Parallel communication

In telecommunication and computer science, parallel communication is a method of sending several data signals simultaneously over several parallel channels. It contrasts with serial communication; this distinction is one way of characterizing a communications link.
The basic difference between a parallel and a serial communication channel is the number of distinct wires or strands at the physical layer used for simultaneous transmission from a device. Parallel communication implies more than one such wire/strand, in addition to a ground connection. An 8-bit parallel channel transmits eight bits (or a byte) simultaneously. A serial channel would transmit those bits one at a time. If both operated at the same clock speed, the parallel channel would be eight times faster. A parallel channel will generally have additional control signals such as a clock, to indicate that the data is valid, and possibly other signals for handshaking and directional control of data transmission.


Examples of parallel communication systems


Computer peripheral buses: ISA, ATA, SCSI, PCI and Front side bus, and the once-ubiquitous IEEE-1284 / Centronics "printer port"
Laboratory Instrumentation bus IEEE-488




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 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 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.