A ModbusTCPGateway is a network-level object that also represents a particular device: a Modbus TCP-to-Serial gateway, where this device has an IP address reachable by the station. On the gateway’s “far side” are serially-connected Modbus devices (typically Modbus RTU via RS-485), as shown in Figure 14. Those serial Modbus devices (RTU or ASCII) are represented by child devices under the gateway (network) object.

In addition to the IP address and TCP port used by the gateway, there are global properties on the network’s property sheet specific to Modbus, for example the default order for “float” and “long” numeric data (overrideable within each child device).

The station acts as Modbus master to the serially-connected Modbus devices on the gateway’s far side. Each child device is represented by a ModbusTcpGatewayDevice, and has a unique Modbus address (1 to 247), as well as other Modbus config data and starting addresses for Modbus data items (coils, inputs, input registers, holding registers). There are typically many child ModbusTcpGatewayDevices.

Modbus Tcp Gateway configuration is straightforward, using the property sheet of the network object.