COM (hardware interface)

History

COM port came with the original IBM PC. IBM called the four well-defined communication RS-232 ports the "COM" ports, COM1 through COM4. In BASICA and PC DOS these ports can be opened as "COM1:" through "COM4:", and all PC compatibles using MS-DOS used the notation. Most PC-compatible computers in the 1980s and 1990s had one or two COM ports.

By 2007, most computers shipped with one or no physical COM ports. Today, few consumer-grade PC-compatible computers include COM ports, though some include a COM header on the motherboard.

After the RS-232 COM port was omitted from consumer-grade computers, adapters from remaining interfaces such as PCI Express and USB could be used to install a COM port. One supplier chips for external USB-to-UART serial adapter cables is FTDI.

I/O addresses

The COM ports are interfaced by an integrated circuit such as 16550 UART. This IC has seven internal 8-bit registers that hold information and configuration data about which data is to be sent or was received, the baud rate, interrupt configuration and more. In the case of COM1, these registers can be accessed by writing to or reading from the I/O addresses to .

If the CPU, for example, wants to output information on COM1, it writes to I/O port , as this I/O port is "connected" to the UART IC register that holds the information.

Implementations

Image:FTDI USB SERIAL.jpg|USB to RS-232 adapter with one 9-pin COM port (FTDI US-232R) Image:RS232 PCI-E.jpg|PCI-E card with one 9-pin COM port Image:Tarjeta PCI con 2 puertos serie RS-232.jpg|PCI card with two 9-pin COM ports Image:IBM PC Serial Card.jpg|ISA card with one 25-pin COM port

References

References

1. (August 14, 2008). "Configuring a communication port". IBM Lotus Domino and Notes Information Center.
2. Stephen Byron Cooper. "What Is a Com1 Port?".
3. "Serial port".
4. "Motherboard Port Guide: Solving Your Connector Mystery".
5. "22.2.3. Resources Required by Serial Ports - PC Hardware in a Nutshell, 3rd Edition [Book]".