As Chapter 13 describes, an interface is either classified as using serial data
transfer or parallel data transfer. Although it is possible to devise a serial bus, almost
all buses used in computer systems are parallel. That is, a bus is capable of transferring
multiple bits of data at the same time†.
The most straightforward buses are classified as passive because the bus itself does
not contain electronic components. Instead, each device that attaches to a bus contains
the electronic circuits needed to communicate over the bus. Thus, we can imagine a
bus to consist of parallel wires to which devices attach‡.