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From: toma@tekgvs.LABS.TEK.COM (Tom Almy)
Newsgroups: comp.sys.ibm.pc
Subject: Re: extension cable for EGA/VGA monitor
Keywords: extension monitor/keyboard cable
Message-ID: <6196@tekgvs.LABS.TEK.COM>
Date: 20 Oct 89 15:05:02 GMT
References: <[253d2160:289]comp.sys.ibm.pc@nstar.UUCP> <4241@ncsuvx.ncsu.edu>
Reply-To: toma@tekgvs.LABS.TEK.COM (Tom Almy)
Organization: Tektronix, Inc., Beaverton,  OR.
Lines: 45

In article <4241@ncsuvx.ncsu.edu> unkydave@shumv1.ncsu.edu (David Bank) writes:
>In article <[253d2160:289]comp.sys.ibm.pc@nstar.UUCP> akcs.larry@nstar.UUCP (Larry Snyder) writes:
>>What is the longest one could build a keyboard/monitor
>>extension cable for a VGA or EGA monitor and have it work
>>without damaging any of the equipment?  

>>Could it be 40 feet long?  50 feet?

>    That still doesn't answer your question, does it?? Depending
>on how well built the PC is, and what quality of components went
>into it, I'd say you have about an 80% chance of doing what you want
>with the keyboard [...]
>the keyboard is not a concern here.

>    Now, the monitor is a different story. Depends entirely on the
>interface card and the monitor itself. Chances are, however, that
>50' will be a mite too long to go without significant signal
>loss.

Well I beg to disagree on several points. In both the case of the keyboard
and an EGA display, all signals are digital, and the distance you can drive
(with a given type of cable) is dependent on the driver current (yes, the
keyboard matters), the hysteresis of the receiver (noise immunity), and
impedence matching.  "Quality" is not really the consideration; the shabbiest
display card might do the best because they used a cheap driver that just
happened to have greater drive capability at the cost of bandwidth (clarity).

The cable used will make a major difference. The longer the cable the
more you have to worry about crosstalk, shielding (the cable becomes a
dandy radio antenna), and capacitive loss. Just connecting up a bunch of
keyboard and display extension cords won't hack it -- you'll suffer one or
more of these problems.

A VGA display makes the situation worse since the analog video levels have
little noise immunity and any impedence mismatching will certainly lead
to ghost images.

I have no idea how far you can really go, but I can tell you that it will
be *expensive*, requiring custom cabling. It might be cheaper, if you
application can support it, to use a remote terminal on an RS-232 port. 
This will have no trouble going 50'.

Tom Almy
toma@tekgvs.labs.tek.com
Standard Disclaimers Apply