titus-larsen-titus-1981-apple-interfacing

In this new progra, the A array stores the light patterns, while th
M array stores the time intervals.
Step 1
So far, the computer has served only as a sequencer, generati
the proper lamp patterns and time delays. In this step, some contr
steps will be added to the traffic-light control program.
The traffic on Main Street is usually heavy, so the normal mod
for the traffic light should be green on Main and red on Elm.
program should be able to detect a single car waiting on Elm,
that it may be given the green light. However, Main Street m
be given at least 30 seconds of "green time," before any cars a
sensed on Elm Street. This means that every car waiting on El
Street will not automatically trigger a green-on-Elm sequence. T
make things even more interesting, there is a sensor on Main Stree
too. If five or more cars are waiting on Main Street at a red ligh
Main Street will be given the green light, and the cars on Elm wi
have to wait.
In order to program this, you may wish to draw a simple flowch
of the problem. An input port could be used to simulate the two road
sensors, but to teach you a bit more about the Apple, the keyboard
·
will be used instead.
The keyboard uses two memory addresses for control. Address
49152 contains the keyboard data, and address 49168 is used as a
flag-clear pulse output.
Enter the following program into your computer and run it:
2000 PRINT PEEK(49152): GOTO 2000
Press some of the keys on the keyboard and note what happens
the display. What do you observe
There is a new decimal value displayed whenever a new key is
pressed, and the value continues to be displayed until a new key
is actuated. Thus, the information at input port 49152 represents the
code of the last key that was pressed.
Step S
We would like to have the computer input a value from the keyboard
input port only when a key has been pressed. To do this, you
must use the keyboard flag bit, which is bit D7 at input port 49152.
If this bit is a logic zero, all values from this port will be less than 128.
If this bit is a logic one, the values will be 128 or greater, up to 255.