Radio Shack Telephone Interface II Technical Manual

INTRODUCTION
"We've
done so much,
with so
little, for so
long,
that now
we can
do anything,
with nothing,
except
weld
the Crack
of
Dawn
and
put wheels on
a
Carriage
Return!"
Armed
with only
a
schematic,
attacking
a TRS-80 com-
puter
can be like
trying to weld
that crack. You
may
know
what
"CPU"
stands for.
You
may have the
knowledge
of
how
a microprocessor
system
works. So
you grab
the
sche-
matic's
book and attack
that TRS-80
with
soldering
iron
smoking
and a determined
gleam
in your eye.
After
a while,
you find you have
problems. You
know a 4K
RAM
needs
12 address lines. But
you
only found 7.
You
know
a computer
keyboard
gives you
ASCII. Yet,
you
find the
keyboard is shorting
out
an address line
to a data
line. You
know how
a TV typewriter
scrolls
characters
on
the
screen,
and you do find
the video
memory
wired
to do
the
job. But
where is all the
hardware to make
the
display
scroll?
You
know what
a
NAND
gate symbol
looks
like and
you
also
know what
an OR
gate symbol looks
like.
But some
sadist has
gotten all his
symbols backwards.
A
NAND
gate
is
shown like
an
OR
gate,
and the OR
gate looks
like a
NAND
gate.
To
top
it
all
off, the
power supply
consists
of
two
large rectangles
with
transistors
and resistors
sticking
out
of them;
and the only
voltages
shown
are the
resulting
outputs.
Welcome
to the
wonderful
world
of computer
electronics.
Now
admit it. You
don't really
know your
ASCII from
a
scroll
when
it
comes
to the TRS-80
computer.
come
back
to the CPU. The
address
lines
are outputs from
the CPU.
They never
receive
data or addresses
from other
sections. The
data lines
on the
other hand can
give
or
receive
data.
ROM
The
ROM
(Read
Only Memory)
could be considered
the
brains,
if the CPU
has
to
be
the heart,
of the system. The
ROM
tells the CPU
what
to do, how
to do it and where to
put it after
it's done. Without
the ROM,
the
CPU
would
just sit there
and oscillate.
When
power is first applied
to
the system,
the CPU
has just enough
smarts to output
an
address to
the ROM that
locates
the
CPU's
first instruction.
The ROM
shoots back
the first
instruction and
then the
two really
start communicating.
In less
than a second,
the
CPU,
under ROM
supervision,
performs
all the house-
keeping
necessary
to get
the system alive
and a
"READY"
flashes
on the screen;
If
the
CPU
misses
that first
piece of ROM
data, then it may
go bananas.
It may tell
the ROM that
it is ready
to
load
a
tape
so the ROM
tells it
how to do that. The
tape recorder
turns on. But
since the CPU
is now
playing games in the
video
memory, who
cares about the
tape? The
CPU
operates
at
about
2 MHz; therefore, digital screw-ups seem
instantaneous.
Remember
that the
CPU
is the work horse
and the
ROM
is
the boss. The
ROM tells
the
CPU
how to do it, when
to
do
it,
and where
to put it.
RAM
The
purpose
of this
manual is
to give you
a practical
knowledge
of system operation
as it
pertains to
the
TRS-80.
This manual will
show
you why there
are only
seven
address inputs
to a 4K RAM. You
will
be shown
when the
microprocessor
inputs data from
the
keyboard
the
CPU
thinks
the keyboard
is a memory,
of all
things!
You'll
learn
how the
CRT
screen is
scrolled
and you
might
even
learn to appreciate
the
backward
symbolization.
As
far
as the
power supply
is concerned,
you might
find
that it's
not nearly
as
complex
as
you thought.
So, grab
your sche-
matics
and let's
take a tour of
the
TRS-80
computer.
SYSTEM
BLOCK
DIAGRAM
The
80
integrated circuits
contained
in the
TRS-80
can be
broken
down
into
about
10 major sections.
Figure
1 shows
these sections
as they relate
to other
sections.
The heart
of
the
system is definitely
the CPU
(Central Processing
Unit).
You
might
consider the CPU
as being
a very
dumb calcula-
tor circuit. It
may be dumb, but
it's
a
fast
dummy.
Most of
the leads
on the CPU
are data lines
and address
lines.
The
CPU
tells
the address bus
where the
data it wants
is located,
and
the data bus is
a good place
for the
information
to
The next
major section
in Figure
1 is the
RAM
(Random
Access
Memory).
This memory
is where the CPU
may place
data it
doesn't need
until later.
The
RAM
is also the
place
where
the programs
are kept
for use. If you tell
the compu-
ter to count
to
10,000,
then the CPU
stores your instruc-
tions
in the RAM. If
you tell
the computer
to do
it NOW,
here
is what happens:
The CPU
tells the ROM
someone
wants in. The ROM
tells
the CPU
to
go to the keyboard
and find out
who.
The CPU
finds
out, tells the ROM
that it's
the boss. The ROM
tells
the
CPU
to find out
what he wants. The CPU
tells the ROM
that the boss
wants US
to RUN. The ROM
tells the
CPU
to
go to
RAM
and
find out what the
boss wants done. The
CPU
says the
boss wants
to count to
10,000.
The
ROM
tells
the CPU
how
to do it. After
it's done, the ROM
tells the
CPU
to
find out what
to do with it. The CPU
informs
the
ROM
that the
10,000
has got to
go on
the display
and must
be
saved. The
ROM tells
the
CPU
how
to
put
it on the dis-
play
and then
tells it
to store the
10,000
somewhere in
RAM;
but it
had better
remember
where it is. The CPU tells
the ROM
that
the job is done. The
ROM tells the CPU
to
monitor the keyboard
in case
the boss wants something
else.

Radio Shack Telephone Interface II Specifications

General

Brand	Radio Shack
Model	Telephone Interface II
Category	Desktop
Language	English