Editing COM (hardware interface)

{{Short description|Computer serial port}}
[[Image:Serial plug1.jpg|thumb|COM port ([[DE-9]] connector).]]

'''COM''' (communication port)<ref>{{cite web|title=Configuring a communication port|url=http://publib.boulder.ibm.com/infocenter/domhelp/v8r0/index.jsp?topic=%2Fcom.ibm.help.domino.admin.doc%2FDOC%2FH_SETTING_UP_A_COMMUNICATION_PORT_STEPS.html|publisher=IBM Lotus Domino and Notes Information Center|date=August 14, 2008|archive-url=https://archive.today/20130917043522/http://publib.boulder.ibm.com/infocenter/domhelp/v8r0/index.jsp?topic=/com.ibm.help.domino.admin.doc/DOC/H_SETTING_UP_A_COMMUNICATION_PORT_STEPS.html|archive-date=17 September 2013}}</ref><ref>{{cite web |title=What Is a Com1 Port? |url=https://www.techwalla.com/articles/what-is-a-com1-port |author=Stephen Byron Cooper |access-date=2021-09-30}}</ref> is the original, yet still common, name of the [[serial port]] interface on [[IBM PC compatible|PC-compatible computers]]. It can refer not only to physical ports, but also to [[Hardware emulation|emulated]] ports, such as ports created by [[Bluetooth]] or [[USB adapter]]s.

==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 [[MS-DOS]] used the same denotation.{{citation needed|date=February 2022}}  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,<ref>{{cite web |url=https://www.computerhope.com/jargon/s/seriport.htm |title=Serial port |access-date=2020-09-14}}</ref> though some of them do still include a COM [[Pin header|header]] on the motherboard.<ref>{{cite web |url=https://www.pcworld.com/article/254998/motherboard_port_guide_solving_your_connector_mystery.html |title=Motherboard Port Guide: Solving Your Connector Mystery |access-date=2020-09-14}}</ref>

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]].{{cn|date=September 2020}}

==I/O addresses==
The COM ports are interfaced by an integrated circuit such as [[16550 UART]]. This [[Integrated circuit|IC]] has seven internal 8-bit [[Hardware register|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 address]]es {{mono|0x3F8}} to {{mono|0x3FF}}. 

If the CPU, for example, wants to send information out on COM1, it writes to [[I/O port]] {{mono|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:<ref>{{Cite web |title=22.2.3. Resources Required by Serial Ports - PC Hardware in a Nutshell, 3rd Edition [Book] |url=https://www.oreilly.com/library/view/pc-hardware-in/059600513X/ch22s02s03.html |access-date=2024-10-15 |website=www.oreilly.com |language=en}}</ref>
*'''COM1''': [[I/O port]] {{mono|0x3F8-0x3FF}}, [[Interrupt request|IRQ]] 4
*'''COM2''': I/O port {{mono|0x2F8-0x2FF}}, IRQ 3
*'''COM3''': I/O port {{mono|0x3E8-0x3EF}}, IRQ 4
*'''COM4''': I/O port {{mono|0x2E8-0x2EF}}, IRQ 3

==Implementations==
<gallery widths="220px" heights="165px">
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|[[Conventional PCI|PCI]] card with two 9-pin COM ports
Image:IBM PC Serial Card.jpg|[[Industry Standard Architecture|ISA]] card with one 25-pin COM port
</gallery>

==See also==
*[[Device file]]
*[[Parallel port]]

==References==
{{Reflist}}

==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; {{ISBN|978-1-931-44806-2}}.

==External links==
{{Commons category multi|RS-232|DE-9 connectors}}
*{{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}}

[[Category:Computer buses]]