titus-larsen-titus-1981-apple-interfacing

1/0 STROBE
AlO
MEMORY ENABLE
SIGNAL
A9-AO
l K Or
ROM
07-DO
TO DATA BUS
circuit, the I/0 STROBE signal has been used to select a lK block
of ROM on an interface card. The remaining 1024 addresses could
be divided among the other interfaces as you wish. We urge that you
use caution in using this signal, however, since you may find that
some manufacturers have used this line to decode memory and I/0
device addresses in just this manner. Thus, you may find that you
have a conflict in addressing between a commercial interface that
you wish to add to your system, and one that you have already designed,
built, and installed.
DEVICE S!:l!Et'f
This signal is specific to each interface slot, and it has a range of
only 16 addresses for each slot, as shown in Table 7-3. The DEVICE
SELECT signal is active in the logic zero state. Since the DEVICE
SELECT signal is active for only a 16-address block, its use will be
fairly well limited to I/ 0 device addressing, as shown in Fig. 7-4. In
this circuit, the DEVICE SELECT signal has been used to enable a
4-to-16-line decoder. If a particular interface has only a single function,
and only requires a single enable signal, you may decide to use
the DEVICE SELECT signal by itself, without any further decoding.
This is permissible, as long as you realize that the device selected
in this way will be active at 16 different addresses, COnOH to COnFH,
SN74154
AO A 15
Al
B
A2
c
A3
D
2
DEVICE SELECT G l
G 0
1
16 DECODED DEVICE
ADDRESSES