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From: mjw06513@uxa.cso.uiuc.edu (Mary Winters)
Newsgroups: comp.sys.ibm.pc
Subject: Re: INPUT PARALLEL DATA OVER THE PRINTER PORT?
Summary: Last word?
Message-ID: <1990May31.141830.11312@ux1.cso.uiuc.edu>
Date: 31 May 90 14:18:30 GMT
Article-I.D.: ux1.1990May31.141830.11312
References: <7730@canterbury.ac.nz> <1990May30.153755.29766@ux1.cso.uiuc.edu> <1990May31.020814.26271@gpu.utcs.utoronto.ca>
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Organization: University of Illinois at Urbana
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Last night I received some email telling me that my earlier comments were "full
of baloney." Well, definitely not FULL....

I did some more digging, and here's what I came up with. Apparently, the 
original IBM PC had an output-only printer port. The procedure I described
yesterday (where a certain pin's ground connection was removed and the pin
tied high) comes from the book "Interfacing to the IBM Personal Computer" by
Lewis Eggebrecht, and appears to apply to the original PC. Steve Ciarcia, in
a Circuit Cellar article appearing in the September, 1988 issue of Byte 
magazine, has this to say about PC parallel ports:

"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. [...] Figure 5 shows the single cut and addition to
convert a standard parallel 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."

The person who emailed me said he has tried bidirectional I/O on various types
of parallel ports, and it always works for him. This seems to suggest that 
while the original IBM PC and clones which copy the hardware of the PC's 
parallel port exactly suffer from the output-only problem, many (most?) third
party ports will work just fine. I found a parallel port card made by 
"Magitronics" or something like that for $20 in Computer Shopper which is
bidirectional (according to the docs), so if your printer port isn't bi, it's
easy and cheap to get one that is.


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