titus-larsen-titus-1981-apple-interfacing

ue in sequence (in binary), 255, 255, 0, 1, 2 ... 254, 255, 0, 1, etc.
is program will be seen again, in the experiments.
Example 3·1. An 8-Bit Binary Counting Program for Port 49320
FOR N = 0 TO 255
POKE 49320,N
NEXT N
GOTO 10
Output ports are rather easy to construct. Most parallel-in, paral­
·
-out lo ic evice inte h ca abilities can be used as
c Examples of devices that can be use as latches are the
74193 programmable binary counter, the SN74LS194A universal
ift register, the SN74198 shift register, etc.
Most output ports are readily configured with standard inteated
circuits, but some of the newer integrated-circuit devices that
· meant specifically for use with microcomputers are becoming
ilable with built-in latch functions. An example is the Signetics
5018 8-bit digital-to-analog converter chip which contains a latch
tion.
Typical applications for output ports include the following:
Transfer data to a printer
Transfer data to a video display
Control a traffic light
Transfer data to a floppy disk
Actuate switches on a model railroad
Control valves and pumps in a chemical process
Control a plotter
Transfer data to a seven-segment display
Control another computer
n some applications, the value of the information is actually used,
· e in others, the on or off state of each bit is used. Some devices
as a printer may use a combination: ports for the transfer of the
to be printed and ports for the control of the printer functions.
lays made up of seven-segment LEDs frequently require the
•··of several output ports, even though the display is considered to
only one "device."
INPUT PORTS
pput ports are used with 1/0 devices so that they may transfer
mation to the computer in 8-bit bytes. Unlike output ports that
be able to accept and hold information that is placed on the bus
specific time, and may be continuously connected to the data
input ports must be able to "disconnect" themselves from the
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