Attaching a device to a bus is nontrivial. Recall that a bus uses an access protocol
to determine when a given device can use the bus. Therefore, each device must have an
additional digital circuit that attaches to the bus and follows the bus protocol. Known
as a bus interface or a bus controller, the circuit implements the bus protocol and allows
the device to access the bus. If the bus protocol is complicated, the interface circuit
can be large; many bus interfaces require multiple chips.
How does an interface plug into the socket of a bus? Interestingly, the sockets of
many buses are chosen to make it possible to plug a printed circuit board directly into
the socket. The circuit board must have a region cut to the exact size of a socket, and
must have metal fingers that touch metal contacts in the socket. Figure 14.3 illustrates
the concept.
The figure helps us envision how a physical computer system can be constructed.
If the mother board lies in the bottom of a cabinet, individual circuit boards that plug
into the mother board are vertical. The arrangement is used in a typical PC — the back
of the cabinet contains a series of slots that can each be occupied by a circuit board.