COM (hardware interface)

<h2 id="history">History</h2>
<p>The name for the COM port started with the original <a href="/facts/IBM_PC/yhcJGMPL">IBM PC</a>. IBM had called the four well-defined communication <a href="/facts/RS-232/NCpvGuyq">RS-232</a> ports the "COM" ports, starting from COM1 through COM4. In <a href="/facts/BASICA/aTec7AQY">BASICA</a> and <a href="/facts/PC_DOS/ScsfqAwQ">PC DOS</a> you can open these ports as "COM1:" through "COM4:", and all PC compatibles using <a href="/facts/MS-DOS/fVrrz1uN">MS-DOS</a> used the same denotation.  Most PC-compatible computers in the 1980s and 1990s had one or two COM ports.
</p><p>By 2007, most computers shipped with only one or no physical COM ports. Today, few consumer-grade PC-compatible computers include COM ports,<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> though some of them do still include a COM <a href="/facts/Pin_header/dnsYKYpn">header</a> on the motherboard.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a>
</p><p>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 <a href="/facts/FTDI/KYbBHMlQ">FTDI</a>.
</p>
<h2 id="io-addresses">I/O addresses</h2>
<p>The COM ports are interfaced by an integrated circuit such as <a href="/facts/16550_UART/WlLkR7eh">16550 UART</a>. This <a href="/facts/Integrated_circuit/fjn6vg7C">IC</a> has seven internal 8-bit <a href="/facts/Hardware_register/V1roVh0p">registers</a> which hold information and configuration data about which data is to be sent or was received, the <a href="/facts/Baud_rate/YlQ3JHVr">baud rate</a>, <a href="/facts/Interrupt/Mm6C4rpc">interrupt</a> configuration and more. In the case of COM1, these registers can be accessed by writing to or reading from the <a href="/facts/I%2fO_address/PmLNFq01">I/O addresses</a> 0x3F8 to 0x3FF. 
</p><p>If the CPU, for example, wants to send information out on COM1, it writes to <a href="/facts/I%2fO_port/PmLNFq01">I/O port</a> 0x3F8, as this I/O port is "connected" to the UART IC register which holds the information that is to be sent out.
</p><p>The COM ports in PC-compatible computers are typically defined as:<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a>
</p>
<ul><li>COM1: <a href="/facts/I%2fO_port/PmLNFq01">I/O port</a> 0x3F8-0x3FF, <a href="/facts/Interrupt_request/n2VvmL4K">IRQ</a> 4</li>
<li>COM2: I/O port 0x2F8-0x2FF, IRQ 3</li>
<li>COM3: I/O port 0x3E8-0x3EF, IRQ 4</li>
<li>COM4: I/O port 0x2E8-0x2EF, IRQ 3</li></ul>
<h2 id="implementations">Implementations</h2>

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Device_file/t8ISnXft">Device file</a></li>
<li><a href="/facts/Parallel_port/qpkjH8ts">Parallel port</a></li></ul>

<h2 id="further-reading">Further reading</h2>
<ul><li><i>Serial Port Complete: COM Ports, USB Virtual COM Ports, and Ports for Embedded Systems</i>; 2nd Edition; Jan Axelson; Lakeview Research; 380 pages; 2007; <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-1-931-44806-2.</li></ul>
<h2 id="external-links">External links</h2>

Wikimedia Commons has media related to <i>RS-232</i> and <i>DE-9 connectors</i>.

<ul><li><a href="https://web.archive.org/web/20170913121748/http://www.thaiio.com/serialportinfo.html">How to Interface Hardware in COM ports</a> at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a> (archived 2017-09-13)</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>"Configuring a communication port". IBM Lotus Domino and Notes Information Center. August 14, 2008. Archived from the original on 17 September 2013. <a href="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" target="_blank">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</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Stephen Byron Cooper. "What Is a Com1 Port?". Retrieved 2021-09-30. <a href="https://www.techwalla.com/articles/what-is-a-com1-port" target="_blank">https://www.techwalla.com/articles/what-is-a-com1-port</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>"Serial port". Retrieved 2020-09-14. <a href="https://www.computerhope.com/jargon/s/seriport.htm" target="_blank">https://www.computerhope.com/jargon/s/seriport.htm</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>"Motherboard Port Guide: Solving Your Connector Mystery". Retrieved 2020-09-14. <a href="https://www.pcworld.com/article/254998/motherboard_port_guide_solving_your_connector_mystery.html" target="_blank">https://www.pcworld.com/article/254998/motherboard_port_guide_solving_your_connector_mystery.html</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>"22.2.3. Resources Required by Serial Ports - PC Hardware in a Nutshell, 3rd Edition [Book]". www.oreilly.com. Retrieved 2024-10-15. <a href="https://www.oreilly.com/library/view/pc-hardware-in/059600513X/ch22s02s03.html" target="_blank">https://www.oreilly.com/library/view/pc-hardware-in/059600513X/ch22s02s03.html</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
</ol>

COM (hardware interface) open-in-new

COM (hardware interface)