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SCIENTIFIC APPLICATIONS OF THE APPLE GAME PORT- PART III
Abstract:
Kenneth Ratzlaff
Note: This article was scanned using OCR from the 1984 Winter CCCE Newsletter. Please contact us if you identify any OCR errors.
In previous notes, we looked at methods of gathering data using the Apple and Commodore 64 game ports. The analog-to-digital converter in each case measures resistance and we noted some useful resistive transducers, particularly the thermistor, the photoconductive cell, and the potentiometer.
A Project SERAPHIM disk entitled "Data Collection/Retrieval System" developed by John K. Estell and John W. Moore is available. This disk desribes how the Apple II Game I/0 port can be used to collect data, store it on floppy disk and retrieve it for analysis. Software for data collection, retrieval, and graphical and numerical display is contained on the disk. A tutorial on the disk gives an overview of the system and contains instructions for using the game port, building an interface cable and using the software routines. The necessary hardware can be purchased for under $20 in local .electronics stores. The disk can be purchased for $4 from Project SERAPHIM (Department of Chemistry, Eastern Michigan University, Ypsilanti, MI 48197; (313) 487-0368) and will be available early in 1985.
Robert F. Tinker and Diana Malone have developed an experiment package entitled "Experiments in Chemistry". The kit consists of hardware and software which can be used with an Apple II, II+ or IIe having 48k or more of core. Fifteen experiments have been developed using the package which involve temperature measurements (enthalpic titrations, cooling curves and measuring the specific heat of metals), pH measurements or titrations, EMF measurements or poteniometric titrations. Additional experiments can be devised by the user. The software is menu driven and interactive. Graphs and tables can be generated. First and second derivatives of titration curves can be obtained. The complete package can be purchased from HRM software for $335 (.175 Tompkins Avenue, Pleasantville, NY 10570; (800) 431-2050). A demonstration disk is available to those contemplating purchase.
In this final note, we want to look at some experiments which use the game port to teach some fundamentals of data acquisition. In this regard, there are two general principles to be demonstrated: the effects of aliasing and of resolution.
ALIASING. The problem of aliasing the signal is best understood when one attempts either to sample a sinusoidal signal or to sample a de signal when sinusoidal noise is present. The sampling theorem states that a signal must be sampled at the rate of at least twice the highest frequency present. Failure to heed this rule will lead to the appearance of a component of the signal with a frequency which is an integral traction of the actual frequency.
We can demonstrate this phenomenon with the game port by using a potentiometer as the pivot of a pendulum as outlined in the last installment. If the pendulum is allowed to swing, the resistance will vary sinusoidally with time. Set up the pendulum so that the period is short, about 1 second. If you sample several times per second and plot the results, a sine wave will result with the same period that you observe visually.
To demonstrate an aliased signal, repeat the experiment a large number of times, each time increasing (with a FOR-NEXT wait loop) the time between acquisitions. When the acquisition rate. is equal to the period of the pendulum, the readings will be constant, and as the acquisition rate is slowed even further, a sine wave will reappear, but at a lower frequency.
In this experiment, the objective is to measure the frequency and amplitude of the sine wave, but the aliased·signal has the wrong frequency even while having the appearance of being correct. A similar experiment with a voltage-input ADC might entail acquisition of a spectrum from a spectrophotometer when a 60 Hz noise signal is super-imposed on the signal. If an analog filter is not used to remove the 60 Hz noise, it could very easily appear as a much lower frequency sine wave superimposed on the spectrum. The effect of the acquisition of gaussianshaped peaks is similar but is less intuitive and will not be discussed here.
RESOLUTION. The resolution must be controlled in a manner similar to the scale on a strip-chart recorder. The signal span should be from nearly zero to nearly full-scale. If not, precision
is lost.
The measurement of heat transfer is a simple experiment which simulates a first-order process. Take a beaker of cool water stirred by magnetic stirrer .with a thermistor to measure the temperature. Make a "hot finger" by blowing steam into a test tube which is partially submersed in the beaker. The temperature of the water will rise to a steady-state temperature.
Date:
12/04/84 to 12/07/84
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