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 parallel. ...
interface on
PC-compatible computers. It can refer not only to physical ports, but also to
emulated
In computing, an emulator is Computer hardware, 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 so ...
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 limi ...
or
USB adapter
A USB-to-serial adapter or simply USB adapter is a type of protocol converter that is used for converting USB data signals to and from serial communications standards (serial ports). Most commonly the USB data signals are converted to either ...
s.
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'') such a ...
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
Future Technology Devices International Limited, commonly known by its acronym FTDI, is a Scottish privately held semiconductor device company, specialising in Universal Serial Bus (USB) technology.
It develops, manufactures, and supports dev ...
.
I/O addresses
The COM ports are interfaced by an integrated circuit such as
16550 UART. 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
In telecommunication and electronics, baud (; symbol: Bd) is a common unit of measurement of symbol rate, which is one of the components that determine the speed of communication over a data channel.
It is the unit for symbol rate or modul ...
, interrupt configuration and more. In the case of COM1, these registers can be accessed by writing to or reading from the
I/O address
Memory-mapped I/O (MMIO) and port-mapped I/O (PMIO) are two complementary methods of performing input/output (I/O) between the central processing unit (CPU) and peripheral devices in a computer. An alternative approach is using dedicated I/O p ...
es 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
Memory-mapped I/O (MMIO) and port-mapped I/O (PMIO) are two complementary methods of performing input/output (I/O) between the central processing unit (CPU) and peripheral devices in a computer. An alternative approach is using dedicated I/O pro ...
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
Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply (interfacing) between computers, peripherals and other computers. A broad ...
to RS-232 adapter with one 9-pin COM port (FTDI
Future Technology Devices International Limited, commonly known by its acronym FTDI, is a Scottish privately held semiconductor device company, specialising in Universal Serial Bus (USB) technology.
It develops, manufactures, and supports dev ...
US-232R)
Image:RS232 PCI-E.jpg, PCI-E
PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe or PCI-e, is a high-speed serial communication, serial computer expansion bus standard, designed to replace the older Conventional PCI, PCI, PCI-X and A ...
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 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 special files allow a ...
*
Input/output base address
*
Parallel port
In computing, a parallel port is a type of interface found on early computers (personal and otherwise) for connecting peripherals. The name refers to the way the data is sent; parallel ports send multiple bits of data at once ( parallel ...
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