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LPT (parallel port) hardware question

Discussion in 'Hardware' started by EtherJones, Jun 5, 2017.

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  1. EtherJones

    EtherJones Thread Starter

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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 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.
     
  2. zx10guy

    zx10guy Trusted Advisor Spam Fighter

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    What are you trying to do? It might be simpler to just crack open the laptop and trace the wiring to the I/O chip from the LPT port.
     
  3. EtherJones

    EtherJones Thread Starter

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    Several things:

    a) determine the driver chip so I can search for and download the datasheet and see if my measurements are consistent with the numbers in the datasheet

    b) determine if the driver chip is bad or if the numbers I'm getting are actually what the driver chip is supposed to be doing

    c) possibly write to the authors of those books and share my findings with them.

    d) determine what kinds of external buffer circuit(s) might be suitable for this particular LPT port.

    e) find out if someone on techguy has more extensive information about the driver chips used in LPT port designs than the authors of those books, and how he/she found that information.

    That's Plan Z. I tried taking a laptop apart before and it did not end well.
     
    Last edited: Jun 5, 2017
  4. EtherJones

    EtherJones Thread Starter

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    I designed, built, and tested the attached circuit, and it works well (Vs is drawn from the power pin of the laptop's USB port).

    But without knowing the specs for the particular LPT driver chip in this laptop, I can't be sure I'm not stressing the chip and risking burning it up.
     

    Attached Files:

  5. Gr3iz

    Gr3iz

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    The LPT port is typically integrated with many other devices in a PIO chip, which itself may be part of another integrated chip, especially in a laptop. Consider this, though ... Printer cables have been known to be fairly lengthy, 10-15', or more. This would tend to imply that the signal would need to be fairly substantial.
     
  6. EtherJones

    EtherJones Thread Starter

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    See attached graph of measured LPT data pin output mA vs Volts.
     

    Attached Files:

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