RevPi as Modbus RTU Slave

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, e.g. QModmaster on a Windows PC.
✓ USB-RS485 Converter
✓ Cables and connectors for RS485 communication

Software

✓ Modern web browser (e.g. Google Chrome or Mozilla Firefox). ✓ QModmaster: downloadable from SourceForge.

RS485 Signal Connections

Signal	Pin
RS485-	D+
RS485+	D-

Signal

RS485-

D+

RS485+

D-

usb RS485

Step 1: Setting up the Hardware #

▷ Connect the USB-RS485 Converter to the RevPi Connect.

▷ Wire the RS485+ and RS485- signals appropriately to your devices.

▷ Ensure power is supplied to the RevPi Connect.

Step 2: Configuring Modbus RTU Slave in PiCtory #

▷ Start pictory.

▷ Add the base module and the virtual Modbus RTU Slave to your configuration, see Arrange Devices.

▷ 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, see Configuring Inputs and Outputs

▷ Select the Modbus RTU Slave in the configuration.

▷ Set the following parameters in the Value Editor:

Parameter Description

Parameter	Description
Input	Configure up to 32 input values, each 16 bits wide.
Output	Configure up to 32 output values, each 16 bits wide.
device_path	Path to the Linux device file, default: `/dev/ttyRS485`.
baud_rate	Speed of the serial connection, default: `19200`.
parity	Configure parity bit: None, Even (default), or Odd.
data_bits	Number of data bits, default: `8`.
stop_bits	Number of stop bits, default: `1`.

Input

Configure up to 32 input values, each 16 bits wide.

Output

Configure up to 32 output values, each 16 bits wide.

device_path

Path to the Linux device file, default: /dev/ttyRS485.

baud_rate

Speed of the serial connection, default: 19200.

parity

Configure parity bit: None, Even (default), or Odd.

data_bits

Number of data bits, default: 8.

stop_bits

Number of stop bits, default: 1.

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 #

▷ Start QModmaster on the Windows PC.

▷ Set the parameters baud rate, parity, stop bits and data bits for communication to match the Modbus RTU Slave configuration.

▷ Use QModmaster to write values (e.g., 1, 2, 3) into the Modbus registers.

▷ 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)

Note	Ensure the master and slave devices are configured with matching baud rates for proper communication.