What are Serial Ports?

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  • Written By: R. Kayne
  • Edited By: Niki Foster
  • Last Modified Date: 13 October 2019
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Serial ports are a type of computer interface that complies with the RS-232 standard. They are 9-pin connectors that relay information, incoming or outgoing, one byte at a time. Each byte is broken up into a series of eight bits, hence the term serial port.

These ports are one of the oldest types of interface standards. Before internal modems became commonplace, external modems were connected to computers via serial ports, also known as communication or "COM" ports. Computer mice and even keyboards also used them. Some used 25-pin connectors, but the 9-pin variety was more common. They are controlled by a special chip call a UART (Universal Asynchronous Receiver Transmitter).

Serial ports differ from 25-pin parallel ports in that the parallel ports transmit one byte at a time by using eight parallel wires that each carry one bit. With data traveling in parallel, the transfer rate was greater. A parallel port could support rates up to 100 kilobytes per second, while serial ports only supported 115 kilobits per second (kbps). Later, enhanced technology pushed serial speeds to 460 kbps.

In traditional computers, serial ports were configured as follows:

Serial Ports Interrupt Memory Address
COM 1 IRQ 4 0x3f8
COM 2 IRQ 3 0x2f8
COM 3 IRQ 4 0x3e8
COM 4 IRQ 3 0x2e8

Devices configured to use ports COM 1 and COM 3 could not be active at the same time, as they shared interrupt IRQ 4. The same was true of COM 2 and COM 4 port devices. Often this led to manually reconfiguring the ports, which frequently caused more trouble than it was worth and posed a special challenge for the dyslexic.

Today, serial ports are mainly used for dial-up modems and current operating systems handle configuration automatically. Newer, faster technologies of USB (Universal Serial Bus) and Firewire have otherwise replaced both serial and parallel ports. USB supports speeds from 1.5 megabits per second to 60 megabytes per second. Firewire boasts transfer rates between 100 and 400 megabits per second.


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Post 3

I had to download free serial port monitor software to check the flow of data through a COM port. I have a very old computer and sometimes wish I could still use it without having to use adapters. It is so slow that there is no way I can use it on the internet.

Such a shame to have something sitting around that could be used but cannot due to technology surpassing it. I had to break down and buy a new computer for all my daily internet usage. I am an internet junkie, so having a computer that can handle the web was a priority for me.

Post 2

I have some old hardware that has a serial connection, will it work OK if I get a serial port to USB adapter, or will I need some special drivers?

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