Creating virtual Modbus RTU element

Connecting Modbus devices

Related articles are:

Connecting Modbus devices

Connecting "VT85 / Modbus extension board"

Connecting Modbus sensors using "RS485 to USB adapter"

Description

Virtual "Modbus RTU" sensor is used to read data from external equipment via Modbus RTU protocol (RS-485 line). An external converter to the USB bus is used to provide RS-485 communication.

Activating Modbus

After connecting RS485 bus, according to the "Slave" hardware documentation, you can now activate the Modbus.

The Modbus operates in "Master" mode. To enable the Modbus, go inside the web interface of Vutlan monitoring unit and go to Preferences menu >> Modbus RTU. Set up the bus according to the documentation for the "Slave" equipment. Please note "VT85 / Modbus extension board"  only support 1 stop bit.


Adding the Modbus virtual element

To create a "Modbus RTU" element, press add button "  " inside "Group tree" or "System tree" menu. Then choose "Modbus RTU". A modal window will appear:

Click on the "Modbus RTU". Modbus element form will be opened:

Fields in the form available for change:

#NameDescription
1NameCreated element name
2User defined typeDefines the icon to be displayed in the "System tree"
3Units of measurementThe unit of measure in which the value is displayed in the "System tree"
4Minimum valueDetermines the minimum measurement range, when the value is out of range, the sensor goes into "Not connected" state
5Maximum valueDetermines the maximum measurement range, when the value is out of range, the sensor goes into "Not connected" state
6Alarm and warning levelsDefines the alarm and warning thresholds, as for other sensors, see Sensor configuration
7HysteresisOption of setting the hysteresis state, see Sensor configuration
8Expression

Function of the form f (x). The default value for the sensor is equal to the measured value: "x", ie corresponds to the expression "x". To calculate the indirect value of the sensor is possible to use an arbitrary expression, which are permissible "(", ")", as well as:

    • operators: "+", "-", "*", "/", "%" (remainder of the division), "^" (exponentiation);
    • functions: "abs","sqrt","exp","ln","log","sin","cos","tan","asin","acos","atan";
    • constants: "pi" (3.1415926...), "e" (2.7182818...)

        For example: "0.1*x+0.5"


The Modbus RTU bus configuration available in "Additional" tab:

Fields in the form available for change:

#NameDescription
1Slave addressModbus RTU slave address of external equipment
2Register index

Modbus protocol register start addres in hexademal view, like 0046

3Function codeThe following functions are supported:
  • 0x01 - Read Coil Status
  • 0x02 - Read Input Status
  • 0x03 - Read Holding Registers
  • 0x04 - Read Input Registers
4Data type

Determines how to present the data:

  • 16 bits signed int
  • 16 bits unsigned int
  • 32 bits signed int
  • 32 bits unsigned int
  • 32 bits IEEE floating point
5Data ordering

Determines byte order in Modbus protocol response message:

  • low byte first, low word first
  • low byte first, high word first
  • high byte first, low word first
  • high byte first, high word first
6Polling periodDefines the time of polling the sensor value

The Test button allows you to make a test connection with a modbus device. In additional fields, the bytes sequence of connection data is displayed, in hexadecimal form. Modbus request - data of the sent request, without a checksum. Modbus raw answer - data of the received answer, without a checksum. Modbus value - contains the data value interpreted in accordance with the selected Data type.

Example, adding input register "Active power" for Eastron SDM220 Modbus slave

For this example we will be using "Eastron SDM220-Modbus Single Phase Two Module DIN rail Meter" as a slave Modbus device.

SDM220ModbusEN.pdf

SDM220Modbus_protocol_V1.1.pdf

Let's create a "Modbus RTU" element for input register "Active power", which is measured by SDM220.

Press add button "  " inside "Group tree" or "System tree" menu. Then choose "Modbus RTU". A Modbus element form will be opened:

Let's change values in Setting tab:

#NameNew value
1Name

We are adding input register "Active power",

so we write: Active power

2User defined typeLet's choose "power" for a nice icon of a meter.
3Units of measurement

"Active power" is measured in Watts.

so we write: Watts

4Minimum value, Maximum value, Alarm and warning levelsDefine it according to Your needs
7HysteresisDefine it according to Your needs
8Expressionx


Now let's change the values in Additional settings tab tab:


#NameDescription
1Slave address

on page 2 of SDM220ModbusEN.pdf it is stated in the table in row 16 that:

"How to find Modbus address. By default it is 001"

But in our case it is different, a slave address has been manually altered. So,after following the procedure and navigating to slave address onscreen instruction, we find out that the Salve address for this device is currently set to 31 in our example. It is advised to have have a slave address according to position in a Modbus chain. So if the sensor is currently 1st in a Modbus chain, set it to 1.


So we write 31.

2Register index

on page 2 of SDM220Modbus_protocol_V1.1.pdf it is stated in the table that:

"Hi Byte = 00

+

Lo Byte = 0C"


So we write 000C
3Function code

on page 2 of SDM220Modbus_protocol_V1.1.pdf it is stated that:

"Modbus Protocol function code 04 is used to access all parameters."

So we write:

0x04 - Read Input Registers

4Data type

on page 1 of SDM220Modbus_protocol_V1.1.pdf it is stated that:

"The format for each byte in RTU mode is:

Coding System: 8-bit per byte

Data Format: 4 bytes (2 registers) per parameter.

Floating point format ( to IEEE 754)"


or


on page 2 it is stated that:

"the data used by the Eastron Digital meter is in 32 bit IEEE 754 floating point format."


So we write:

32 bits IEEE 754 floating point

5Data ordering

on page 1 of SDM220Modbus_protocol_V1.1.pdf it is stated that:

"Most significant register first (Default). The default may be changed if

required -See Holding Register "Register Order" parameter."


So we write:

low byte first, low word first

Save the virtual Modbus element.

Now You can add next virtual Modbus element using register index for the same Eastron SDM220 Modbus slave. For example, Current, Apparent power, Reactive power and so on.

If You decide to add next Modbus slave device, make sure that it has a different slave address, different from other slave devices in the chain.








Developer notes: