Radio Shack Quick Printer II Reference Handbook

a wait status until it goes back high. Once high,
the CPU continues with the operation. For
example: Assume you have a memory system
that takes 100 microseconds before addressed
data can be guaranteed to be present at the out-
put. When the memory logic sees that the CPU
wants data, it will make the WAIT line low.
After 100 microseconds, the logic will make the
WAIT pin high, and the CPU will input the data.
The INT (Interrupt Request) is at pin 16 of Z40.
When low, this input will force the CPU into an
interrupt request section of the memory. It
would then perform some instruction associated
with the interrupt. An example of this use
would be as follows: Assume there is a door on
the back of the TRS-80 that should always be
closed.
There is a switch connected to the door
~uch that when opened, the switch contacts are
shorted. The switch is connected to ground and
to pin 16 of Z40. If the door is opened, the
Computer stops what it is doing and prints on
the screen "Close Door." The CPU would be
interrupted, and it would henpeck you until you
closed that door! As you can see, pin 16 is tied
to VCC through a resistor and is not used. How-
ever,
it is used with the TRS-80 Expansion Inter-
face.
The TEST input may be quite useful in your
troubleshooting. Pin 25 of Z40 is labeled
tBUSRQ (Bus Request). When this pin is brought
low, it will force the data, the address and the
control lines into the disabled or floating state.
Although it is not used by the TRS-80 in normal
operation, it is quite useful when someone wants
to "shut down the CPU". We'll talk about this
input when we discuss the Control Logic Group.
tNote that we use two types of signal names. If a bar is placed
over a signal name. as in SUSRQ. then read it as "NOT SUSRQ"
(which means it IS an active low signal or pin). BUSRQ* also
reads
"NOT SUSRQ", and refers to an active low signal or pin.
The reason the asterisks are used Instead of the bar, is because
computer printouts write asterisks better than bars. Throughout
thiS book we use both methods of naming signals (we'll use the
method which appears on the Schematic [which typically is the
manufacturer's name· label] ).
16
CPU Address Lines
There are system outputs of the microprocessor
labeled
Af/J through A 15 that start the address
bus. Since
these lines must go to ROM, RAM,
the Keyboard and the Video RAM, they must be
buffered for two reasons.
First,
the buffers must be able to supply the
address bus with proper logical levels. The
microprocessor cannot supply the current neces-
sary to drive all of the sections connected to the
address bus, and buffers are needed for current
gain.
Secondly, it may be necessary to switch off the
address bus. For example, if an Expansion Inter-
face
is connected to the bus, it may be neces-
sary to address RAM in the main unit for a data
transfer. Therefore, there must be some method
to take the CPU off the data bus. The buffers
are tri-state devices. This
means they will either
act as buffers or as opened switches.
Z38, Z39 and part of Z22 and Z55 are the
address line buffers. Notice that in Z38 and Z39
there are two sections of buffers. The first sec-
tion contains four buffers and the second sec-
tion contains only two buffers. Each section is
controlled by a single pin. The first is controlled
by pin 1 and the second by pin 15. When these
control pins are at a logical low, the buffers are
enabled and will
operate normally. When the
control pin is at a logical high, the buffers are
disabled,
and will show a high impedance from
input to output. The signal that controls the
address buffers is labeled ENABLE* and is
sourced at Z52, pin 4. Pin 3 is the input for
control line inverter, Z52, and is tied to the
TEST* line. Notice that R58 keeps this line
pulled high. Thus,
the address buffers' control
line will always be at a logical low; and there-
fore,
operating as buffers. If TEST* is shorted to
ground, the address buffers will be disabled. This
feature could be very useful in troubleshooting.

Radio Shack Quick Printer II Specifications

General

Brand	Radio Shack
Model	Quick Printer II
Category	Desktop
Language	English