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Interfacing electronics project to a PC

Discussion in 'Electronic Basics' started by Stephen Schmitt, Dec 20, 2003.

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  1. Hey everyone,

    I have a hobby project I'm trying to control with my PC. It consists mostly
    of reading inputs and driving nothing more difficult than some LEDs.

    I've read multiple FAQs about the parallel port and so forth, and I can
    figure out the circuit design for what I want to do easily enough, but what
    I _don't_ know about is what kind of protection I need to buildin for the

    First of all, since I will have both an external +5v regulated wall-wart and
    the +5v from the parallel port, what's the normal way to reconcile the two
    seperate power supplies? I thought I could just tie the grounds together and
    be OK, but I've heard vague rumblings that that could sometimes lead to
    disaster. How is this normally done, since it's a very common situation but
    one that I never really hear addressed.

    Secondly, what kind of protection do I need on the input/output lines for
    the parallel port? Should I be using opto-isolators, or am I ok running the
    inputs right into my control pins, and the outputs through TLL driver ICs?

    Thanks for any advice!

  2. Bob Masta

    Bob Masta Guest

    Go ahead and connect the grounds together, as long
    as you are certain the wall-wart ground is in fact
    isolated from the powerline (UL approved, etc).
    Unplug and check with an ohmmeter if any doubts.

    The port can drive TTL-type loads, so you will be OK
    running them directly to TTL drivers for the LEDs.

    The port inputs can handle 0-5V, so if your external
    supply is in fact regulated to 5V, you should be OK.

    Note that if you want to run this under Win2K, NT, or XP
    you may need a special driver since you can't do direct
    I/O. Check at <> for links and discussion.
    Win9x and older you can talk to the port directly.

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  3. Stephen,

    Make sure that the frame of your PC has been connected to the protective
    Never connect or disconnect something to/from the parallel port when the PC
    is running.
    Connect the ground of your external 5V powersupply to the ground of the
    parallel port.
    Parallel ports do not have a 5V on their connectors. Some printeres may
    Keep the cable to the parallel port short.
    Buffer the signals using LS245, LS364 or LS14 depending on the type and
    direction of the signal. Similar CMOS types will do as well.
    As long as you use only 5V external supply and do not use long lines, there
    is no need for optocouplers.
    If you want to stay on the save side, get an extra parallel port card and
    use that for your expirements.

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