How Serial Ports Work

Serial Ports

Serial ports are the connectors on the back of your PC that enable your computer to control devices such as modems, scanners, and printers. Although some serial ports use a 25-pin interface, most only have nine pins. One pin serves as the ground while the others handle the flow of data between a computer and its peripherals. Serial ports are also known as communication (COM) ports, and most PCs have at least one.

Unlike parallel ports which send a full byte (8 bits) of data at once, serial ports can transfer information only one bit at a time. They use a bidirectional form of communications that allows information to travel back and forth across a cable. RS-232 is the standard serial port design determined by the Electronics Industry Association (EIA). Among its other specifications, a RS-232 serial port can connect a computer to a device located more than 50 meters away.

Serial devices use an asynchronous process to control the transfer of information, relying on special start and stop bits to signal when serial data should or shouldn’t be transmitted. The asynchronous method divides information into packets of one byte each, sometimes including a special parity bit to verify the integrity of the data in each packet.

In your computer, the UART (Universal Asynchronous Receiver/Transmitter) converts the parallel data moving throughout your PC into a serial format that can be used by other serial devices. The procedure is reversed, however, when your computer receives serial data it must work with. The UART is also directly responsible for regulating the flow of serial bits.

In most serial communications, a device such as a modem is called the data communications equipment (DCE), while the computer itself is referred to as the data terminal equipment (DTE). The DCE and DTE (or two DTEs) establish a connection described as a handshake prior to exchanging data.

Your computer assigns an input/output (I/O) address and an interrupt request line (IRQ) to each serial port in your PC. This designation gives devices priority over each other as they attempt to communicate with your computer. Unfortunately, trouble occurs when a device is given the same I/O address or IRQ. This conflict can lead to your system shutting down or other PC problems. Therefore, make sure new serial devices are installed on only available I/O addresses and interrupts.

If you think your computer is experiencing a conflict, you can usually change your port configurations through software installed on your system. Consult your PC owner’s manual or activate your operating system’s help feature for instructions on how to fix port conflicts. Also, there are several shareware programs available that may help you detect possible conflicts as well.

Because many devices now use universal serial bus (USB) ports, computer users deal with fewer conflicts. USB ports use a different technology that offers much greater data transfer speeds than both parallel and serial ports. Also, USB ports support the connection of over 100 devices to your PC. But as long as computers continue to use modems, serial ports will probably be needed. Serial ports, despite their conflict issues, use a reliable form of data communications and have performed quite well over the years.

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