RS485 vs RS232 is the comparison every integrator meets on the plant floor: they are the two serial interfaces industrial devices still use most for serial communication, and getting them onto Ethernet is how their data finally reaches a monitoring platform. Neither is an Ethernet port — both are serial electrical levels — so the bridge between them and your network is a serial device server, and an industrial switch carries the traffic from there.
RS232 is point-to-point and short (under 15 m); RS485 is differential, multi-drop, and reaches 1200 m — that is why RS485 won on the factory floor.
A serial device server wraps the serial stream in TCP/IP; an industrial switch then aggregates many of those streams and uploads them.
Termination resistors, isolation, slave addresses, and baud rate cause more field failures than the hardware choice itself.
RS232 is the oldest point-to-point serial link — one PC COM port to one device, one transmitter, one receiver. Its signal is single-ended, so it rejects interference poorly, its distance is capped near 15 m, and its speed is limited. You still find it on legacy instruments and debug ports.
RS485 uses a differential signal on two wires (A/B), which shrugs off common-mode noise. One bus carries many devices (32 nodes by the standard; 256 with repeaters or high-impedance transceivers) and reaches up to 1200 m. It is the workhorse serial interface in the field: half-duplex, with every device taking its turn on the same pair.
Both are serial voltage levels used for serial communication on the floor — TX/RX or A/B lines — and not the same thing as an RJ45 Ethernet port. You cannot plug either straight into a switch.
| Dimension | RS232 | RS485 |
|---|---|---|
| Topology | Point-to-point, 1 to 1 | Multi-drop bus |
| Max nodes | 2 | 32 / 256 |
| Distance | ≤ 15 m | ≤ 1200 m |
| Signal | Single-ended | Differential (A/B pair) |
| Noise immunity | Weak | Strong |
| Typical use | Legacy instruments, debug ports | PLC, meters, access control, sensors |
On the floor, PLCs, meters, access controllers, and temperature or humidity sensors are often pure serial devices — data you could only read by standing next to the cable with a laptop. But the server room, the monitoring platform, and the cloud all speak Ethernet and TCP/IP.
Tip
The point of serial-to-Ethernet conversion is remote read, centralized collection, and cloud analytics — not wiring a laptop to every device. A typical case: dozens of RS485 smart meters in a distribution room share one bus, and one serial device server sends the whole bus up to the monitoring network.
A serial device server (Serial-to-Ethernet) takes RS485 or RS232 on one side and presents an RJ45 Ethernet port on the other, packaging the serial data into TCP/IP. The common pattern is Modbus RTU over TCP — the Modbus RTU frame is dropped unchanged into a TCP segment, and the host polls devices as Modbus TCP.
Besides Modbus conversion, the device server offers a transparent mode: the raw byte stream from the serial port is wrapped in one TCP connection and sent on, and the host parses its own private protocol. That mode suits older non-Modbus equipment especially well.
Termination resistor. The two ends of an RS485 bus need 120 Ω termination to match the impedance. Miss it or fit it wrong and long-distance communication turns flaky — works, then fails, then works.
Grounding and isolation. Different devices often sit at different ground potentials, and the A/B lines pick up ground-loop noise. An isolated serial device server and opto-isolated transceivers block most of it.
Address conflict. Every device on one RS485 bus needs a distinct slave address; Modbus polls by address. Two devices with the same address and the polling falls apart.
Baud rate and parity. The whole bus must agree on baud, data bits, stop bits, and parity. One device set wrong and the entire bus misbehaves.
Can RS485 connect directly to a switch port?
No. RS485 is a serial differential level and a switch is RJ45 Ethernet — different physical layers. Something in between, a serial device server or gateway, has to do the conversion.
How many devices on one RS485 bus?
The standard driver handles 32 nodes. In real projects leave margin and stay around 16–24 for stability; reaching the full 256 needs high-impedance transceivers or repeaters that segment the bus.
What is the difference between Modbus RTU and Modbus TCP?
RTU — often written as modbus rs485 — runs on serial (RS485/RS232) and is polled by address; TCP runs on Ethernet and is addressed by IP and port. The frame format differs but the register meaning is the same. The serial device server performs that conversion.
What about a serial device server outdoors?
Pick an industrial-grade, wide-temperature (−40 to 75 °C), isolated model and house it with the industrial switch in a protected enclosure. A desktop unit will not last long in a distribution room or pump station.
Written by Sara, Customer Manager at Rayin — 10+ years in industrial communications. She helps integration engineers choose and deploy switching and PON equipment.
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