Modbus TCP Channel Tag

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The Modbus TCP Channel Tag Channel Tag Icon provides a TCP/IP SCADA communication channel to Modbus Devices.

Configuration Properties

The Modbus TCP Channel Tag supports the properties of the Active Channel Tag plus these properties:

Host WSTRING 1.0 The network address of the Modbus Device. This can be any of the following:
  • An IP Address. For example:
  • A node name. For example: MODBUSDEVICE
  • A fully qualified domain name. For example:
Port WSTRING 1.0 The port number used for communications. The default port number for Modbus is 502.
Protocol USINT 1.0 The variant of Modbus TCP to use:
  1. Modbus TCP
  2. Modbus RTU over Modbus TCP
  3. Modbus RTU over TCP
For most applications use the default: Modbus TCP.
TransactionIdMin UINT 3.89 The minimum value to use for the Modbus TCP Transaction Id.
TransactionIdMax UINT 3.89 The maximum value to use for the Modbus TCP Transaction Id.
GatewayMode BOOL 3.99 Whether the TCP Connection is a gateway to other Modbus Devices.
  • False: A connection to a single device.
  • True: A connection to a gateway that communicates with other devices.

Protocol Property

The Protocol property defines which variant of Modbus to use on the communication channel:

Modbus TCP Standard Modbus TCP.
Modbus RTU Over Modbus TCP A rarely used and non-standard variant of Modbus TCP that includes a Modbus RTU CRC at the end of the message.
Modbus RTU Over TCP The standard Modbus RTU protocol is sent over a TCP communication channel. Use this variant if you are using a TCP to serial port adapter.

TransactionIdMin and TransactionIdMax Properties

Each Modbus TCP message sent by Fernhill SCADA includes a 16-bit Transaction Id. Fernhill SCADA uses the Transaction Id to match responses with requests. By default, Fernhill SCADA uses the full range of Transaction Id values 0 - 65535.

To restrict Fernhill SCADA to use a limited range of Transaction Id values, enter the range in the TransactionIdMin and TransactionIdMax Properties.

These properties are not used if the Protocol property is set to Modbus over TCP.

GatewayMode Property

The GatewayMode property controls whether a reply timeout disconnects the TCP socket.

Setting GatewayMode to True helps when:

In this scenario communication with the remaining online Modbus Devices will continue without interruption.

Tag Commands

The Modbus TCP/IP Channel Tag supports the tag commands of the Modbus Channel Tag.

Example Configuration

This is an example of a channel configured to support Modbus TCP protocol to host :

ProtocolModbus TCP

Relationships to Other Tag Types

The Modbus TCP/IP Channel Tag builds on the features of these tag types:

Further Information

Modbus Protocol

To learn more about the Modbus Protocol.

Active Channel Tag

To learn about the properties common to all active channels.

Modbus serial channel

To learn about Modbus Serial Channels.

Modbus Device Tag

To learn about Modbus Device Tags that can be associated with Modbus TCP Channel Tags.

Alarm Conditions

To learn about abnormal conditions in SCADA applications.


For the meaning of terms used in Fernhill SCADA.