In
data transmission
Data transmission and data reception or, more broadly, data communication or digital communications is the transfer and reception of data in the form of a digital bitstream or a digitized analog signal transmitted over a point-to-point o ...
, parallel communication is a method of conveying multiple binary digits (
bit
The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...
s) 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 informa ...
is the number of
electrical conductors
In physics and electrical engineering, a conductor is an object or type of material that allows the flow of charge ( electric current) in one or more directions. Materials made of metal are common electrical conductors. Electric current is gen ...
used at the
physical layer 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 uni ...
) simultaneously, whereas a serial channel would convey those same bits sequentially, one at a time. If both channels operated at the same
clock speed
In computing, the clock rate or clock speed typically refers to the frequency at which the clock generator of a processor can generate pulses, which are used to synchronize the operations of its components, and is used as an indicator of the pro ...
, 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 circuits, in
peripheral buses, and in memory devices such as
RAM
Ram, ram, or RAM may refer to:
Animals
* A male sheep
* Ram cichlid, a freshwater tropical fish
People
* Ram (given name)
* Ram (surname)
* Ram (director) (Ramsubramaniam), an Indian Tamil film director
* RAM (musician) (born 1974), Dutch
* ...
. 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
In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This e ...
,
system bus, and
front-side bus
A front-side bus (FSB) is a computer communication interface (bus) that was often used in Intel-chip-based computers during the 1990s and 2000s. The EV6 bus served the same function for competing AMD CPUs. Both typically carry data between the ...
*
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
IEEE 1284 is a standard that defines bi-directional parallel communications between computers and other devices. It was originally developed in the 1970s by Centronics, and was widely known as the Centronics port, both before and after its IEEE ...
/
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 division ...
"printer port"
* Laboratory Instrumentation bus
IEEE-488
IEEE 488 is a short-range digital communications 8-bit parallel multi-master interface bus specification developed by Hewlett-Packard as HP-IB (Hewlett-Packard Interface Bus). It subsequently became the subject of several standards, and is ...
* (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 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 advanced ...
reduces the speed of every link to the slowest of all of the links.
* Cable 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
A latch or catch (called sneck in Northern England and Scotland) is a type of mechanical fastener that joins two (or more) objects or surfaces while allowing for their regular separation. A latch typically engages another piece of hardware on t ...
to copy data onto a
data bus
In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This e ...
. In contrast, most serial communication must first be converted back into parallel form by a
universal asynchronous receiver/transmitter
A universal asynchronous receiver-transmitter (UART ) is a computer hardware device for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least significa ...
(UART) before they may be directly connected to a data bus.
The decreasing cost and better performance of
integrated circuits
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
has led to serial links being used in favor of parallel links; for example,
IEEE 1284
IEEE 1284 is a standard that defines bi-directional parallel communications between computers and other devices. It was originally developed in the 1970s by Centronics, and was widely known as the Centronics port, both before and after its IEEE ...
printer ports vs.
USB
Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply (interfacing) between computers, peripherals and other computers. A broad ...
,
Parallel ATA vs.
Serial ATA, and
FireWire or
Thunderbolt
A thunderbolt or lightning bolt is a symbolic representation of lightning when accompanied by a loud thunderclap. In Indo-European mythology, the thunderbolt was identified with the 'Sky Father'; this association is also found in later Hel ...
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 pr ...
(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 and
BPSK
Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency reference signal (the carrier wave). The modulation is accomplished by varying the sine and cosine inputs at ...
), well-known techniques such as
PSM,
PAM, and
Multiple-input multiple-output
In radio, multiple-input and multiple-output, or MIMO (), is a method for multiplying the capacity of a radio link using multiple transmission and receiving antennas to exploit multipath propagation. MIMO has become an essential element of wir ...
communication send a few bits in parallel. (Each such group of bits is called a "
symbol"). Such techniques can be extended to send an entire byte at once (
256-QAM).
See also
*
Data transmission
Data transmission and data reception or, more broadly, data communication or digital communications is the transfer and reception of data in the form of a digital bitstream or a digitized analog signal transmitted over a point-to-point o ...
*
Serial port
In computing, a serial port is a serial communication interface through which information transfers in or out sequentially one bit at a time. This is in contrast to a parallel port, which communicates multiple bits simultaneously in parallel. ...
*
Bit-level parallelism Bit-level parallelism is a form of parallel computing based on increasing processor word size. Increasing the word size reduces the number of instructions the processor must execute in order to perform an operation on variables whose sizes are grea ...
References
*
{{Computer-bus
Data transmission