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PC port question

Discussion in 'Electronic Basics' started by Randy Day, Nov 10, 2006.

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  1. Randy Day

    Randy Day Guest

    Before I plug this circuit into my PC, I'd like
    to know if it's going to blow things up.

    I'd like to use a 9v battery to power it,
    rather than a 5v supply, since I'm using spare
    parts for this. Can I use 9 volts and 1k
    resistors, rather than the 5v and 330 ohms I've
    seen in schematics? I know the current won't
    blow the LEDs, but will the 9v cause problems
    for the port circuitry? Are PC ports active
    pull-up, or open-collector devices?


    printer 1k LED
    port ___
    ___ |
    ___ |
    D2-|___|--|<--|-- +9v
    ___ |
    ___ |
    ___ |

    --- | |--
    --- ---
    - -
    (created by AACircuit v1.28 beta 10/06/04
  2. Baron

    Baron Guest

    Its a few years since I did any of this stuff, but my advice would be,
    If you are going to use 9V, to use a buffer between the LED and port.
    Individual transistors will work fine without risking damage to your
  3. If nothing else, the buffer protects the parallel port. So if there's
    an accidental short on the output, the parallel port survives. It's
    easier to put some another buffer IC in a socket than it is to fix
    the parallel port. In the old days, it was merely tedious (you'd
    have to desolder the dead IC), but nowadays it may not even be possible
    (because the parallel port is part of a much larger IC that is
    unavailable to the end user).

    Of course, there is always the risk that the wiring to the buffer
    IC is done wrong.

  4. Eeyore

    Eeyore Guest


  5. Randy Day

    Randy Day Guest

    Rats. Back to the parts bins.

    Thanks everyone.
  6. Gareth

    Gareth Guest

    Not sure if it will do any damage, but I'm sure it wont work (assuming
    you want to switch the LEDs on and off?)

    When the parallel port pin is low current will flow from the +9v though
    the LED and the LED will be on. That bit should be OK.

    When the parallel port pin is high, it will be 3.3V (unless you have a
    very old PC). You then have 9V - 3.3V across the LED and resistor, so
    current will still flow and the LED will still be on, though not quite
    as bright. You also have a problem that this current is going the wrong
    way (into the parallel port when the pin is high). This may do damage.

    PC parallel ports are NOT open collector.

    Are you saying that you want to use 1k resistors just because you don't
    have 330 ohms? You could try the original schematic with 1k resistors
    in series with the LED in place of 330 ohm. The LEDs wont be as bright,
    but probably still visible. Another thing you could try is paralleling
    up the resistors - 3 X 1k in parallel would give 333 Ohms.


  7. Randy Day

    Randy Day Guest

    Gareth wrote:

    I didn't know if they were or not; I was kinda
    That's what I'll be doing. I found an old 7805,
    and I have lots of 1k resistors. Thanks.
  8. Baron

    Baron Guest

    I know its academic, but where did you get 3.3 volts from ?
  9. Jamie

    Jamie Guest

    and where did you get those standards if may ask?
    i think many people have forgotten what the real
    standards are.

    if memory serves, -12 = low, +12 high.
    and the threshold points of
    +2.5 = high, -2.5 = low.
    anything between that is undefined.

    or has my years of work failed me?
  10. Yes, they've failed you.

    "High" and "Low" is in reference to the logic family, rather than
    an absolute. so your 12V bits would never apply to the TTL family
    that can never see more than 5v. (And given those voltages, it
    seems more like you are thinking of RS-232 that does have something
    like that.)

    The logic family likewise defines the threshold point.

    But, once this is all in place, the fact that there is a range of
    voltages for a "high" and a "low" means that you wouldn't know
    absolutely what a "high" out of a ttl gate would be. It just has
    to be above the threshold, and it might vary from IC to IC (within
    a certain small range) and from function to function (because the
    internal circuitry varies and that affects the exact voltage when
    the output is high).

  11. Jamie

    Jamie Guest

    if you say, so, i was talking about the RS-232 port, not the
    5 Volt logic family..

    have it your way, i'll stick with what i do.
  12. Tim Auton

    Tim Auton Guest

    You almost described (inverted) RS-232. But not quite. Parallel ports
    ("Centronics", IEEE 1284 etc.) are a different beast all together.

  13. Baron

    Baron Guest

    You are talking serial ports !
  14. Baron

    Baron Guest

    If you want to discuss serial ports then fine ! The OP was talking about
    a Parallel port device !
  15. Jamie

    Jamie Guest

    yes, your right, i was..
  16. Baron

    Baron Guest

    Succinctly put !
  17. Sjouke Burry

    Sjouke Burry Guest

    You have been playing with serial ports to much :) :)
    LPT port is ttl, with a pull up to Vcc minus one or two
    diode voltages(between 1 and 2 volts).
  18. Jamie

    Jamie Guest

    yeah well, at least i have something to play with.
  19. Gareth

    Gareth Guest

    Determined experimentally using a multimeter and a few PCs.

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