Radio Shack Quick Printer II Reference Handbook

CPU Data Bus
The data bus is buffered like the address bus,
except for one area. Notice that there are only
eight data lines at the CPU, labeled D0 through
D7. But there at 16 buffers. Remember that the
CPU must receive data as well as send data. The
address lines are strictly CPU outputs. while the
data lines are inputs and outputs. Therefore.
there must be two sets of buffers for the data
line. One set handles CPU output data while the
other set takes care of the CPU input data.
The output data buffers consist of Z75 and one
section of Z76. The input buffers consist of one
section of Z55 and the last section of Z76.
Notice that the input and output buffers are
connected "head to toe". This could cause pro-
blems if
both were on at the same time! The
control inputs to the output buffers are all con-
nected together on the line labeled DBOUT*,
and are in
turn tied to Z53, pin 6. Likewise, the
input buffers' controls are tied together on the
line labeled DBI N*, and are connected to Z53,
pin 8. DBOUT*, is tied to pins 9 and 10 of Z53,
the gate wh ich generates DB IN *.
As
you can see, Z53, pin 6, is the major source
of input or output data control. If pin 6 is high,
DBOUT*
is high and DBIN* is low. Therefore,
the input buffers are enabled and the output
buffers are disabled. If Z53, pin 6, is low,
DBOUT*
is low and DBIN* is high. In this case,
the output buffers are enabled and the input
buffers are off.
Pin 4
of Z53 is tied to TEST*. If TEST* is
grounded, not only will we disable the address
buffers,
but we will also cause pin 6 of Z53 to
go high. Thus, the data output buffers will be
off, robbing the CPU's control over the data
lines. Since DBIN* is now held low, the input
data buffers would be active. But, this would
not cause any problem since the address bus
from the CPU has been disabled.
When
TEST* is left alone, it is held high. If pin
21 of the CPU (the Memory Read output) is
high, Z53, pin 6, will be low. The low causes
DBOUT*
to be low and DBIN* to be high.
Therefore, the CPU is outputting data; and the
buffers are switched accordingly. When pin 21
of Z40 goes low, Z53, pin 6, will be high. We
now have almost the same condition as if TEST*
went low. DBOUT* is high and DBIN* is low
but the address buffers are still enabled. The
data buffers are now ready for the CPU to
accept data.
17

Radio Shack Quick Printer II Specifications

General

Brand	Radio Shack
Model	Quick Printer II
Category	Desktop
Language	English