Peripheral devices typically are connected to the system unit of a per- sonal computer by means of a cable, one end of which is plugged into the device and the other end plugged into a port, or interface, on the system unit. Also, recall that an add-on board, such as an internal modem, contains a port that allows a computer to be connected to a medium, such as a telephone line. Older computers are equipped with two types of ports: parallel and serial. A parallel cable is needed to connect a device to a parallel port, whereas a serial cable is needed to connect a device to a serial port. Many older peripheral devices, including printers and mice, may be either parallel or serial. Universal
Serial Bus (USB) is a type of high-speed serial transmission that has replaced serial and parallel ports on newer computers. Data travels within a computer system and over long distances in either parallel or serial form. During transmission, 8 bits (representing a single byte) plus 1 bit called a parity bit are transmitted. A parity bit is an extra bit added to a byte, character, or word to ensure there is always either a predetermined even or odd number of bits and therefore an accurate transmission. The parity bit also indicates the end of the 8-bit byte.
In serial transmission, the byte plus the parity bit are transmitted one bit after another in a continuous line.
In parallel transmission. 8 bits (a byte) plus a parity bit are transmitted at the same time over nine separate paths Thus, parallel transmission is generally faster than serial transmission. A modem that connects the system unit to a line contains a serial port because the telephone line expects the data being transmitted to be in serial form. Serial ports are designed and built according to either the RS-232 or RS-422 standard, which specifies the number of pins required for the port's connector. Two common connectors for ports are a 9-pin connector and a 25-pin connector.