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

Using the RevPi as a Modbus RTU Client is suitable for scenarios where devices such as sensors, actuators, or other industrial devices need to be controlled or monitored via a serial RS485 connection.

This tutorial applies to RevPi base modules that either have an integrated RS485 interface or are connected via a USB-RS485 adapter.

Prerequisites #

Devices

✓ RevPi base module

✓ Modbus Server device(s)

✓ Option: 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 server devices are connected to the RevPi via the RS485 interface or a USB-RS485 adapter.

See also:

Connecting a USB-RS485 Adapter:

▷ Connect a USB-RS485 adapter to the USB port of the RevPi.

▷ Connect a D-Sub 9 connector to the adapter to connect it to the server devices.

D-sub connector config
Signal D-Sub Pin

RS485-

1

RS485+

2

GND

5

+5V

6

With some USB-to-RS485 adapters, only the RS485 data signal lines (D+ and D-) are connected.

usb RS485
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 address.

✓ The address, baud rate, parity, data bits, and stop bits of all server devices are documented according to the manufacturer’s specifications.

Note

Multiple server devices should not have the same address on the same RS485 bus, 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 RTU 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 Modbus RTU Client from Virtual Modbus Devices in the Device Catalog to the base module in the configuration area.

▷ Optional: Add multiple Modbus RTU Clients to expand the capacity for additional values.

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

pictory config

▷ Select the Modbus RTU Client 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.

Output_

Up to 32 output values, each 16 bits wide.

device_path

Path to the Linux device file. /dev/ttyRS485 is used by default.

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.
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 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 RTU Client in the configuration area.

▷ Select Extended Data to open the command configuration window.

▷ Configure the commands using the following parameters:

Parameter Description

Slave Addr.

This address can be found in the server manual.
Each server device on the same RS485 bus must have a unique address.

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.
Modbus RevPi Connect 4 wiring
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.

Error Codes #

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

▷ To reset an error, manually write 1 to the Client_Status_Reset or Action_Status_Reset register.

Common error codes:

Code Description

0x10

The serial device could not be opened or found. Check device_path_ and the wiring.

0x11

No valid response received from the Modbus server.

1

ILLEGAL FUNCTION: Invalid function code.

2

ILLEGAL DATA ADDRESS: Invalid or write-protected register address.

12

INVALID CRC: Corrupted packet. Check wiring, connection, and serial parameters.

110

Connection timeout.

Possible error causes:

  • RS485 connection interrupted or A/B lines swapped

  • Incorrect path in device_path_

  • Incorrect server address, baud rate, parity, data bits, or stop bits

  • Invalid register address

  • Overlapping or conflicting Modbus commands

  • Action interval too short under high bus or system load

Further Resources #