Connecting Modbus RTU sensors/meters/devices (legacy)

Owned by Vladimir Tayzhev
Last updated: Mar 19, 2024
3 min read

Latest Vutlan drivers (from August 2021) support Modbus RTU read and write functions for all monitoring systems.

Physical description

VT336, VT855i, VT960i, VT960ii monitoring and control systems have:

"RS-485 MODBUS RTU" - port for connecting Modbus RTU / RS-485 sensors and devices. Read instructions at "Connecting Modbus devices", "Configuring Modbus devices".

  • "RS-485 Status" - LED for displaying Modbus RTU port status.

By default, the terminator on the board of VT336 and VT336PoE is always ON. The cap is inserted between the two contacts providing 120R resistance. The terminator is a J16 connector on the board.

 

Connecting Modbus RTU sensors:

Example 1: Daisy chain connection (the main controller is located at the beginning of the chain)

  • Sensors are connected in a daisy chain connection.

  • 120Ω resistor is used at the end of the chain which is equal to the resistance of the cable.

  • A twisted pair cable is used for A(+) and B(-).

  • "VT336 and VT336PoE / DIN monitoring units" have an internal termination and by default, it is switched to ON position.

  • Up to 32 devices can be used.

  • Overall cable length is no more than 1000 meters.

 

Example 2: (Not recommended) Daisy chain connection (the main controller is located in the middle of the chain)

"VT336 and VT336PoE / DIN monitoring units" have an internal termination and by default, it is switched ON.

  • Switch off the Vutlan monitoring unit.

  • Open the enclosure’s bottom and pull the mainboard from the enclosure.

  • Find a Modbus jumper and take the cap off.

  • Put everything back together.

Now it is possible to connect the Modbus sensors in a daisy chain with the Vutlan monitoring unit in the middle of the chain.

  • Sensors are connected in a daisy chain connection.

  • 120Ω resistor is used at the ends of the chain which is equal to the resistance of the cable.

  • A twisted pair cable is used for A(+) and B(-).

  • Up to 32 devices can be used.

  • Overall cable length is no more than 1000 meters.

 

Activation and Configuration of Modbus interface:

After all Modbus devices have been connected to it, you need to activate it and configure: Creating virtual Modbus RTU element

Accessories:

You can use a range of Sensors/Meters/Devices for Modbus.