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From: oferf@shum.huji.AC.IL (ofer faigon)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Using the printer port for input
Summary: See BYTE Sep 88, page 305 and 310
Message-ID: <69@shuldig.huji.ac.il>
Date: 26 Jan 89 01:05:58 GMT
References: <704@krafla.rhi.hi.is> <691@hgcvax.uucp> <274@laas.laas.fr> <485@octopus.UUCP>
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: oferf%shum.UUCP@humus.Huji.AC.IL (ofer faigon)
Organization: Hebrew University of Jerusalem, ISRAEL
Lines: 48


There waere several queries recently about reading input through
the parallel printer port, but I don't recall seeing any definitive
reply.

The following is quoted from Steve Ciarcia's Circuit Cellar that
appeared in BYTE, vol 13, No.9 (September 1988), page 305:

    Unfortunately, an irritating problem with the design of the PC
    is that its standard parallel printer port is output only.
    All the hardware needed to read or write 8 bits of data is
    already in place, but it lacks a connection to enable that
    function.  While you can buy a custom parallel I/O board, I think
    changing a single trace on the existing board might be worth it
    to some readers.  Figure 5 shows the single cut and addition to
    convert a standard paraller printer port to bidirectional operation.
    Of course, all the clone boards (even the IBM boards) seem to use
    different IC numbers and assign the bits to different pins on the
    ICs, so this modification can be a real mystery.  If you are unwilling
    to chop up your printer port card, buy a $50 clone printer port card.
    They are usually bidirectional.  Some clone boards also omit the input
    connection between the data bus and pin 14 of the LS174.
    ...[some text deleted]...
    ...(a modified port will stil work correctly with all your other
    software, simply because the code doesn't know about the change
    and won't take advantage of it).

Figure 5 appears on page 310, and is a bit hard to reproduce here.
What it shows is the three chips involved (74LS374, 74LS244 and 74LS174),
and the one (or two) modifications needed:

- Disconnecting the output-enable pin (1) of the 374 from ground,
  and connecting it to the unused output pin on the 174 (the figure
  shows a connection to pin 15 - Q6, assuming Q1-Q5 are in use).

- If the connection between BDATA5 ant the input side of the 174
  is missing, then connect them (the figure shown connection to
  pin 14 - 6D, assuming 1D-5D are connected to BDATA0-4).

And a final quotation: "Actual pin numbers on your board will vary;
Trace the wiring from the DB25 connector to identify the right traces
and pins".

Have fun.

- Ofer Faigon                                         TEL: +972-2-669-834
  MAIL: P.O.B 7347, Jerusalem 91072, ISRAEL
  BITNET: oferf@HUJINIX         CSNET & INTERNET: oferf@shum.huji.ac.il