Introduction
Armed with only a Schematic, attacking a TRS-80 Computer may at
first seem to be the ultimate exercise in futility. But that's where we
come in. This book has been written with those kinds of feelings in
mind (matter of fact, the writer went through many of the feelings of
trepidation as he was first asked to trouble-shoot and repair an early
TRS-80
...
).
You may know what
"CPU"
stands for. You may even
have some knowledge of how a microprocessor system works.
After a while, you find you have problems. You know a 4K RAM needs
12 address lines. But you only find
7.
You know a computer keyboard
givesyou 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'treally know your ASCII from a scroll when it
comes to the TRS-80 Computer
...
and so once again we get back to
this book. Join the crowd; line up and let's all learn together. Grab this
book, the Schematics and together we'll attack that TRS-80 with
soldering iron smoking.
The purpose of this book is to give you a practical knowledge of
system operation a'ia TRS-80. This book will show you why there are
only seven address inputs to a 4K RAM. We'll show you 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
Schematics and let's take a tour of the TRS-80 Computer.
Let's start off easy
...
with a
...
7
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