Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!ucsd!brian
From: brian@ucsd.Edu (Brian Kantor)
Newsgroups: sci.electronics
Subject: Re: Is PC parallel port latched?
Message-ID: <15568@ucsd.Edu>
Date: 15 Jul 90 03:17:07 GMT
References: <33965@ut-emx.UUCP> <1990Jul15.014354.12657@ux1.cso.uiuc.edu>
Organization: The Avant-Garde of the Now, Ltd.
Lines: 12

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.

There are other strobe and control lines, some in each direction.  Best
you check a schematic if you're planning to hack it.
	- Brian