Re: [PARPORT] Question about reading from input pins.

From: Blaise Gassend (
Date: Thu Nov 21 2002 - 21:11:34 EST

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    > For example im connecting the 11pin (which is first input pin) with ground
    > to the 9v battery
    > and, in while(1) loop i am reading the status of inputs in parraler port.
    > I cant see any change when connecting/disconnecting the input pin from
    > battery ;((
    > Of course, i ses a change when i connect the pin to the ground, but this is
    > not what i need :(

    What you are experiencing is perfectly normal. The input pins of your
    parallel port are designed to have a very high impedance. The logic of
    the parallel port looks at the voltage of the pin, and depending on the
    value of the voltage, it decides that it is seeing a 1 or a 0.

    If you connect the pin to a 5V potential (the - of a 5V power supply to
    the ground, and the + terminal to the input pin), you change the voltage
    between ground and the input pin to 5V, and the input is a 1.

    If you connect the input pin directly to ground, you are putting a 0V
    voltage between ground and the input pin, and the input is a 0.

    However, if you don't connect anything at all to the input pin, it stays
    at its old value, which is exactly what you are experiencing.

    To solve your problem, I would suggest using a pulldown resistor. That
    is, you put a resistor between the input pin and ground. That way, if
    nothing else is connected, the input pin discharges to 0V through the
    resistor and you get a 0. However, when you supply 5V to the pin, the
    pin is brought to 5V (and there is a little leakage current through the
    resistor that you shouldn't worry about) and you get a 1. Bingo, this is
    what you want.

    The pulldown resistor acts a bit like a spring that brings your pin back
    to 0 when you are done driving it (don't push the analogy too far
    though). I would suggest a few kilo-ohms for the resistor.

    As a side note, I am worried about the 9V you are using on your parallel
    port. You could damage it as it is only supposed to be used with up to
    5V. You could put a resistor that is the same size as your pulldown
    resistor between the battery and the input pin to get that voltage
    divided by 2, which should be fine.

    The circuit I would do is presented below. Of course I decline all
    formal responsibility should your computer blow up or your stock
    suddenly crash... :-)

         R R

    where A is the place where your battery is connected or not, B is the
    input pin on the parallel port, and GND is the ground (- terminal of the
    battery, and ground of the parallel port).

    Have fun,
    Blaise Gassend

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