Caple C871i Manual do Utilizador descarregar pdf (Página 208)

Apple II Computer Info

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### FILE : interfacing

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### Created : Sunday, January 14, 1996 Modified: Sunday, January 14, 1996

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Subject: versatile interface

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GAME PORT, VERSATILE INTERFACE

A multitude of interface cards exist for the Apple II But, few

people realize that the Apple II game port can be a very

versatile interface.

A "paddle" input is internally connected to a trigger chip and

a .022 MFD capacitor. The paddle is a variable resistor and a

voltage source. In use, the capacitor is discharged, a timer

(counter) is reset, and then the capacitor is allowed to charge

by current passing through the external resistor while the

counter is incrementing. When the charge on the capacitor reaches

3.2 volts, the trigger circuit stops the counter. The number

stored in the counter at that time gives an indication of the

time required for the capacitor to charge up to the trigger

voltage, and that time relates the value of the external resistor

(paddle) as follows;

V = (input voltage)*(1-EXP(-t/R*C))

From experiment, we find that with a 5 volt input and a 120K

resistor, the counter will reach a value of 250. From this

fact, and using the above formula, we can plot the count as

a function of resistance

Knowing all of this, we can design interface circuits. Such

devices as photocells, photo resistors, thermisters, and other

things could be connected to the game port. With appropriate

software, we could use the Apple to monitor a variety of

real-world events, and to display the results.

For example, a local chemistry lab wanted to measure the "time

of solution" when a chemical was dropped into a test tube

Apple II Computer Technical Information : Apple II Family Hardware Info

ftp://ground.ecn.uiowa.edu/2/apple2/miscinfo/hardware : May 2001 : 208 of 572

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Caple C871i Manual do Utilizador Página 208

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