Function and purpose
This is an expansion board that must be purchased separately.
It enables the monitoring of up to 32 Modbus RTU devices/counters/sensors. The maximum daisy-chain distance is 1000 meters. Each measured value from a Modbus device is individually addressed via input registers and displayed by the system as a virtual sensor. You can configure thresholds and symbol representations for these elements.
The expansion board can be hot-swapped into the monitoring unit. It has an internal termination resistor and is enabled by default (it can be disabled if required).
This module is compatible with: 100T, 300T, 500T, and 550T
Physical description
By default, the termination resistor on the PCB of 100T, 500T and 550T is always enabled.
a.) "RS-485 Status" – LED to indicate Modbus RTU port status.
b.) "B-, A+, GND" – Port for connecting Modbus RTU devices.
Description: Modbus RTU extension specifications.
| Function & Purpose | Description |
|---|---|
| Device type: | Extension PCB slot. |
| Purpose: | For connecting industrial Modbus RTU devices, sensors, systems & meters. |
| Communication | Description |
|---|---|
| Control: | In the Didactum web interface. |
| Interface: | Modbus RTU, RS485. |
| Maximum distance: | Up to 1000 m. |
| Connections: | 3P terminal block (B-, A+, GND), 3.5 mm pitch. |
| Terminator: | On PCB, enabled by default. |
Example 1
Series Connection (Main Control Unit at the Beginning of the Chain)
The sensors are connected in series.
At the end of the chain is a 120 Ω resistor whose resistance matches the cable resistance. A twisted pair of wires is used for A(+) and B(-). The 100T, 500T, and 550T monitoring units have an internal terminating resistor that is enabled by default. (It can be disabled if needed.) Up to 32 devices can be connected. The maximum cable length is 1000 meters.
Description: Modbus cable pinout for Eastron SDM220-MODBUS.
| Cables | Eastron SDM220-MODBUS |
|---|---|
| B- | red |
| A+ | blue |
| GND | white |
Example 2:
Example 2: (Not recommended)
Series connection (main controller in the middle of the chain)
The 100T, 300T, 500T, and 550T monitoring units have an internal termination resistor that is enabled by default.
- Switch off the Didactum monitoring unit.
- Open the bottom of the housing and remove the main board.
- Locate a Modbus jumper and remove the protective cap.
- Reassemble everything.
The Modbus sensors can now be connected in series to the Didactum monitoring unit in the middle of the chain.
- The sensors are connected in series.
- A 120 Ω resistor is used at each end of the chain to match the cable resistance.
- A twisted pair of wires is used for A(+) and B(-).
- Up to 32 devices can be connected.
- The total cable length is a maximum of 1000 meters.
Activation and configuration of the Modbus interface
A virtual Modbus RTU sensor is used to read and write data from external devices via the Modbus RTU protocol (RS-485 interface). These devices can include sensors/measuring instruments or other systems/units.
Some Didactum devices have a built-in Modbus RTU port. For others, a separately available expansion module is required. Some older Didactum devices use an external converter to enable RS-485 communication via the USB bus.
Activation
After connecting the RS485 bus according to the slave hardware documentation, you can now activate Modbus.
Modbus operates in master mode. To activate Modbus, access the web interface of the Vutlan monitoring unit and select "Modbus RTU" from the "Settings" menu. Configure the bus according to the slave device documentation.
Please note that the VT85/Modbus expansion card only supports one stop bit.
Adding the virtual Modbus reader
To create a "Modbus RTU (Read)" element, click the "Add" button in the "Group Structure" or "System Structure" menu.
Then select "Modbus RTU (Read)". A modal window will appear.
Click on “Modbus RTU (Read)”. The Modbus element form will open:
The following fields in the form can be changed:
Description: Modbus sensor configuration fields.
| # | Name | Description |
|---|---|---|
| 1 | Name | Created element name. |
| 2 | User-defined type | Defines the icon displayed in the "System tree". |
| 3 | Unit of measurement | Unit in which the value is displayed in the "System tree". |
| 4 | Minimum value | Determines the minimum measurement range. When exceeded, the sensor changes to "Not connected" status. |
| 5 | Maximum value | Determines the maximum measurement range. When exceeded, the sensor changes to "Not connected" status. |
| 6 | Alarm and warning levels | Defines alarm and warning thresholds like other sensors. |
| 7 | Hysteresis | Option for setting the hysteresis state. |
| 8 | Expression | Function of the form f(x). Default sensor value is the measured value: "x".
Example: "0.1*x+0.5" |
The Modbus RTU bus configuration is available on the "Additional Information" tab:
Description: Modbus RTU element configuration fields.
| # | Name | Description |
|---|---|---|
| 1 | Slave address | Modbus RTU slave address of the external device. |
| 2 | Register index | Modbus protocol register start address in hexadecimal notation, e.g. 0F4A. |
| 3 | Function code | The following functions are supported:
|
| 4 | Data type | Determines data representation:
|
| 5 | Data ordering | Determines byte order in Modbus protocol response:
|
| 6 | Polling period | Defines sensor value polling time. From 10 seconds to 5 minutes. |
With the "Test" button, you can establish a test connection to a Modbus device. Additional fields display the byte sequence of connection data in hexadecimal format. Modbus request – data of the sent request without checksum. Modbus raw response – data of the received response without checksum. Modbus value – contains the data value interpreted according to the selected data type.
Adding the virtual Modbus write element:
To create a "Modbus RTU (Write)" element, click the "Add" button in the "Group Tree" or "System Tree" menu.
Then select "Modbus RTU (Write)".
The Modbus element form will open.
Fields in the form available for change:
| # | Name | Description |
|---|---|---|
| 1 | Name | Created element name. |
| 2 | Slave address | Modbus RTU slave address of the external device. |
| 3 | Register index | Modbus protocol register start address in hexadecimal notation, e.g. 0F4A. |
| 4 | Function code | The following functions are supported:
|
| 5 | Writing value | Decimal integer value that will be written to the specified register. |
The Test button allows you to immediately write a value to the Modbus register.
The Modbus RTU write operation can now be inserted into logic diagrams as a THEN statement: