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  • #2341693

    LPT (parallel port) voltage and current

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    by steve19814 ·

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    Hello.

    I am wanting to re-purpose an old laptop computer to use its parallel port (LPT printer port) to output a +5V/0V signal on one of the data pins. The load requires about 9 mA at 5V.

    I know how to program this; my question is about the LPT hardware.

    I hooked up a data pin to a simple circuit with a variable resistance load, an ammeter, and a voltmeter… and got the following mA vs Volts at the data pin by varying the load resistance:

    Volts mA
    4.53 0
    4.23 0.15
    3.97 0.28
    3.93 0.3
    3.78 0.38
    3.5 0.38

    From this data, is there any way to identify the output driver chip(s) being used in the LPT port?

    I’ve read two books:
    – Parallel Port Complete by Jan Axelson, and
    – Interfacing the PC for Process Control by Dhananjay Gadre

    … and the data I got above is not consistent with the information in either of those books.

    Thank you.

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