Connecting Modbus Devices to viewLinc 5.1 - viewLinc 5.1 - VNET

Connecting Modbus devices to viewLinc 5.1 Technical Note

Document code
viewLinc 5.1
Document type
Technical note

Vaisala’s viewLinc Enterprise Server software supports measurement data acquisition from Modbus TCP and RTU devices. This technical note describes methods that will allow Modbus devices to communicate with viewLinc. Adding a Modbus device to viewLinc involves two steps: first establish communication between viewLinc and the device, and then define the device in viewLinc.

Establishing communication Modbus TCP Modbus RTU
The connection process differs depending on whether the device is Modbus TCP or Modbus RTU. Modbus TCP devices communicate over Ethernet. Turn on the Modbus device and connect it to the same network as your viewLinc server. Follow the steps on page two to define the device in viewLinc. Modbus RTU devices communicate serially (typically over RS-485). In order to establish communication between a Modbus RTU device and your viewLinc server, the Modbus device needs to be connected to the network by an Ethernet-to-serial adapter device. Vaisala recommends using a Digi Portserver TS device for this purpose.

Connecting RTU Modbus Devices

In order to connect the Modbus device to a Digi Portserver, you need a cable with appropriate connectors. The Digi Portserver TS uses an RJ45 connector with the pinout below (Source: PortServer® TS Family Quick Start Guide). The pinout on the Modbus device’s side will depend on the connectors available on that device. Please refer to the manufacturer’s documentation for specifications. Different Modbus RTU devices may require different settings or connections, depending on the model and adapter required. Always ensure the device is connected and communicating properly before proceeding.

Review serial pinouts

The following table shows the pin positions for connecting serial devices using either 10-wire or 8-wire connectors.

Pin positions for connecting serial devices
PortServer TS products use an RJ-45 10-wire jack (female), with Pin 1 in the following location. For a 10-wire connector, Pin 1 of the connetor is shown here. For an 8-wire connector, connect the pins to the center 8 pins of the 10-wire jack.

The following table lists the serial port pin assignments for different connector types.

Serial port pin assignments
Pin # on 10-wire connector Pin # on 8 wire connector EIA-232 MEI versions only
EIA-422/485 Full-duplex EIA-485 Half-duplex
1 RI TxD- N/A
2 1 DSR* RxD- DATA-
3 2 RTS RTS+ N/A
5 4 TxD TxD+ N/A
6 5 RxD RxD+ DATA+
8 7 CTS CTS+ N/A
9 8 DTR RTS- N/A

* Use the Altpin setting to swap these two signals.

The CTS and RTS control signals are available as separate differential signals in the EIA- 422/EIA-485 4-wire mode. Do not use these differential signals in 2-wire mode. The CTS and RTS differential signals are not terminated or biased internally. Any termination or biasing must be done externally.