Microcomputer Circuits and Processes

(b)
D
II
CPU
_____ ~----- w;'es althe bus
Figure 2.1(b)
Interconnecting computer devices: the efficient bus connection.
It saves a lot of wires, but there isjust one problem. What happens if
two devices put their signals onto the bus simultaneously? This is
shown in figure 2.2, where just one wire of the bus is shown, and two
logic inverters are shown connected. The output of one is high and the
output of the second is low. What happens?
The bus wire cannot
be both low and high
Figure 2.2
There is a problem when two devices put different signal levels on to the bus. This
situation must be avoided.
Almost certainly, something will get very hot and be damaged; at
the very least the system will not work. This is called bus contention, and
steps must be taken to ensure that no two devices put their signals on
the bus together. In technical jargon, no two devices may 'talk' to the
bus simultaneously. This is achieved using a type oflogic gate called the
'Tri-State' gate (which is a trademark of National Semiconductor
Corporation). A tristate gate does not have three different logic states,
but is like a normal logic gate in se'rieswith a switch. Look at figure 2.3
which shows a tristate inverter gate. The idea is that a control signal is
able to connect the gate to its output. This is called 'enabling' the gate. If
the enable is low, then there is no output from the gate, neither high nor
low. The gate is in a 'high-impedance' state, meaning that anything
connected to its output does not know it is there. When the gate is
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