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RevPi as Modbus RTU Slave

Introduction​

This Tutorial explains how to configure a RevPi base module as a Modbus RTU Slave using a USB-RS485 Converter. It includes detailed steps to set up the connection and query data from the RevPi using a Modbus RTU Master.This Tutorial is for industrial automation professionals, IoT developers, and others needing to integrate RevPi as a Modbus RTU Slave in their systems.

Prerequisites​

Hardware​

  • RevPi base module (eg.RevPi Connect 4)
  • Master device or software: For example, “qModMaster” on a Windows PC.
  • USB-RS485 Converter
  • Cables and connectors for RS485 communication

Software​

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

RS485 Signal Connections​

SignalPin
RS485-D+
RS485+D-
usb RS485

Step 1: Hardware Setup​

  1. Connect the USB-RS485 Converter to the RevPi Connect.
  2. Wire the RS485+ and RS485- signals appropriately to your devices.
  3. Ensure power is supplied to the RevPi Connect.

Step 2: Configure Modbus RTU Slave in PiCtory​

▷ Start pictory.

Webstatus login

▷ Add Base Module and Add the virtual Modbus RTU 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 RTU Slave to the base module on the virtual DIN rail.
    ➜ The Modbus RTU Slave will now appear in the configuration

▷ Configure Modbus TCP Slave

  • Select the Modbus RTU Slave in the configuration.

Webstatus login

  • Set the following parameters in the Value Editor:
ParameterDescription
InputConfigure up to 32 input values, each 16 bits wide.
OutputConfigure up to 32 output values, each 16 bits wide.
device_pathPath to the Linux device file, default: /dev/ttyRS485.
baud_rateSpeed of the serial connection, default: 19200.
parityConfigure parity bit: None, Even (default), or Odd.
data_bitsNumber of data bits, default: 8.
stop_bitsNumber of stop bits, default: 1.

Webstatus login

Note: If using multiple devices of the same type, configure unique udev rules to prevent device path conflicts after reboot.

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

Step 3: Querying Data Using qModMaster​

  1. Start qModMaster on the Windows PC.

  2. Configure Serial Interface Parameters
    Set the parameters for communication to match the Modbus RTU Slave configuration:

    • Baud rate, parity, stop bits, and data bits.

    qmodmaster

  3. Transfer Data to Modbus Registers
    Use qModMaster to write values (e.g., 1, 2, 3) into the Modbus registers.

    qmodmaster

  4. Query Data from RevPi Shell
    Use the piTest command to query data from the Modbus RTU Slave:

    piTest -1 -r Input_1

    Example Output:

    2 Byte-Value of Input_1: 1 dez (=0001 hex)
    piTest -1 -r Input_2
    2 Byte-Value of Input_2: 2 dez (=0002 hex)
    piTest -1 -r Input_3
    2 Byte-Value of Input_3: 3 dez (=0003 hex)

Notes and Best Practices​

  • Device Path Conflicts: To avoid issues when rebooting, configure udev rules to assign consistent paths to the device files.
  • Baud Rate: Ensure the master and slave devices are configured with matching baud rates for proper communication.

Further Resources​