Radio Shack CCR-81 Reference Handbook

-8
and
he Outside World
As you use your Computer more and more, you
will begin to see more and more applications for
it. There will come a time when you will want to
do some tasks that can only be handled with
external circuitry; or the BASIC language does
not include such functions as "TU RNCP.X"
(turn on coffee pot at time x). You know that
a Computer should be able to do such things but
you may not know how to go about designing
a
circuit to do it.
An added function consists of two basic parts;
the first part is the software. Will you have to
write a short machine language program to do
the task? If you have a Level I machine, more
than likely the answer is yes. If you have a Level
II machine then maybe you can use the POKE
or PEEK or the OUT or IN instructions. If so.
you're writing the program in BASIC and the
software part will not give you any problems.
Software written in machine language can also
be painless if you use an Editor/Assembler to
generate your object code. At any rate you're
going to need software to control the device that
you build.
The next part you'll need to think about is the
hardware. That's a necessity. Neither the
TRS-80 nor the Expansion unit incorporate a
relay
to turn on power to your coffee pot (or
whatever). Hooking
up a relay to an output line
isn't going to do the job, because your coffee
pot may turn on every time you do a cassette
load.
When designing
hardware for your special task,
you should have defined the software already.
How you design your hardware will be deter-
mined by the instructions you will use to
operate it. There are two approaches you may
use to control your home-built device:
1. A memory mapped system
2. A port system
84
If you decide to memory map your hardware,
you will specify a memory address that will be
the location your system is in the map. To write
data to your system you will address it via the
16 address lines and write data to it using the
data lines. You will do this by using the address
lines
and the WR* line at the Expansion Port
Connector. If you need to read data back into
the TRS-80, you will use the address lines and
the RD* line. The machine language software to
do the job will be the LD (load) instruction or in
Level
II the POKE and PEEK instruction.
For example, let's say your coffee pot is
memory-mapped to address 8FFF (Hex) and
binary data 02 will turn it on. An assembler
instruction that controls it will be:
LD 8F FFH,02H ; turn on coffee pot.
(Address 8FFF will be loaded with binary 02
Hex.)
Note:
This assembler routine will generate the
object machine code which will activate the
function.
A POKE statement would look like this:
POKE
36863,2
(decimal address 36863 is 8FFF in Hex.)
In a port-based system. you will specify a port
address out of the 256 ports the CPU will
address. Once again the address of your system
is selected using the address lines but this time
only eight (8) lines are used instead of all 16.
The data bus is used to pass information back
and forth between the CPU and the selected
port.

Radio Shack CCR-81 Specifications

General

Brand	Radio Shack
Model	CCR-81
Category	Desktop
Language	English