Plenty of field instruments and controllers still speak only serial - RS-232 or RS-485 - because they were installed long before Ethernet reached the pad. A serial-to-Ethernet converter bridges that gap, putting a serial device on an IP network so a modern SCADA can reach it. This guide explains what the device does and how it is used.
Serial-to-Ethernet Converter in one line: A serial-to-Ethernet converter, also called a serial device server, is a device that connects an RS-232 or RS-485 serial device to an Ethernet/IP network. It wraps the serial data in TCP or UDP packets in one direction and unwraps them in the other, letting software reach a legacy serial device over the network as if it were locally attached.
A serial-to-Ethernet converter has a serial port on one side and an Ethernet port on the other. Bytes that arrive on the serial port are encapsulated into IP packets and sent out the network port; packets arriving from the network are stripped back to raw bytes and clocked out the serial port. To the serial device, the converter looks like whatever was on the other end of the cable; to the network, the serial device becomes an IP endpoint with its own address and port.
Most converters offer a few operating modes. Raw TCP (or TCP server) mode simply tunnels bytes over a TCP socket - the SCADA opens a connection to the converter's IP and port and reads the serial device directly. Real-COM or virtual-COM mode installs a driver on a host so the serial device appears as a local COM port over the network, useful for software that only knows how to open a COM port. UDP mode is used where low overhead matters more than guaranteed delivery.
The converter moves bytes; it does not usually understand the protocol riding on them. If a serial device speaks Modbus RTU, the converter tunnels those RTU frames unchanged over TCP - the SCADA must still speak Modbus RTU, just wrapped in a socket. This is different from Modbus TCP, which is a distinct framing. Some smarter gateways do translate Modbus RTU to Modbus TCP; a plain serial-to-Ethernet converter typically does not, and mixing the two up is a common commissioning mistake.
RS-485 adds a wrinkle: it is a multidrop bus where several devices share one pair of wires and take turns. A converter serving an RS-485 segment must manage the timing of when it drives the line versus listens, and the master (the SCADA or its poller) must still poll each device address in sequence. The converter puts the whole bus on the network, but it does not remove the need to poll addresses one at a time.
Field sites are full of long-lived serial equipment - older flow computers, gas chromatographs, tank gauges, RTUs, and analyzers - that will run for years yet. Rather than replace working instruments, operators drop in a serial-to-Ethernet converter so those devices join the site's Ethernet network and become reachable from a remote head end. It is one of the cheapest ways to bring legacy assets into a modern monitoring scheme.
A cloud SCADA such as Merobix connects to a serial device through this bridge: the converter exposes the device at an IP and port, and Merobix polls it over Modbus (RTU tunneled or TCP, depending on the setup) or DNP3, exactly as if it were an IP-native controller. The converter is what lets a decades-old serial instrument appear on a browser dashboard alongside brand-new equipment.
A serial-to-Ethernet converter changes the protocol and framing - it puts serial data onto an IP network. A media converter keeps the same protocol but changes the physical medium, for example Ethernet copper to Ethernet fiber. One bridges serial to IP; the other bridges copper to fiber.
Usually not. A plain converter tunnels the raw serial bytes - including Modbus RTU frames - over a TCP socket, so the SCADA still speaks Modbus RTU. Only a protocol gateway that specifically advertises RTU-to-TCP translation converts the framing. Check the datasheet before assuming.
Yes. RS-485 is a multidrop bus, so a converter on that segment exposes all the devices sharing those wires. The master must still poll each device address in turn - the converter puts the whole bus on the network but does not let you address several devices simultaneously.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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