Advanced TCP/IP Data Logger

Copy settings from the MODBUS Poll utility to our Modbus Data Logger

Problem scenario:

I can successfully test and read data from my MODBUS RTU device using the MODBUS Poll software. How can I copy a MODBUS query settings to your logger because I would like to export data to a database?


It is assumed that:

You can successfully read data from your MODBUS device using MODBUS Poll (fig. 1).


1. Create a new configuration from the main window using the "Green Plus" button. (fig. 3). This example shows the connection settings for MODBUS RTU. If your device uses MODBUS TCP, look here. First, copy the communication settings from MODBUS poll (Connection → Connect, fig. 1, 2):

  • COM port, Baud, Data bits, Parity, Stop bits.
  • Spy mode - disable this option.
  • RTS toggle (fig. 2) → RS485 mode. Enable this option, if you've connected your MODBUS devices to an RS-485 bus, and you use RS485-USB or RS485-RS232 converter.
  • DTR → Data flow control → Use DTR.
  • DSR/CTS → Data flow control → Require DST/CTS.
  • Other options - leave the default value as shown below.

MODBUS Poll Connection Setup
Fig. 1: MODBUS Poll Connection Setup.

MODBUS Poll Advanced Setup
Fig. 2: MODBUS Poll Advanced Setup.

MODBUS Data Logger Connection Setup
Fig. 3: MODBUS Data Logger Connection Setup.

2. Activate the "MODBUS RTU" plugin in the settings in both fields "Data query module" and "Parser module". Please do not confuse it with "MODBUS RTU [Passive]" or "MODBUS RTU [Slave]" because they realize other functionality. We configure the MODBUS master station.

Activating the MODBUS RTU plugin
Fig. 4: Activating the MODBUS RTU plugin.

3. In the MODBUS RTU plugin settings, click the "Action → Add new request" button to add a new query request (fig. 6). Copy basic request settings from MODBUS poll (Setup → Read/Write Definition, fig. 5):

  • Slave ID → Device address.
  • Function → Function.
  • Address (Dec) → First register - Value.
  • Quantity → Registers to read.
  • Response timeout (fig. 1) → Request timeout (ms).
  • Scan rate → Polling interval.

Note: The figure 6 is an example. It does not match the settings on figure 5.

Basic MODBUS query settings
Fig. 5: MODBUS poll: Basic MODBUS query settings.

Basic MODBUS query settings
Fig. 6: Logger: Basic MODBUS query settings.

4. Add only one response item by clicking the "Action → Add response item" button if it does not exist in the list (fig. 8).

  • Name - any name of this value.
  • Offset - it is the byte offset in response data without a device address, function code and data size. Set this value to "-1", and the program will calculate an offset automatically.
  • Count - the number of sequential values with the identical data type and name. This value depends on "Registers to read" and "Data type". One register contains 16 bits. So, Count = Registers to read * 16 / Bits in value.
  • Append counter - it is used when "Count" is greater than 1.
  • Data type - copy the data type from the "Display" menu (fig. 7).
  • Unsigned - enable this option if a data type in MODBUS Poll is unsigned.
  • Little-endian - it is the same as the "Little-endian" option in MODBUS Poll.
  • Swapped - it is the same as the "Byte swap" option.

Note: The figure 8 below is an example. It does not match response items on fig. 5 and 7. The correct definition for the "Signed 32 bit, big-endian" data type in MODBUS Poll:

  1. Name: VALUE, Offset: -1, Append counter: On, Count: 5, Data type: Decimal 32 bit, Unsigned: Off, Little-endian: Off, Swapped: Off.

MODBUS Poll: Display
Fig. 7: MODBUS Poll: Display.

MODBUS response items
Fig. 8: MODBUS response items.


How can I poll multiple MODBUS RTU devices?

MODBUS RTU allows you to connect up to 254 devices on one RS-485 bus. You may specify several addresses like 1,2,3,4,5 in the "Device address" field (fig. 3).

Can I poll MODBUS TCP devices?

Yes, of course. The settings are almost identical. You should configure the TCP connection settings and select other parser type (MODBUS TCP instead of MODBUS RTU on step 2).

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