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Parallel port pins 18-25

Discussion in 'Electronic Basics' started by [email protected], Jul 14, 2005.

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  1. Guest

    Most PC parallel port literature seems to state that pins 18-25 are
    connected to ground, however, I have a laptop with parallel port pin 25
    not connected to anything IMO. At first I thought it was a faulty cable
    but that wasn't the case. Pins 18-24 are all connected to ground.
    What's the deal here?
  2. Historically, pin 25, was reserved for the parity bit on one high speed
    IBM printer connection, on the first port only. This was specific to a
    couple of IBM machines, and 99% of other people never implemented this.
    The line is either left N/C (floating), or connected to ground. The latter
    IEEE specification for the port has the line used as the ground connection
    for three of the handshake lines. Normally the ground connections are all
    simply shorted together, so having the pin N/C, should not be a problem.
    The reason is commonly that a 24pin PCB header has been used to a ribbon
    cable, feeding the rear plug, so the manufacturers leave this ground

    Best Wishes
  3. Guest

    The reason is commonly that a 24pin PCB header has been used to a ribbon
    Thanks, makes sense.
  4. w_tom

    w_tom Guest

    Some implementations of the Centronix interface used pin 25
    from printer as a +5V power source with limited current.

    Remember one side of the standard IBM cable is a 25 pin
    connector. The other side is a 36 pin connector. Therefore
    all 25 pins on the first connector would be connected to a
    each wire in the ribbon cable. But when talking about cables,
    first define with connector is being used - the 25 pin D
    connector or the 36 pin Centronix connector.
  5. oparr

    oparr Guest

    Remember one side of the standard IBM cable is a 25 pin
    And in case you forgot, it plugs into what is called the parallel port which
    leads to the subject above and what was being discussed until your post.
  6. w_tom

    w_tom Guest

    Centronix parallel port. The signal called Error is pin 32
    on the printer; pin 15 on the PC. The signal called
    Initialize Printer is pin 31 on the printer; pin 16 on the
    PC. Both ends of the cable are standard parallel port
    connectors. And yet pin numbers are different on both ends of
    the cable. I did not forget anything. The pin numbered 18 on
    the printer parallel port connector sometimes connected to pin
    25 on the parallel port connector that might connect to PC.
  7. Iwo Mergler

    Iwo Mergler Guest

    Some laptops can connect an external floppy drive to
    the parallel port connector.

    It's an alternative function. Switching between parport
    and floppy function could be done by pulling high an
    'unconnected' ground pin.


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