5
STAR*TECH JOURNAL/JANUARY 1983
SEGA ELECTRONICS'
ROM LINE (PART 4)
1/0 Ports
So far in our discussion of a computer, we've
looked at circuits that cannot communicate
meaningfully with a human.
There must be additional circuits that allow
a computer operator to control the flow of
information to and from the computer. That is
exactly what the input and output ports do in
any computer. They are the direct means
through which an operator communicates with
the computer, or the computer with the operator.
Let us look at an input port first.
In a computer, the data bus is the bi-
directional path for the flow of computer infor-
mation. The data on the data bus is used by the
microprocessor to perform certain functions.
When the data is input by a human operator, it
must be placed on the data bus at the proper
time for use by the computer. So, an input port
is tied directly to the data bus and the informa-
tion available at the port is "read" by the
computer and acted on. (See Figure I.)
The input switches are operated by the
player ( in the case of video game computers),
then, at a predetermined time, the computer
enables the input port IC so that the switch
levels ( either high or low) can be placed on the
data bus. Now, the switch levels can be under-
stood by the computer and acted on accordingly.
Note that the input port is not enabled all the
time. Through the input port select IC, the
computer "looks at" this input port only and
takes in the data. The "AD" and "IN" signals
must both be active high for the "ENABLE"
signal to be active LO and select and input port
IC. Both" AD" and "IN" are signals generated
by the microprocessor under control of the
computer program. Remember, both signals
must be high to enable the port.
In the case of output ports, the exact
opposite procedure occurs. Data to be output
to the player (for example, a game sound), is
placed on the data bus. At the right moment,
again controlled by the program, the output
port is selected and the data is sent to the sound
board to trigger the appropriate sound board
circuit. Figure 2 shows a typical output port
configuration.
Again, the computer must be told by the
program when to select the output port. This is
done by the" AD" and "OUT" signals applied
to the output port select IC. Both signals must
go high to make "ENABLE" active low.
This is, of course, a general introduction to
the 1/0 ports, but it describes common designs
in 1/0 ports. More complex 1/0 designs are
often necessary because of the larger number of
1/0 ports available in a particular computer.
The important points to remember in any 1/0
circuit are that 1) the computer must select the
1/0 port under program control and 2) the
computer, in selecting a port, must then enable
that port. Keep these points in mind when
troubleshooting a game that, for example, does
not output data to a sound board, or does not
allow the player switches to activate the game.
In the next issue, we'll see the need for
power. Keep tech'ing!
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