Introduction to Serial Ports

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. Serial ports are one of the oldest types of interface standards.

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 serial 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. The serial port is much more than just a connector. It converts the data from parallel to serial and changes the electrical representation of the data.
Serial flow is a stream of bits over a single wire (such as on the transmit or receive pin of the serial connector). For the serial port to create such a flow, it must convert data from parallel (inside the computer) to serial on the transmit pin (and conversely).
The advantage is that a serial port needs only one wire to transmit the 8 bits (while a parallel port needs 8). The disadvantage is that it takes 8 times longer to transmit the data than it would if there were 8 wires. Serial ports lower cable costs and make cables smaller. Serial ports, also called communication (COM) ports, are bi-directional. Bi-directional communication allows each device to receive data as well as transmit it.
Serial ports rely on a special controller chip, the Universal Asynchronous Receiver/Transmitter (UART), to function properly. The UART chip takes the parallel output of the computer's system bus and transforms it into serial form for transmission through the serial port. In order to function faster, most UART chips have a built-in buffer of anywhere from 16 to 64 kilobytes. This buffer allows the chip to cache data coming in from the system bus while it is processing data going out to the serial port.