[PARPORT] Strange behavior from the parallel port

From: David Tarkowski (dtarkows@rcn.com)
Date: Thu Mar 14 2002 - 22:12:07 EST

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    I have what I think is a strange problem, and I just can't figure it
    out. Hopefully somebody here can help me.

    I have essentially eight LEDs connected to the data lines of my parallel
    port. On my windows PC at work I have software that allows me use the
    parallel port as a digital I/O device. When I plug my LEDs into the
    parallel port on that computer I can turn on or off any LED as I please.

    When I plug the LEDs into my the parallel port of my Linux box I see the
    strange behavior. Some of the lines are active intermittently. D0 is
    modulated with a very high duty cycle. Lines 3,4 and 7 seem to be high
    all the time, and lines 1 and 2 pulse about ten times a second. Several
    of the lines flicker whenever there is a hard drive access. Near as I
    can tell, the lines are a reflection of the interrupt status of the PC.

    If I write data out on the lines, the LEDs show it, but the data is
    immediately "erased." If I put the program into a tight loop, the lines
    will reflect the data I am trying to write (i.e.
    for(;;){ioctl(fd,PPWDATA,0)} will turn off all of the lines).

    If I do 'cat file.txt > /dev/lp0' the LEDs go out, but still flicker
    like they are being turned on and immediately turned off by the lp

    I know the parallel port works because I had a printer connected to it
    until recently.

    Is my description clear? Any thoughts?


    "Do not taunt Happy Fun Ball"  -SNL

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