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Fun with the parallel (LPT) port

Recently I participated in an applied electronics seminar at the University of Ruse, which aimed at promoting the electronics "sub-subject" to high school and first-year students. I decided to talk a bit about CMOS integrated circuits and let the students play with the parallel port of a PC and make them play some music over the LPT port through an R2R DAC. It ended-up being quite fun, let me briefly show you what I used and how easy it was :) Below you can watch a video with the very first enlivening of the "player".

I used randomly picked (not even exactly R and 2R) resistors to form an R2R ladder which I then connected to the parallel port's 8-bit data bus. For the rest (LPT control) I used octave and the instrument-control package provided to directly access the LPT port and dump vector streams to it. Unfortunately "instrument-control" does not provide us with sampling rate functions as this is purely OS/octave handling dependent. Nevertheless we can measure it and provide interpolated samples to the DAC to suit our needs, or to keep a somewhat reasonable sample-time base close to the real one in the files which we are playing. Here is a minimalistic code to make things work.

%clear all;

pkg load instrument-control

[y0 rate bits] = wavread("/usr/share/skype/sounds/CallRingingIn.wav");

N = 8;

y=round(y0*(2^N/2)+(2^N/2)); % scale to 8 bits 0-255 and round to nearest integer

nSmpl = length(y);

pp = parallel("/dev/parport0", 0);

for k = 1:nSmpl
%pp_data(pp,255);  % flash port to measure LPT port sampling rate


Also, here are some pictures from the seminar we organized.


Date:Tue Apr 16 10:02:34 CET 2015


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