Modbus TCP Client to RTU Slave Gateways

Your Ethernet Controller Needs to Reach Serial Field Devices

When a modern PLC, PAC, or software controller operates as a 
Modbus TCP Client on an Ethernet network actively initiating 
requests and polling data but the field devices it needs to 
reach are Modbus RTU Slaves on a serial RS-485 bus, a Modbus 
TCP Client to RTU Slave gateway is the correct solution.

The gateway acts as the bridge between these two roles: it 
receives Modbus TCP requests from your Ethernet controller, 
translates them into Modbus RTU commands, polls the serial 
slave devices on the RS-485 network, and returns the responses 
back to the TCP Client over Ethernet all transparently and 
without any changes to your existing field wiring or serial 
device configuration.

Understanding Client and Slave Roles

In modern Modbus terminology, Client is equivalent to Master 
and Server is equivalent to Slave. A Modbus TCP Client to RTU 
Slave gateway therefore connects a device that initiates 
requests on the Ethernet side to devices that only respond 
on the serial side. This distinction matters when configuring 
the gateway: the Ethernet port operates in Client mode, 
actively connecting to the gateway's TCP interface, while 
the serial port operates as RTU Master internally, polling 
the connected slave devices on the RS-485 bus on behalf of 
the Ethernet controller.

Typical Use Cases

This gateway configuration is used when a new Ethernet-based 
PLC or controller must integrate with existing RS-485 serial 
instruments such as energy meters, temperature controllers, 
drives, and flow meters already installed in the plant, when 
a SCADA or MES system operating as Modbus TCP Client needs 
direct access to serial slave devices across multiple 
production lines, when replacing a legacy serial master 
controller with a modern Ethernet-based system while keeping 
all existing field wiring intact, and when building new 
automation systems where the control layer runs on Ethernet 
but field instrumentation uses serial Modbus RTU.

Engineering Support from Archonwell

Correctly mapping TCP Client requests to RTU Slave addresses 
requires careful planning of Slave IDs, register maps, and 
polling intervals especially when multiple serial devices 
share a single RS-485 bus. Archonwell's engineering team 
provides full application support from device selection and 
network design through to on-site installation and 
commissioning across Thailand.