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Using Unserver to Connect to PLCs over Modbus RTU

What is Unserver?

[Unserver](https://unserver.xyz) 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]

Using Unserver to Connect to PLCs over Modbus RTU

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](https://unserver.xyz).

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:

“`json

“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:

“`json

“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:

“`json

[

{

“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:

“`html

var unserver = new Unserver('http://localhost:9000');

unserver.startPolling(1000);

var value = unserver.watchProperty('getting-started.test-property', function(value){

document.write('tag value: ' + value);

});