Re: Apple II ADCs

[dempson@atlantis.actrix.gen.nz (David Empson)]

In article <1995Jan31.142337.14317@cuok.cameron.edu>,
Thomas A. Compter <tacompte@cuok.cameron.edu> wrote:
> John L. Graham (fl332@cleveland.Freenet.Edu) wrote:
> 
> > Just a thought...a friend of mine at work are getting ready to
> > prototype a new ADC that would work on an Apple II, or nearly any
> > computer with a serial port. Our current setup is an 8 channel,
> > 12 bit ADC with hardware selectable ranges of +/-5 or +/-10 volts.
> ----< Snipped>-------
> Sounds interesting...... but;
> Just one question from the ignorant masses, (or at least this
> ignorant mass)  What the heck is an ADC? 

Analog to Digital Converter.  It is a device which reads a voltage (or
current, in some designs) and returns a binary number proportional to
the input.

The number of "channels" indicates how many analog inputs can be
connected.  The resolution indicates how many discrete voltages can be
measured (e.g. a 12-bit ADC can read 4096 distinct voltages within its
range, assuming the full resolution is available - this is not usually
the case; there are also accuracy limits in most ADCs).

In this case, the voltage range of the analog inputs can be set to
(-5V..+5V) or (-10V..+10V).


Analog to Digital Converters are usually used with some kind of
"transducer", which is a device that converts a physical measurement
into an analog voltage or current.  You can measure almost anything
given the right type of transducer.  Some examples: wind direction
(angle), temperature, pressure, humidity, ambient light, power supply
or battery voltages and currents.

Some physical measurements are easier to read as a digital pulse
count, e.g. wind speed (measuring the rotation rate of an anemometer),
power usage.  These quantities can be converted into an analog voltage.

The Apple's paddles/joystick are an example of a simple
analog-to-digital converter.  The position of the paddle (or one axis
of the joystick) controls a variable resistance, which affects the
time required to discharge a capacitor in the game I/O circuitry.
This time is measured by software, and the time taken indicates the
paddle or joystick setting.

The main problem with this method is that it is slow and somewhat
inconvient (reading a paddle can take up to three milliseconds).  A
"real" ADC will generally take a fixed and small amount of time to
generate a digital value (most are in the order of 10 to 200
microseconds), a much more accurate reading is possible, and the
conversion process doesn't tie up the computer's microprocessor.


Then there are digital to analog converters, which allow the computer
to output an analog voltage or current proportional to a digital
value.  This can be used to control a variety of items, such as motor
speed, light intensity, etc.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand