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 data registers of the RevPi.

Prerequisites #

Hardware

✓ RevPi base module (e.g. RevPi Connect 4)
✓ Master device or software, e.g. QModmaster running on a Windows PC
✓ Matching cables with RJ45 connectors
✓ Power supply for RevPi Connect

Software

✓ Modern web browser (e.g. Google Chrome or Mozilla Firefox). ✓ QModmaster: downloadable from SourceForge for use in this example.

System Setup

Ensure that:

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

Step 1: Setting up the Hardware #

▷ Connect the RevPi Connect to the master device using an RJ45 cable.

▷ Power on the RevPi Connect by connecting it to a suitable power supply.

Step 2: Configuring Modbus TCP Slave in PiCtory #

▷ Start pictory.

Webstatus login

▷ Add the base module and the virtual Modbus TCP 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 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, see Configuring Inputs and Outputs.

▷ 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 #

▷ 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
Configuring QModMaster

▷ Select Menü › Optionen › 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
Connecting to the Modbus

▷ Select Commands › Connect to establish a connection with the Modbus Slave.

qmodmaster config
Querying Data

▷ Select 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:

Function Description

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 #