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From: grege@gold.gvg.tek.com (Greg Ebert)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: VGA cable lengths
Message-ID: <1687@gold.gvg.tek.com>
Date: 20 Nov 90 18:10:38 GMT
References: <13304@encore.Encore.COM>
Organization: Grass Valley Group, Grass Valley, CA  95945
Lines: 18

In article <13304@encore.Encore.COM> elliot@xenna.encore.com writes:
>Is there a rule-of-thumb on how long a (S)VGA cable can be?  I attempted
>extend a 2-foot cable with a shielded db9-to-db9 6'cable.  This causes
>(not-too-unexpectedly) shadows, ghosts, and funny lines.  (BTW, ignore my
>earlier query on VGA memory tests -- the problem was the cable length!)
>
>What is the difference between a VGA cable and a shielded serial cable
>(besides the 15-pin connector)?
>

The shielded cable is terminated with it's characteristic impedance, hence
there will be minimal (ideally none) signal reflections (ie 'ghosts'). The
printer cable has a shield around all conductors collectively, whereas the
video cable has individual shielded wires. Guess what ? You have an
impedance mismatch with the printer cable. Another problem with the printer
cable is that there will be coupling between the R,G, and B signals; don't
worry about HSYNC and VSYNC because video is probably blanked when these
guys wiggle.