Objective: Use a RevPi base module as a Modbus TCP Client to read data from or write data to server devices. The configuration is performed in the RevPi application PiCtory using a practical example.

Using the RevPi as a Modbus TCP Client is suitable for scenarios where devices such as sensors, actuators, or other industrial equipment need to be controlled or monitored in a networked environment.

This tutorial applies to all RevPi base modules.

Prerequisites #

Devices

✓ RevPi base module

✓ Server device(s)

✓ Cable with RJ45 connectors

Hardware Installation

✓ The RevPi base module is connected to the network.

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

✓ The server devices are connected to the RevPi via the RJ45 connector.

Software Installation

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

✓ The RevPi and the server devices are on the same network.

✓ Each server device has a unique IP address.

✓ The IP addresses and ports of all server devices are documented according to the manufacturer’s specifications.

Note

Multiple server devices should not have the same IP address on the same port, as this causes communication errors.
Note

As of RevPi Bookworm (08/2025) the firewall on the RevPi must also be enabled to allow the default ports of the Modbus server:

▷ Add the service revpi-modbus-default to the firewall settings in Cockpit, see Firewall.

1. Configuring the Modbus TCP Client 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 ModbusTCP Master [1] from Virtual Modbus Devices in the Device Catalog to the base module in the configuration area.

❯ The virtual device is now displayed in the configuration area.

pictory config

▷ Select the Modbus TCP Client in the configuration area.

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

  • slave_IP_address: IP address of the Modbus TCP server.

  • slave_tcp_port: Default is 502, as specified in the Modbus specification. This can be changed if required.

▷ In the Value Editor, you can also 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. Configuring Modbus Commands #

▷ Open the context menu of the Modbus TCP Client in the configuration area.

▷ Select Extended Data to open the command configuration window.

▷ Configure the commands using the following parameters:

Parameter Description

Unit ID

This address can be found in the server manual.
For pure Modbus TCP, the Unit ID is ignored by many devices. In this case, 255 or 1 can often be used.

Function Code

READ_COILS: Read digital outputs (bits).
READ_DISCRETE_INPUTS: Read digital inputs.
READ_HOLDING_REGISTERS: Read 16-bit data registers.
READ_INPUT_REGISTERS: Read 16-bit input registers.
WRITE_SINGLE_COIL: Write a single bit.
WRITE_SINGLE_REGISTER: Write an entire register.
WRITE_MULTIPLE_COILS: Write multiple bits.
WRITE_MULTIPLE_REGISTERS: Write multiple registers.

Register Addr.

Define the Modbus register or bit address to be accessed.

Quantity of Registers

Number of registers or bits to read/write.

Action Interval (ms)

Time interval for sending the command in milliseconds.

Device Value

Name of the variable in the process image.
The read values are stored in the process image and can then be read, for example, with piTest or from your own applications.
Note

For devices that use 0-based addressing, the register address must be incremented by 1 during configuration.

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

▷ Select Tools › Reset driver to apply the configuration.

3. Task Management and Error Codes #

The Modbus TCP Client can manage up to 32 tasks. Each task has its own status and reset registers available in the process image.

If an error occurs while a task is being executed, an error code is stored in the corresponding status register: Modbus_Action_Status[1…​32].

The error remains stored until the corresponding reset register is set to 1: Action_Status_Reset[1…​32].

The status registers can contain both Modbus exception codes from the server and communication or driver errors.

▷ If communication problems occur, check the Modbus_Master_Status register in the Value Editor.

Common error codes:

Code Description

0x10

The TCP connection to the Modbus server could not be established.

0x11

No valid response received from the Modbus server.

110

Connection timeout.

Possible error causes:

  • Network connection interrupted

  • Incorrect IP address or port number

  • Invalid register address

  • Overlapping or conflicting Modbus commands

  • Firewall blocks communication

Example: Temperature Measurements with RevPi as Modbus TCP Client #

The Inveo NANO TEMP temperature sensor serves as the Modbus server in this example.

Configuration

▷ Refer to the sensor manual for the following data:

Parameter Value

IP address

192.168.0.103

Port

502

Modbus register

4004

Modbus function

READ_HOLDING_REGISTERS

Server address

1

▷ Configure the settings accordingly in PiCtory in the Extended Data window:

  • Unit ID: 1

  • Function Code: READ_HOLDING_REGISTERS

  • Register Addr.: 4004

  • Quantity of Registers: 1

  • Action Interval (ms): 1000

  • Device Value: Input_Word_1

pictory config

▷ Save the configuration and reset the driver.

Querying Data

▷ Open a terminal and run:

piTest -r Input_Word_1

❯ You receive the data from your server:

2 Byte-Value of Input_Word_1: 284 dec (=011c hex)

The value 284 corresponds to 28.4 °C.

Further Resources #

1. In PiCtory, the Modbus TCP Client is referred to as ModbusTCP Master.