What Is RS-232?

RS-232 is a standard for serial data between two devices, first published in 1960 and now maintained as TIA-232-F. RS stood for Recommended Standard. It defines the voltage levels, timing, signal names and connectors for a link where bits are sent one after another on a transmit line and received on a separate receive line, so data can flow in both directions at once.

Each signal is single-ended: it is measured against a shared ground wire. A logic 1, called a mark, is a voltage between −3 V and −15 V, and a logic 0, called a space, is between +3 V and +15 V. The band between −3 V and +3 V is undefined. Those relatively large swings give RS-232 reasonable noise margins over short cables, but because there is no differential pair, noise and ground differences limit how far it can go.

Inside the computer, a UART chip turns bytes into a serial bit stream, and an RS-232 transceiver converts the logic levels to RS-232 voltages. Both ends must agree on the baud rate, data bits, parity and stop bits, for example 9,600 bit/s, 8 data bits, no parity and 1 stop bit, written 9600 8N1.

Most RS-232 ports today use the 9-pin D-sub connector, often called DB9 (properly DE-9). The table shows the standard pinout on a computer, which RS-232 calls data terminal equipment (DTE).

A device such as a modem is data communication equipment (DCE) and has the directions reversed, so a straight cable joins a DTE to a DCE. Two computers need a crossed cable, covered in our serial port FAQ. Many simple devices use only pins 2, 3 and 5.

The original standard covers data rates up to 20 kbit/s, but modern transceivers commonly run at 115,200 bit/s and some reach 1 Mbit/s. Cable length is limited by capacitance rather than a fixed distance: the standard allows up to 2,500 pF, which works out at roughly 15 m (50 ft) with typical cable. Shorter, low-capacitance cable allows higher speeds or longer runs, and slower rates tolerate more. For longer or noisier runs, RS-422 and RS-485 take over; our RS-232 vs RS-485 FAQ compares them.

Flow control stops one device sending data faster than the other can handle it. RS-232 supports two methods. Hardware flow control uses the RTS and CTS lines: the computer raises RTS when it wants to send, and the device raises CTS when it is ready to receive. Some older equipment also expects DTR and DSR to be active before it will talk at all. Software flow control, often called XON/XOFF, sends special characters in the data stream to pause and resume transmission, which saves wires but cannot be used with binary data that might contain those characters.

Many modern devices use no flow control, relying on small messages and buffers. When a link connects but data is lost or nothing arrives, a mismatch in flow control settings is one of the first things to check.

RS-232 has largely disappeared from office PCs, but it remains common on industrial equipment because it is simple, deterministic and supported by decades of installed machines.

Industrial PCs and panel PCs keep native COM ports because USB-to-serial adapters can change port numbers, add latency and come loose. Look for ports that can be switched between RS-232, RS-422 and RS-485, ESD protection, optional 5 V or 12 V power on pin 9 for small peripherals, and electrical isolation where cables run near motors or drives. On rugged and vehicle systems, serial is often brought out on locking M12 connectors; our M12 A-coded FAQ covers the usual connector. The British Steel case study shows serial on M12 connectors linking panel PCs to plant equipment, and our serial I/O modules add RS-232 ports where a system needs more.

We specify and fit RS-232 ports, cables and connectors on industrial computers as standard, set the port modes in the BIOS and test communication with your equipment before each system ships.