What Is a Serial Port?

A serial port is an interface that transmits data one bit after another over a single signal line in each direction. Inside the computer, a chip called a UART (universal asynchronous receiver-transmitter) turns each byte into a stream of bits framed by a start bit and a stop bit, and the receiving UART rebuilds the bytes at the other end.

Both ends must agree on the speed and the frame format. The usual description is a string such as 9600 8N1: 9,600 baud, eight data bits, no parity bit and one stop bit. Optional flow control, either hardware (RTS/CTS lines) or software (XON/XOFF characters), stops the sender when the receiver’s buffer is full.

The serial port is the opposite of the parallel port, which sends eight bits at once over eight wires. Parallel was faster over short printer cables, but serial needs fewer wires, runs further and is simpler to isolate, which is why it outlasted parallel in industry.

A COM port, short for communications port, is the name the operating system gives a serial port. Windows numbers them COM1, COM2 and so on, and Device Manager lists each one under Ports (COM and LPT). Linux names on-board ports /dev/ttyS0, /dev/ttyS1 and USB adapters /dev/ttyUSB0. The terms are used interchangeably: the serial port is the hardware, and the COM port is how software addresses it.

A USB-to-serial adapter creates a virtual COM port, so older software can still talk to it. Adapters work well for occasional use, but on a production machine a native port avoids driver changes and COM numbers that move when the adapter is plugged into a different USB socket.

The names date from the original IBM PC, which gave the first serial ports fixed resources: COM1 at I/O address 3F8h on interrupt 4 and COM2 at 2F8h on interrupt 3, with COM3 and COM4 at 3E8h and 2E8h sharing those interrupts. Windows now numbers ports from COM1 up to COM256 and remembers each device it has seen, so a computer that has had many adapters plugged in can end up with numbers such as COM17. Older software often opens only COM1 to COM4, and the usual fix is to change the number in Device Manager, under the port’s Port Settings and Advanced options.

Not every COM port is a physical socket. As well as USB adapters, drivers create virtual COM ports for Bluetooth serial links, for modems and GNSS modules that present a serial interface, and for serial-over-network software that maps a remote serial device to a local COM number. Software sees them all the same way, but buffering and handshake lines can behave differently from a native UART, which matters for protocols that depend on exact gaps between characters, such as Modbus RTU.

Serial ports differ in their electrical standard rather than the way data is framed. The same UART can drive any of them through a different transceiver, and many industrial boards let you select the mode per port in the BIOS or with jumpers.

Distances fall as speed rises, so the longest runs assume lower baud rates. Our guide to RS-232 signals and pinouts covers the PC standard, and how RS-485 multi-drop networks work explains the industrial bus.

Consumer PCs dropped serial ports years ago, but factory, transport and utility equipment still depends on them. Serial is simple, deterministic and supported by almost every controller made in the past 40 years.

The port count is only the start. On a long-life installation, the details below decide whether serial links stay reliable.

• Mode per port. Check which ports are RS-232 only and which can switch to RS-422 or RS-485.
• Isolation. Optical or magnetic isolation, typically rated in kilovolts, stops ground loops and surges between buildings or machines damaging the computer.
• Termination and biasing. RS-485 buses need a termination resistor at each end and bias resistors to hold the line in a known state.
• Power on pin 9. Some industrial boards can supply 5 V or 12 V on pin 9 to power a scanner or touch controller, set in the BIOS.
• Connector. DE-9 is standard, but terminal blocks suit RS-485 wiring in a cabinet.
• Speed. Check the highest baud rate each port supports if a device needs more than 115,200 baud, as some industrial ports run at 921,600 baud or faster.

We configure every COM port on the industrial computers we supply as standard, setting the RS-232, RS-422 or RS-485 mode, termination and pin 9 power to suit the devices on site. Send us your device list and we will specify the ports to match.