data transmission Data communication, including data transmission and data reception, is the transfer of data, signal transmission, transmitted and received over a Point-to-point (telecommunications), point-to-point or point-to-multipoint communication chann ...

, parallel communication is a method of conveying multiple binary digits ( bits) simultaneously using multiple conductors. This contrasts with serial communication, which conveys only a single bit at a time; this distinction is one way of characterizing a communications link. The basic difference between a parallel and a serial

communication channel A communication channel refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel in telecommunications and computer networking. A channel is used for infor ...

is the number of electrical conductors used at the

physical layer In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer: the layer most closely associated with the physical connection between devices. The physical layer provides an electrical, mechani ...

to convey bits. Parallel communication implies more than one such conductor. For example, an 8-bit parallel channel will convey eight bits (or a

byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable un ...

) simultaneously, whereas a serial channel would convey those same bits sequentially, one at a time. If both channels operated at the same

clock speed Clock rate or clock speed in computing typically refers to the frequency at which the clock generator of a processor can generate pulses used to synchronize the operations of its components. It is used as an indicator of the processor's ...

, the parallel channel would be eight times faster. A parallel channel may have additional conductors for other signals, such as a clock signal to pace the flow of data, a signal to control the direction of data flow, and handshaking signals. Parallel communication is and always has been widely used within

integrated circuit An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...

s, in

peripheral A peripheral device, or simply peripheral, is an auxiliary hardware device that a computer uses to transfer information externally. A peripheral is a hardware component that is accessible to and controlled by a computer but is not a core compo ...

buses, and in memory devices such as RAM. Computer system buses, on the other hand, have evolved over time: parallel communication was commonly used in earlier system buses, whereas serial communications are prevalent in modern computers.

Examples of parallel communication systems

* Internal buses: memory bus, system bus, and front-side bus, including various generations of DDR SDRAM and GDDR SDRAM * IBM System/360 ''Direct Control Feature'' (1964). Standard System/360 had an eight-bit wide port. The process-control variant Model 44 had a 32-bit width. * Legacy computer peripheral buses: ISA, ATA, SCSI, PCI, and the once-ubiquitous IEEE-1284 /

Centronics Centronics Data Computer Corporation was an American manufacturer of computer printers, now remembered primarily for the parallel interface that bears its name, the Centronics connector. History Foundations Centronics began as a divisio ...

"printer port" * Laboratory Instrumentation bus IEEE-488 * (see more examples at computer bus)

Comparison with serial links

Before the development of high-speed serial technologies, the choice of parallel links over serial links was driven by these factors: * Speed: Superficially, the speed of a parallel data link is equal to the number of bits sent at one time times the

bit rate In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction ...

of each individual path; doubling the number of bits sent at once doubles the data rate. In practice,

clock skew Clock skew (sometimes called timing skew) is a phenomenon in synchronous digital circuit systems (such as computer systems) in which the same sourced clock signal arrives at different components at different times due to gate or, in more advanc ...

reduces the speed of every link to the slowest of all of the links. However, parallel lines have lower latency than serial lines, this makes parallel lines is still used on memory bus like DDR SDRAM. * Cable length or link length: Crosstalk creates interference between the parallel lines, and the effect worsens with the length of the communication link. This places an upper limit on the length of a parallel data connection that is usually shorter than a serial connection. * Complexity: Parallel data links are easily implemented in hardware, making them a logical choice. Creating a parallel port in a computer system is relatively simple, requiring only a latch to copy data onto a data bus. In contrast, most serial communication must first be converted back into parallel form by a universal asynchronous receiver/transmitter (UART) before they may be directly connected to a data bus. The decreasing cost and better performance of integrated circuits has led to serial links being used in favor of parallel links; for example, IEEE 1284 printer ports vs. USB,

Parallel ATA Parallel ATA (PATA), originally , also known as Integrated Drive Electronics (IDE), is a standard interface designed for IBM PC-compatible computers. It was first developed by Western Digital and Compaq in 1986 for compatible hard drives and C ...

vs. Serial ATA, and FireWire or Thunderbolt are now the most common connectors for transferring data from

audiovisual Audiovisual (AV) is electronic media possessing both a sound and a visual component, such as slide-tape presentations, films, television programs, corporate conferencing, church services, and live theater productions. Audiovisual service provide ...

(AV) devices such as digital cameras or professional-grade scanners that used to require purchasing a SCSI HBA years ago. One huge advantage of having fewer wires/pins in a serial cable is the significant reduction in the size, the complexity of the connectors, and the associated costs. Designers of devices such as smartphones benefit from the development of connectors/ports that are small, durable, and still provide adequate performance. On the other hand, there has been a resurgence of parallel data links in RF communication. Rather than transmitting one bit at a time (as in

Morse code Morse code is a telecommunications method which Character encoding, encodes Written language, text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code i ...

and BPSK), well-known techniques such as PSM, PAM, and Multiple-input multiple-output communication send a few bits in parallel. (Each such group of bits is called a "

symbol A symbol is a mark, Sign (semiotics), sign, or word that indicates, signifies, or is understood as representing an idea, physical object, object, or wikt:relationship, relationship. Symbols allow people to go beyond what is known or seen by cr ...

"). Such techniques can be extended to send an entire byte at once ( 256-QAM).

References

* {{Computer-bus Data transmission

Examples of parallel communication systems

Comparison with serial links

See also

References