Xref: utzoo comp.sys.ibm.pc:40935 comp.sys.mac:45262
Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!hellgate.utah.edu!helios.ee.lbl.gov!ucsd!rutgers!mephisto!mcnc!rti!bcw
From: bcw@rti.UUCP (Bruce Wright)
Newsgroups: comp.sys.ibm.pc,comp.sys.mac
Subject: Re: Multiple monitors (was: Xerox sues Apple!)
Summary: Possible, sort of
Message-ID: <3378@rti.UUCP>
Date: 30 Dec 89 22:44:20 GMT
References: <2938@infmx.UUCP> <21548@mimsy.umd.edu>
Distribution: usa
Organization: Research Triangle Institute, RTP, NC
Lines: 31

In article <21548@mimsy.umd.edu>, folta@tove.umd.edu (Wayne Folta) writes:
> I don't think it is so easy to do this in general.  Your Mono/color system
> is pretty easy.  What about a multi-color system?  How would you change
> monitors then (co80a, co80b?)?  And when you do change monitors, you have to
> give a special command.  On a Mac, you simply move the cursor to the screen
> you want (you define the geometry: how the two monitors logically abut).

You're right -- it isn't so easy to do this in general.  For most video
adapter combinations (that's probably more accurate than "monitors" in
this context), the only thing that works is one of {CGA | EGA | VGA} 
combined with one of {MDA | Hercules}.  That is, you can't mix CGA and 
EGA (unless maybe you run the EGA in monochrome-text-only mode and don't
use it for color or graphics ... I haven't tried this combination because
it's sort of perverse, so I don't know if it would work, but I see no 
obvious reason why it wouldn't;  in any event you could only use one EGA
in this mode -- and no MDA's -- before the available addresses would be 
used up).  So the limit on a PC with "stock" video cards is 2 cards.

That's for standard adapters.  On another discussion thread I mentioned 
that I had seen a special adapter that would allow multiple EGA or VGA
adapters on a single machine.  The adapters definitely know about each
other -- they have a special "bank switching" technique which makes one
or another of them "go away" from the point of view of the PC software.
I saw this used to make a multiuser PC with an EGA or VGA for each user
(rather than running users off the serial ports).  It's rather expensive
(I seem to remember something over $1000 for each adapter), but it might
be cheaper than multiple PC's if you're running high-end machines.  But
this technique doesn't work in general -- you can't use it with just any
video adapter you find.

						Bruce C. Wright