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Objective: Use a RevPi base module as a Modbus RTU Server so that a Modbus RTU Client can read data from or write data to the RevPi. The configuration is performed in the RevPi application PiCtory.

Using the RevPi as a Modbus RTU Server is suitable for scenarios where a higher-level Modbus RTU Client needs to exchange process data with the RevPi via a serial RS485 connection.

This tutorial applies to all RevPi base modules. Base modules without a built-in RS485 interface can be connected using a USB-to-RS485 adapter.

Prerequisites #

Devices

✓ RevPi base module

✓ Modbus RTU Client, e.g., QModMaster on a Windows PC

✓ Optional: USB-RS485 adapter on the Windows PC if QModMaster is used.

✓ Optional: USB-RS485 adapter on the RevPi if no RS485 interface is available.

USB-RS485 adapter

Hardware Installation

✓ The RevPi base module is connected to the network.

✓ The RevPi base module is connected to the power supply.

✓ The Modbus RTU Client is connected to the RevPi via the RS485 interface or a USB-RS485 adapter.

Signal Pin

RS485-

D+

RS485+

D-

See also:

Software Installation

✓ Web browser (e.g., Chrome, Firefox, or Edge)

✓ QModMaster from SourceForge is installed on your Windows PC.

✓ RevPi and Modbus RTU Client are on the same network.

✓ The address, baud rate, parity, data bits, and stop bits of the Modbus RTU Server are defined.

1. Configuring the Modbus RTU Server in PiCtory #

▷ Start PiCtory.

▷ Drag the RevPi base module from the Device Catalog to the empty slot in the configuration area with position number 0.

▷ Drag a Modbus RTU Server from Virtual Modbus Devices in the Device Catalog to the base module in the configuration area.

❯ The Modbus RTU Server is now displayed in the configuration area.

PiCtory config

▷ Select the Modbus RTU Server in the configuration area.

▷ Configure the following parameters at the very bottom of the list in the Value Editor:

Parameter Description

Input_

Up to 32 input values, each 16 bits wide. Values are written by the Modbus RTU Client and can be read in the process image.

Output_

Up to 32 output values, each 16 bits wide. Values can be read by the Modbus RTU Client.

device_path

Path to the Linux device file. /dev/ttyRS485 is used by default. The path may differ for USB-RS485 adapters.

baud_rate

Speed of the serial connection. 19200 is used by default.

parity

Parity bit configuration: None, Even, or Odd. Even is used by default.

data_bits

Number of data bits. 8 is used by default.

stop_bits

Number of stop bits. 1 is used by default.

modbus_address

Address of the Modbus RTU Server on the RS485 bus. 1 is used by default.
Note

Configure udev rules for consistent device file paths when using multiple USB-RS485 adapters of the same type.

▷ In the Value Editor, you can optionally assign suitable I/O names for easier reference, e.g., Modbus_Input_1.

See also Configuring Values.

▷ Select File › Save as Start-Config. to save the configuration.

▷ Select Tools › Reset driver to apply the configuration.

2. Setting Up the Modbus RTU Client #

Configuring QModMaster

▷ Open QModMaster.

▷ Set the following parameters in the main window:

  • Modbus Mode: RTU

  • Slave Addr: value from modbus_address

  • Scan Rate (ms): 1000

  • Function Code: Write Multiple Registers (0x10)

  • Start Address: 0

  • Number of Registers: 3

  • Data Format: Dec

▷ Enter values in the Modbus registers, e.g., 1, 2, and 3.

QModMaster config

▷ Select Options › Modbus RTU…​.

▷ Set the serial parameters to match the Modbus RTU Server configuration in PiCtory:

  • Serial port: COM port of the USB-RS485 adapter

  • Baud: value from baud_rate

  • Data Bits: value from data_bits

  • Stop Bits: value from stop_bits

  • Parity: value from parity

QModMaster config

Starting Serial Communication

▷ Select Commands › Connect to start communication with the Modbus RTU Server.

▷ Select Commands › Read / Write to write the values to the Modbus registers.

3. Checking Data in the Process Image #

▷ Open a terminal.

▷ Query the written values with piTest:

piTest -1 -r Input_1
piTest -1 -r Input_2
piTest -1 -r Input_3

❯ Output:

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

Values that a Modbus RTU Client writes to the registers starting at address 0 are stored in the RevPi process image under Input_*.

Modbus Functions #

The Modbus server module supports the following Modbus functions:

  • Read Holding Registers (0x03)

  • Read Input Registers (0x04)

  • Write Single Register (0x06)

  • Write Multiple Registers (0x10)

Further Resources #