COM (communication port) is the original, yet still common, name of the
serial port
A serial port is a serial communication Interface (computing), 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 Pa ...
interface on
PC-compatible computers. It can refer not only to physical ports, but also to
emulated 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 li ...
or
USB adapters.
History
The name for the COM port started with the original
IBM PC
The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first microcomputer released in the List of IBM Personal Computer models, IBM PC model line and the basis for the IBM PC compatible ''de facto'' standard. Released on ...
. IBM had called the four well-defined communication
RS-232
In telecommunications, RS-232 or Recommended Standard 232 is a standard introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a ''DTE'' (''data terminal equipment'') such as a compu ...
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
MS-DOS
MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few op ...
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 ...
. 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
In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted ...
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 send information out on COM1, it writes to
I/O port , 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 ,
IRQ 4
*COM2: I/O port , IRQ 3
*COM3: I/O port , IRQ 4
*COM4: I/O port , IRQ 3
Implementations
Image:FTDI USB SERIAL.jpg, USB
Universal Serial Bus (USB) is an industry standard, developed by USB Implementers Forum (USB-IF), for digital data transmission and power delivery between many types of electronics. It specifies the architecture, in particular the physical ...
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
See also
*
Device file
In Unix-like operating systems, a device file, device node, or special file is an interface to a device driver that appears in a file system as if it were an ordinary file. There are also special files in DOS, OS/2, and Windows. These s ...
*
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
*{{Webarchive , url=https://web.archive.org/web/20170913121748/http://www.thaiio.com/serialportinfo.html , title=How to Interface Hardware in COM ports , date=2017-09-13
Computer buses