The communication links, across which computers (or parts of computers) talk to one another, may be either serial or parallel. A parallel link transmits several streams of data simultaneously along multiple channels (e.g., wires, printed circuit tracks, or optical fibers); whereas, a serial link transmits only a single stream of data. The rationale for parallel communication was the added benefit of having
Direct Memory Access to the 8-bit or 16-bit registry addresses at a time when mapping direct data lanes was more convenient and faster than synchronizing data serially. Although a serial link may seem inferior to a parallel one, since it can transmit less data per clock cycle, it is often the case that serial links can be clocked considerably faster than parallel links in order to achieve a higher data rate. Several factors allow serial to be clocked at a higher rate: •
Clock skew between different channels is not an issue (for unclocked
asynchronous serial communication links). This can be caused by mismatched wire or conductor lengths. • A serial connection requires fewer interconnecting cables (e.g., wires/fibers) and hence occupies less space. The extra space allows for better isolation of the channel from its surroundings. •
Crosstalk is less of an issue, because there are fewer conductors in proximity. An electrical serial link only requires a pair of wires, whereas a parallel link requires several. Thus serial links can save on costs (also known as the
Bill of Materials).
Differential signalling uses length-matched wires or conductors and are used in high speed serial links. Length-matching is easier to perform on serial links as they require fewer conductors. In many cases, serial is cheaper to implement than parallel. Many
ICs have serial interfaces, as opposed to parallel ones, so that they have fewer pins and are therefore less expensive. ==Examples of architectures==