parallel input/output (PIO) A method of data transfer between devices, typically a computer and its peripherals, in which all the bits associated with a character or byte are presented to the interface simultaneously on separate conductors. There are usually other parallel conductors to carry the control signals. PIO is frequently used since it is compatible with the format used within the processor and enables high rates of data transfer to be achieved. When connection over any significant distance has to be made, the cost of the conductors and the associated drive circuits becomes significant and it is then preferable to convert to a serial input/output. PIO is gradually being replaced by high-speed serial interfaces such as USB, SATA (serial ATA), and Firewire, which offer better performance and lower cost.

parallel port A 25-way input/output socket, called a DB-25 connector, on a computer or other device for a parallel interface (often called a Centronics interface). It includes three groups of lines: eight for data, four for control signals, and five for status signals. A parallel port on a computer is often used for a parallel printer. Compare serial port.

parallel in serial out (PISO) A term used to describe a class of digital device that can accept parallel n-bit data words and convert them into serial sequential n-bit data streams. These devices often consist of an n-bit shift register that is parallel loaded with the data word (see diagram). This data is then clocked out of the register in serial form. Compare serial in parallel out.

parallel in parallel out (PIPO) A term used to describe a shift register that can be loaded in parallel and also read in parallel, in addition to which (by implication) data can enter and leave the device serially.