RS-232 is a long-established serial communication standard, originally published in 1960, that defines how data is exchanged between two devices over a serial cable. Despite its age, RS-232 remains in everyday use across industrial equipment, scientific instruments, PLCs, point-of-sale systems, building services, and legacy equipment that has outlasted the original interface assumptions.

In short: RS-232 is a point-to-point standard: one device talks to one other device, over a cable typically up to 15 metres in length. Common connector types include the DB9 (9-pin D-sub) and the DB25 (25-pin D-sub), with DB9 being far more common today. Data is transmitted asynchronously at speeds typically from 9600 baud to 115200 baud, though both higher and lower rates are widely used. RS-232 supports software flow control (XON/XOFF), hardware flow control (RTS/CTS, DTR/DSR), or no flow control depending on application.

For IoT and industrial deployments, RS-232 is one of the most common ways for a cellular router to connect to legacy equipment that does not speak Ethernet. An industrial router with an RS-232 port can read or write data to a PLC, a barcode scanner, an environmental sensor, or a serial-only piece of equipment, and forward that data over MQTT, Modbus TCP, or a custom protocol to a cloud platform. Teltonika RUT and TRB routers, Proroute H685 series, and most other industrial cellular routers offer RS-232 support as standard or optional.

The practical distinction between RS-232 and RS-485 is important. RS-232 is point-to-point and short-range. RS-485 (a related standard) is multi-drop, supports up to 32 devices on one bus, and runs over longer distances. For new installations connecting multiple devices, RS-485 is almost always the right choice. RS-232 remains common in retrofit applications where the existing equipment is fixed.