titus-larsen-titus-1981-apple-interfacing

DATA
BUS
DATA
A
SN74125
DATA
B
DATA
C
DATA
D
fig. 3·3. Typical three·•awe b.,s fo fom devioes.
A
B BUS ENABLE
C
o
INPUTS
SN74126, is a pin-for-pin replacement for the SN74125, except
it is enabled with a logic one and disabled with a logic zero.
These chips serve to illustrate the action of three-state devices, but
are not generally found in computer interface circuits, since
useful devices are available.
For purposes of illustration, a typical bus is shown in Fig. 3-8. In
circuit, four one-bit devices have been connected to the bus.
a one-bit bus is shown for clarity, although in an 8-bit bus syseight
lines would be required. When one of the BUS ENABLE
is placed in the logic zero state, the corresponding data bit
through the buffer and onto the bus. We will assume that
are no other devices connected to the bus. Thus, the truth table
in Table 3-1 applies to this simple bus circuit.
none of the buffers has been enabled or connected to the
the bus is not connected to anything except the input of the
·i:i