the TCP port to 502 (default for ModbusTCP connections).

qmodmaster config

  1. Connect to the Modbus
    ▷ Click on Commands > Connect to establish a connection with the Modbus Slave.

    qmodmaster config

  2. Query Data
    ▷ Click on Commands > Read/Write to retrieve data.

    ▷ The CPU temperature will be displayed in decimal form, e.g., 487 for 48.7°C.

    qmodmaster config

Modbus Functions​

The Modbus slave module supports the following Modbus functions:

FunctionDescription
Read Holding Registers (0x03)Read input data.
Read Input Registers (0x04)Read output data.
Write Single Register (0x06)Write to a single input register.
Write Multiple Registers (0x10)Write to multiple input registers.

Further Resources​