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. However, parallel lines have lower latency than serial lines, which is why parallel lines are still used on memory buses 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 vs.
Serial ATA, and
FireWire or
Thunderbolt are now the most common connectors for transferring data from
audiovisual (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), 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"). Such techniques can be extended to send an entire byte at once (
256-QAM). ==See also==