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From: emmo@moncam.co.uk (Dave Emmerson)
Newsgroups: comp.sys.ibm.pc
Subject: Re: INPUT PARALLEL DATA OVER THE PRINTER PORT?
Summary: A little knowledge...
Message-ID: <662@marvin.moncam.co.uk>
Date: 30 May 90 00:54:17 GMT
References: <7459@arctic.nprdc.arpa> <2010023@hpctdls.HP.COM> <12548@netcom.UUCP> <3260@galena2.UUCP>
Organization: Monotype ADG, Cambridge, UK
Lines: 37

> >wei@hpctdls.HP.COM (Bill Ives) writes:
> >>     My guess is that
> >>   parallel input has never caught on due to it not being possible
> >>   on original IBM equipment (actually burning it out -- I've done
> >>   it so I know).  


This has been discussed already this year, so I'll keep it very brief this
time..

Many PC comms boards use the standard Centronics output driver configuration
but have NO provision for INPUT other than 2/3/4 control lines, since they
are only expected to be used as printer/plotter ports. The ouput device
is usually an 'open collector' driver (eg 74ls07) with pull-up resistors and 
filter capacitors. If you connect 2 of these cards together, both can normally
survive, since the current sources are pull-up resistors, although they won't
necessarily 'talk' to each other..

Unfortunately, some boards don't use open-collector outputs, they are one-chip 
implementations, from design packages which do not cater for O/C - if you
connect one of these to ANY other prn port, it WILL DIE - pdq.
How can you tell? Sorry, no easy way, but if both boards feature at least 2
74ls07's near the prn connector, they'll likely survive. Consult the manual
or even the box lid - you'll have paid over the odds for that board, so
they should be at pains to tell you why.

Look for boards which are _specifically_ stated to feature PARALLEL I/O, these
will cost about the same as 2 of the other type. (That's the one you fried
and its replacement anyway - seems fair enough). This is the  Centronics
standard board, it automatically disables its output drive when it detects
input, and can only corrupt data if the control lines are broken.

Centronics is a trademark of the Centronics Data Computer Company.

Dave E.

- Rabbiting as only a carrot-cruncher can -