A parallel port is an output device that allows a computer to transmit data to another device using parallel data transmission—several bits sent simultaneously over separate wires. PC parallel ports are usually designated as LPT1, LPT2, and so on. This is in contrast to a serial port, which transmits data sequentially, one bit at a time.
A serial printer is a type of printing device that connects to a computer's serial port, using a communication protocol to send data interspersed with timing signals for printing documents.
