COM (hardware interface)
COM (communication port)
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 MSDOS used the same denotation.[citation needed] 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,[3] though some of them do still include a COM header on the motherboard.[4]
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.[citation needed]
I/O addresses
The COM ports are interfaced by an integrated circuit such as
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[citation needed]:
- COM1: I/O port 0x3F8, IRQ4
- COM2: I/O port 0x2F8, IRQ 3
- COM3: I/O port 0x3E8, IRQ 4
- COM4: I/O port 0x2E8, IRQ 3
Implementations
-
PCI-E card with one 9-pin COM port
-
PCIcard with two 9-pin COM ports
-
ISA card with one 25-pin COM port
See also
References
- ^ "Configuring a communication port". IBM Lotus Domino and Notes Information Center. August 14, 2008. Archived from the original on 17 September 2013.
- ^ Stephen Byron Cooper. "What Is a Com1 Port?". Retrieved 2021-09-30.
- ^ "Serial port". Retrieved 2020-09-14.
- ^ "Motherboard Port Guide: Solving Your Connector Mystery". Retrieved 2020-09-14.
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
- How to Interface Hardware in COM ports at the Wayback Machine (archived 2017-09-13)