Using Unserver to Connect to PLCs over Modbus RTU | PLC, PLC LADDER, PLC EBOOK, PLC PROGRAMMING,

Using Unserver to Connect to PLCs over Modbus RTU

What is Unserver?
[Unserver]( is a software tool designed to simplify communication
with Modbus RTU networks.It's useful as a data acquisition layer for creating HMIs and testing Modbus networks.

How it Works?
Unserver provides an HTTP API over a Modbus network.
The API allows clients to work with Modbus data using predefined objects called "tags".
The user creates tags during configuration.
![HMI applications connect to Modbus networks via Unserver]

Each tag is a group of properties, which correspond to specific Modbus addresses.
Each property is defined with a specific data type, for example int16, BCD16 or float32.
When a client requests read/write of a tag, Unserver "translates" it into appropriate Modbus requests.

![Unserver automatically creates HTTP API based on configuration]

Getting Started
·         First, download the latest build at [Unserver official site](
The trial/non-commercial license of Unserver is free.

·         To install Unserver, simply extract the installation package.

·         Now open "network.json" in a text editor to configure the correct serial port used for Modbus connection:

"port": {
    "name": "COM1",
    "baudRate": 9600,
    "dataBits": 8,
    "parity": "none",
    "stopBits": 1,
    "timeoutMs": 500

·         In the same file specify the address of one or more of your Modbus devices:

"devices": [
      "alias": "plc1",
      "address": "1"

All configuration files use JSON format, which is an easy and programmer-friendly way of specifying text-based configuration.

·         The next step is creating some tags.
Open "tags.json" and review the default tag configuration, which looks like this:

    "name": "getting-started", "device": "device1",
    "properties": [
      { "name": "test-property", "address": "HR0", "type": "numeric", "raw": "int16" }

·         This configuration creates a tag named `getting-started` with one property - `test-property`.The property is mapped to the Modbus address `HR0` as a 16-bit signed integer.

·         You can edit the "address" setting if you wish to use another Modbus register.

·         Make sure to save both configuration files before continuing.

Test Your Tag

·         Now we are ready to run Unserver.

·         First, start `unserver.exe`. During the first run it will ask your permission to bind to an http endpoint.If the configuration is valid, you should not see any error messages in the console.

·         Now make sure you PC is connected to the Modbus device via the port you specified in `network.json`
·         Open a web browser and navigate to http://localhost:9000/tags/getting-started.You should now see the value of the tag in the browser window.

Build a Test Web Page

Create a file named `index.html` with the following content:


This creates an empty web page

The script will call Unserver and update the page every time a new value of `test-property` is received.

What's Next

We have covered the basics of getting started with Unserver.

To use more advanced data acquisition features:

- View the [complete documentation](

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