COM (hardware interface)

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connector).

COM (communication port)

USB adapters
.

History

The name for the COM port started with the original IBM PC. IBM had called the four well-defined communication RS-232 ports the "COM" ports, starting from COM1 through COM4. In BASICA and PC DOS you can open these ports as "COM1:" through "COM4:", and all PC compatibles using MSDOS used the same denotation.[citation needed] Most PC-compatible computers in the 1980s and 1990s had one or two COM ports.

By 2007, most computers shipped with only one or no physical COM ports. Today, few consumer-grade PC-compatible computers include COM ports,[3] though some of them do still include a COM header on the motherboard.[4]

After the RS-232 COM port was removed from most consumer-grade computers, an external USB-to-UART serial adapter cable was used to compensate for the loss. A major supplier of these chips is FTDI.[citation needed]

I/O addresses

The COM ports are interfaced by an integrated circuit such as

I/O addresses
0x3F8 to 0x3FF.

If the CPU, for example, wants to send information out on COM1, it writes to I/O port 0x3F8, as this I/O port is "connected" to the UART IC register which holds the information that is to be sent out.

The COM ports in PC-compatible computers are typically defined as[citation needed]:

Implementations

  • USB to RS-232 adapter with one 9-pin COM port (FTDI US-232R)
    USB to RS-232 adapter with one 9-pin COM port (FTDI US-232R)
  • PCI-E card with one 9-pin COM port
    PCI-E card with one 9-pin COM port
  • PCI card with two 9-pin COM ports
    PCI
    card with two 9-pin COM ports
  • ISA card with one 25-pin COM port
    ISA card with one 25-pin COM port

See also

References

  1. ^ "Configuring a communication port". IBM Lotus Domino and Notes Information Center. August 14, 2008. Archived from the original on 17 September 2013.
  2. ^ Stephen Byron Cooper. "What Is a Com1 Port?". Retrieved 2021-09-30.
  3. ^ "Serial port". Retrieved 2020-09-14.
  4. ^ "Motherboard Port Guide: Solving Your Connector Mystery". Retrieved 2020-09-14.

Further reading

  • Serial Port Complete: COM Ports, USB Virtual COM Ports, and Ports for Embedded Systems; 2nd Edition; Jan Axelson; Lakeview Research; 380 pages; 2007; .

External links