*BSD News Article 10532


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From: mark@roissy.umd.edu (Mark Sienkiewicz)
Newsgroups: comp.unix.bsd
Subject: Re: reading from parallel port
Message-ID: <18212@umd5.umd.edu>
Date: 28 Jan 93 23:31:02 GMT
References: <1jhejv$9l3@Germany.EU.net> <C1ADIM.1s3@sleeper.apana.org.au> <1993Jan26.025455.7032@ucsu.Colorado.EDU>
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In article <1993Jan26.025455.7032@ucsu.Colorado.EDU> galbrait@rintintin.Colorado.EDU (GALBRAITH JOHN) writes:
>
>I have built some hardware that read from the parallel port.  The problem
>is that the main data lines are output only.  You can read from the port,
>but it returns the last value that you wrote, not what is currently
>what you may have put on the port.

and

>Also, it could be that some newer lpt cards do have bidirectionality.  I
>think the MCA cards do for instance, but I may be wrong.

I once saw some PC/XT schematics a _long_ time ago.  I remember thinking
this: if you drive the output with 0xff, the output drivers will go high and
not fight with the input on the same device.  From this, I assume that the
output drivers were open collector, but I'm not sure (maybe was tristate?).
The input and output bits were on the same wire.

It _looked_ like some hacker thought it would be fun to make that channel
available for input, even though nobody has any input to give it.  I've
heard that not all PC parallel ports support it, but apparently several
do.