Radio Shack Quick Printer II Reference Handbook

Notice on Z3 that we can program the system
for
8K of RAM by leaving the shorting bar in-
tact at pins 3 and 14, and at the 2 and 15 posi-
tion. Not only would a 4000 address cause
RAM*,
but a 5000 address would also enable
RAM*. If
we had 12K of RAM, we would leave
pins 4 and 13
shorted. For 16K, we short all
pins we have
mentioned; plus pins 5 and 12.
RAM* would
now be active from addresses
4000 to 7FFF.
As you can see from the RAM discussion, we'll
be
shorting certain outputs of Z21 together. In
most applications using TTL. shorting output
nodes is bad design practice. But there are some
"open collector" type TTL devices. These types
of gates do not have an active pull-up on the
output. Instead, the output transistors have
"open collectors". It is the responsibility of ex-
ternal circuitry
to pull them up. The "open col-
lector" outputs may be tied together for a JJwire
OR" function.
Since Z21 is an open collector decoder, the out-
put may be safely tied together. Notice resistors
R48, R61. R62
and R68. These are the pull-up
resistors for Z21.
Something to remember about open collector
outputs: You cannot tell if one of these outputs
is working unless there is a pull-up resistor tied
to that output. For example, if you placed an
oscilloscope
probe on pin 10 of Z21 as shown in
Figure 3, you would not be able to tell if pin 10
goes low. If the system is working right, it
shouldn't. But if it isn't working right and pin
10 is going low. how are you going to prove it?
Pull it
up with a resistor to +5 volts and see;
that's the only way you can be sure.
23

Radio Shack Quick Printer II Specifications

General

Brand	Radio Shack
Model	Quick Printer II
Category	Desktop
Language	English