COM (communication port) is the original, yet still common, name of the
serial port
In computing, a serial port is a serial communication interface through which information transfers in or out sequentially one bit at a time. This is in contrast to a parallel port, which communicates multiple bits simultaneously in paralle ...
interface on
PC-compatible computers. It can refer not only to physical ports, but also to
emulated
In computing, an emulator is hardware or software that enables one computer system (called the ''host'') to behave like another computer system (called the ''guest''). An emulator typically enables the host system to run software or use peri ...
ports, such as ports created by
Bluetooth
Bluetooth is a short-range wireless technology standard that is used for exchanging data between fixed and mobile devices over short distances and building personal area networks (PANs). In the most widely used mode, transmission power is limit ...
or
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
In telecommunications, RS-232 or Recommended Standard 232 is a standard originally introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a ''DTE'' ('' data terminal equipment'') suc ...
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. 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, though some of them do still include a COM
header on the motherboard.
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.
I/O addresses
The COM ports are interfaced by an integrated circuit such as
16550 UART
The 16550 UART (universal asynchronous receiver/transmitter) is an integrated circuit designed for implementing the interface for serial communications. The corrected -A version was released in 1987 by National Semiconductor. It is frequently u ...
. This
IC has seven internal 8-bit
registers which 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 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:
*COM1:
I/O port 0x3F8,
IRQ 4
*COM2: I/O port 0x2F8, IRQ 3
*COM3: I/O port 0x3E8, IRQ 4
*COM4: I/O port 0x2E8, IRQ 3
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
Isa or ISA may refer to:
Places
* Isa, Amur Oblast, Russia
* Isa, Kagoshima, Japan
* Isa, Nigeria
* Isa District, Kagoshima, former district in Japan
* Isa Town, middle class town located in Bahrain
* Mount Isa, Queensland, Australia
* Mount Is ...
card with one 25-pin COM port
See also
*
Device file
*
Input/output base address
*
Parallel port
References
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
{{Commons category multi, RS-232, DE-9 connectors
How to Interface Hardware in COM ports
Computer buses