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Parallel Port Input

Discussion in 'Electronic Basics' started by Peter, Aug 13, 2003.

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  1. Peter

    Peter Guest

    I have read dozens of postings here and also all the FAQ's at Steve
    Walz's site including zha96lpt.faq but can't find a straight answer on

    1. To input data via S4-S7 can I just wire up a 5v/0v input directly
    to the pins with no resistor in between? I have actually done it and
    it works fine but I was concerned about overloading the port -
    particularly when I found that the pins on the parallel port are 3.6v
    when "1" rather than 5v. (well within the 2.4v-5v TTL high range I

    2. Most circuts I see apply 5v to the Status pins in the steady state
    via a pull-up resistor then pull it low with a switch to input data.
    Why is that preferable to holding them low in the steady state and
    applying 5v to the pins to input data? Is it simply the possible extra
    circuitry required with the latter?
  2. -------------
    Yes, sure. But time-tested methods indicate that some failures in the
    supply can threaten the parallel port if you do, such as if the
    power or ground deviate out of range, greater than 5 or less than 0.

    No, you can't "overload" a port giving it the voltage it expects,
    current flows according to voltage applied and resistance of the
    load, and the parallel port is standard TTL spec, so it can't.

    For a few reasons with TTL involving the actual voltages of the logic
    levels it is better to use a pull-up and ground the pin than using a
    pull-down resistor. Study TTL a while to see this.

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