How to Use RevPi Connect as Modbus TCP Master

You can use your RevPi Connect as Modbus TCP Master to connect slaves.

You need:

  • RevPi Connect
  • Slave
  • Matching cables with RJ45 connectors
  • Internet connection


  • A web browser, e.g. Google Chrome or Mozilla Firefox.
  • RevPi Connect and slave devices are located in your network.

Let’s go!

  • Connect the slaves to your RevPi Connect via the RJ45 jack.
  • Specify the IP address of the slaves and make a note of it. If necessary, you can find information on this in the manufacturer’s operating instructions.
If you are using several slaves, you have to make sure that each slave has its own IP address. If 2 or more slaves have identical addresses on the same port, communication errors occur.
  • Open your web browser.
  • Enter the IP address of your RevPi Connect in the address bar of your web browser.

The login window appears.

  • Log in with the username „admin“.
  • Enter the password. You can find it on the sticker on the side of your RevPi Connect.
  • Click „Login“

You can now see the current device status of your RevPi Connect.

  • Click “Services” tab.
  • Click “Enabled” in the Enable/Disable Modbus Master line.
  • Click the “Save All” button.
Services Reiter Screenshot

Note!: You can also activate the Master function on the command line. Enter the following command: “sudo revpi-config enable pimodbus-master”.

  • Click the „Apps“ tab.
  • Click the start button behind the entry “PiCtory”.
Apps Reiter Screenshot

PiCtory starts.

  • Open the folder “Virtual Devices”in the device catalog.
  • Select Modbus-TCP Master.
  • Hold down the left mouse button and drag the Modbus TCP Master to your RevPi Connect.

Your Modbus-TCP Master will now appear on the far right of the Configuration Board.

Your Modbus TCP master is always arranged on the far right. If you use a RevPi Gate, which you are only allowed to arrange on the far right/left outside, it will still work as before. The RevPi Gate is a device that is physically present and can only be mounted on the outside of the DIN rail. However, virtual devices such as the Modbus TCP master are not physically present. Therefore, you don’t have to create a separate slot for these devices and PiCtory automatically arranges them outside of the devices that physically exist.
  • Select Modbus-TCP-Master.
  • Set the basic settings for your adapters in the “Device Data” window. This entry is optional. If you use a lot of devices and want to process the data later in another program, this input can be very helpful.
Each slave needs a virtual master
  • In the “Value Editor” we define the settings for the TCP connection. We’ve already determined the values from the sensor’s instruction manual:
Parameter Description
slave_IP_address IP address of the Modbus TCP slave
slave_tcp_port The TCP port of the Modbus TCP slave. The default value is “502”according to Modbus specification. You can change this value. This makes for instance sense if you want to implement 2 different controllers in the same network, which should not interfere with each other.
  • Right-click on the Modbus TCP Master on the Configuration Board.
  • A context menu opens.
  • Select the entry “Extended Data”.

An input mask opens.

This is where you can set the Modbus commands.

Parameter Description
Unit ID Please refer to the instruction manual of your slave, if you have to set this address. Most slaves ignore this address. In this case it is recommended to set the value 255 (invalid).
Function Code Modbus has function codes that allow access to data in a certain way. We use the following function codes:


Reading single bits


Read single input bit


Read an whole 16-bit input/output register


Read an whole input register (16 bits)


Write single bit


Writing a whole register


Write several consecutive bits


Write several consecutive registers

Register Address The Modbus register address or bit address of the data you want to access. You will find information on how the registers of the slave are assigned in the operating instructions of the slave. Please note the comment regarding the register address below this table!
Quantity of Registers Number of registers (or bits) that are read/written
Action Interval The interval at which the command is sent (in milliseconds)
Device Value The variable name in the RevPi process image in which the first word or bit of the Modbus command is read or written.

Depending on the selected function code, the dropdown menu offers you the appropriate target addresses of the process image for selection.

Make sure that the different commands do not overlap.

We use the first register address 1 for our products. Some manufacturers use 0 as the first register address for their products.
Check how the addressing is specified at your slave.

If the slave you want to use uses 0 as the first register address, you must add a 1 to the specified value during configuration.
Example: The documentation for your slave states a register address 30053. The addresses start at 0 according to the documentation. You use address 30054 for this register when configuring a Modbus command.
  • Click on „File>Save“ to save the file.
  • Click on “Tools>Reset Driver”. This activates the changes for the adapter.

Do you want to continue using the settings in logiCAD3 or C?

  • Click on „File>Export“.

A window opens. You can specify the format of the file and the filename.

You can choose from 2 file formats:

Export 01 creates a file that is suitable for use in logiCAD3.
Export 02 creates an offset list which you can use as basic information for your own C-program.

  • Select a format you want to continue working with.
  • Specify a file name.
  • Click „Ok“.

Help! Error!

The Modbus master has predefined Modbus registers for status messages.

If a communication error occurs, the error code is written to the Modbus register “Modbus_Master_Status”.

The error stays in this register until the value “1” is written manually into the Modbus register “Master_Status_Reset”.

Error code Description
0x10 The configured device was not found.

Check if wiring is correct.

0x11 The configured device does not respond or you are using a register address which is not allowed by the slave (see note above)).

Check that the IP address is correct and that the cables are plugged in correctly.

The Modbus master can handle up to 32 tasks. For each task there is a register for status and status reset. If an error occurs in a task, the error code is written to the Modbus register “Modbus_Action_Status_[1… 32]”.

The error stays in this register until the value “1” is written manually into the Modbus register “Action_Status_Reset_[1… 32]”.

The error codes correspond to the Modbus exception codes as specified in the Modbus specification.

The following table shows the most important error messages.

Error code Name Description
1 ILLEGAL FUNCTION The function code used is not allowed. Check if you are using the correct function code.
2 ILLEGAL DATA ADDRESS The Modbus register address used is not valid. The register is either write-protected or invalid. Check the register address.
3 ILLEGAL DATA VALUE At least one part of the data values used is invalid. For example, it is possible that you have entered too many registers. Check your values.
13 INVALID DATA The slave has answered my incomplete packet. This can happen after the connection has been interrupted. Check your wiring.
110 CONNECTION TIMED OUT The slave did not respond fast enough or not at all. Check your configuration and wiring.

For more information on this topic, please refer to the official Modbus specifications.