Serial ports are a type of computer interface that transmit information one byte at a time. They were commonly used for external modems, mice, and keyboards. They differ from parallel ports in transfer rate and were often manually reconfigured. They are now mostly used for dial-up modems and have been replaced by USB and Firewire.
Serial ports are a type of computer interface that conforms to the RS-232 standard. They are 9-pin connectors that transmit information, in or out, 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 common, 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 called UART (Universal Asynchronous Receiver Transmitter).
Serial ports differ from 25-pin parallel ports in that parallel ports transmit one byte at a time using eight parallel wires that each carry one bit. With data traveling in parallel, the transfer rate was faster. A parallel port can support speeds of up to 100 kilobytes per second, while serial ports only support 115 kilobits per second (kbps). Later, advanced 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 COM 1 and COM 3 could not be active at the same time, as they shared interrupt 4 IRQ. The same was true for COM 2 and COM 4 devices. Often this led to manual reconfiguration of the ports, which often caused more trouble than it was worth and presented a special challenge for dyslexics.
Today serial ports are mostly used for dial-up modems and current operating systems handle the configuration automatically. The newer and 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 speeds of between 100 and 400 megabits per second.
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