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From: ftpam1@acad3.fai.alaska.edu (MUNTS PHILLIP A)
Newsgroups: sci.electronics
Subject: Re: Is PC parallel port latched?
Message-ID: <1990Jul16.074519.23669@hayes.fai.alaska.edu>
Date: 16 Jul 90 07:45:19 GMT
References: <33965@ut-emx.UUCP> <1990Jul15.014354.12657@ux1.cso.uiuc.edu> <15568@ucsd.Edu> <905@ssc.UUCP>
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In article <905@ssc.UUCP>, markz@ssc.UUCP (Mark Zenier) writes...
>In article <15568@ucsd.Edu>, brian@ucsd.Edu (Brian Kantor) writes:
>> The IBM technical reference manual shows an LS374 in there, which is an
>> 8-bit latch.  The data lines are latched, but there's an LS244 reading
>> them back in when you read from the port so that you can perform a
>> self-test by reading back the byte you had just output.
>> 
>> Thus you can make the port bi-directional if you want.  Just chop the
>> wire from the latch's output enable pin, and run it out to a spare pin
>> on the connector.  The external device would control the direction.
> 
>In a old TTL implementation, you can also route the output enable to an 
>unused bit in the control register.  This is a configuration jumper on
>some boards.  There was a article 2-3 years ago in Computer Shopper
>giving the gory details.
> 
>Markz@ssc.uucp

     My laptop, a T1000, has this mod built into the ASIC already.  Bit 7 of
port 37AH needs to be set to turn off the output drivers.  Port 378H accesses
the data registers.  Coincidentally, I just spent this evening putting an
analog to digital converter on the printer port, so I have all the gory
details if anyone wants them.  (I'm using the port in both directions and some
of the handshaking lines, as well.)

Philip Munts N7AHL
NRA Extremist, etc.
University of Alaska, Fairbanks