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From: gcary@SRC.Honeywell.COM (Greg Cary)
Subject: Re: Programming the printer port to read in data.
Message-ID: <1991May22.175312.4323@src.honeywell.com>
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References: <1991May21.202043.24725@magnus.acs.ohio-state.edu> <1991May22.104513.22169@msuinfo.cl.msu.edu> <1991May22.160613.9942@bronze.ucs.indiana.edu>
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Date: Wed, 22 May 91 17:53:12 GMT

Just as a note about writing to the parallel port, be warned that the
handshaking output pins on the printer port are open collector and are
pulled up by 4.7k resistors.  This is only a problem if you are using
the pins for something high speed and need the output to return high
right away after you have turned them on low (because they are open
collector they float back up at a rate that depends upon the resistor
value).  Also note that some of the the handshaking lines are inverted
so as to reflect the sense of the handshaking line (if a line is
active low, writing a "1" to that pin makes the output go low).  This
is the way I remember it when I worked on and old printer port card.
Newer cards might be somewhat different.
Greg Cary
gcary@SRC.honeywell.com    office: (612)782-7683   FAX: (612)782-7438