titus-larsen-titus-1981-apple-interfacing

The A output, RD 49319, has already been used and RD 49318
not work, since it is decoded for an input port. The WR 49318
output ( C ) would be the choice to use. It provides a positive pulse
which. is the same type of pulse required by the SN7475 latch chips.
This output is ao de£.oded for an output device. You should remember
that the 7 and 6 output pins from the decoder on the printed
circuit board actually correspond to decoded addresses 49319 and
49318, respectively.
Make a connection between pin 13 on the SN7402 and pins 4 and
13 on both of the SN7475 latch chips. This is the DEV SEL connection
shown in Fig. 6-10.
To test the output port, enter the following program into your
computer:
10 A= 0
20 POKE 49318,A
30 END
Preset the variable A to zero, as shown, and run the program. What
happens to the lamp monitors
They should be unlit, since zero has been transferred to the output
pmt Now set A to 255 and nm the program again. You should see
all of the LEDs light. If these conditions have not been found, recheck
your circuit and the test program.
The program may be changed so that you can easily enter new
values from the keyboard. The new program is:
10 INPUT A
20 POKE 49318,A
30 GOTO 10
You may try any values that you choose, but we suggest that you
try powers of two first, 0, 1, 2, 4, 8, etc., since these will test the individual
LEDs.
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