[PARPORT] Re: Printing to off-line printer in 2.4.0-prerelease

From: Gunther Mayer (Gunther.Mayer@t-online.de)
Date: Thu Jan 04 2001 - 14:22:40 EST

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    Jesse Pollard wrote:
    > Originally, (wayback machine on) this was handled by a pull-up resistor
    > in the parallel interface, on the "off-line" signal. ANY time the printer
    > was powered off, set offline, or cable unplugged, the "off-line" signal
    > was raised by the pull-up. No data lost.
    > Now the parallel interface is bidirectional, and can have multiple devices
    > attached - this "fix" cannot be used. The interface is now more of a
    > buss than a single attached interface, and signals from a missing device
    > (powered off or disconnected) are floating. They may float high or low,
    > and depending on the environment (and which end of the cable is unplugged)
    > any thing in between.

    Not true. Electrical characteristics for parallel port implementations/cards
    differ wildly, nevertheless most implementations have:
    - data lines: bidirectional (see datasheets)
    - signal lines: see datasheets, never floating !

    Floating signal lines are a silicon bug/bad engineering and have nothing
    to do with bidirectional interfaces !

    Nowadays most integrated chips have internal signal line pull-ups internally, e.g.
    W83877TF says:
      TTL level input pin. This pin is pulled high internally.
      Open-drain output pin with 12 mA sink capability. Pulled up internally.
    -Data lines:
      TTL level bi-directional with 24 mA source-sink capability.

    Of course I would expect add-in cards to exist, with not so sophisticated chipsets
    and makers that have "forgotten" external pull-ups for economical reasons (2 cents :-)
    We should NOT care for broken hardware !!! I haven't seen any of these yet, even.

    On the other hand printer implmentations vary wildly, too.
    LJ1100: leave signal lines alone if powered off (0x7f)
        i.e. signal printer-not-ready ack-active out-of-paper
    DJ500: signal printer-error and off-line when powered off (0x87) !!!
        => Linux would dump data on this printer, if switched off.

    I think the current linux lp code tries to handle exotic/weird printers
    gracefully and leaves mainstream printers and users alone.

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