Connect with us

PC parallel port, input

Discussion in 'Electronic Design' started by Jem Berkes, Jan 30, 2004.

Scroll to continue with content
  1. Jem Berkes

    Jem Berkes Guest

    I've used the PC parallel port in several projects in order to provide
    output digital signals to an external circuit (the parallel port outputs
    are TTL buffered, and can source max 2 mA and sink considerably more).

    Anyway, now I'm trying to find specs on how to properly use the parallel
    port's level-active input lines - the Status bits (Busy, PaperEnd,
    SelectIn, Error). But I can't find much in the way of specifications for
    how the inputs work. Are these pure TTL as well?

    Since I don't have my own +5 supply on the external circuit, would it be
    safe to set one of the parallell port's output bits high and use that as a
    +5v source? e.g.

    PC gnd ----------------|\\ |
    | | Q1 |
    Error (inp) -------|---|-/ |
    \ | --|
    22k / | LED /\ |
    \ | -- |
    D0 (out) / | --|
    Set to +5 ---------| ----------

    Is this a safe way to do things? Seems to me that in theory, when Q1 turns
    on (due to signal from the other side of the optocoupler) the Error line
    will be pulled low to gnd, and D0 is sourcing < 1 mA (safe). When Q1 is
    off, Error is pulled to the same level as D0 (high). I think this is proper
    for TTL, but I wanted to check before I damaged my motherboard :(
  2. Si Ballenger

    Si Ballenger Guest

    I've got a page below that does something similar with the
    parallel port, using qbasic and a web server to check the status
    of some switches. I used transistors to pull the status lines low
    enough to show a change of state. Using the transistors and 4.7k
    resistors, the data lines can supply enough current to make the
    setup work.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day