Re: Parallel Cables

["Paul Gordon" <paul_gordon@xxxxxxx>]

I was always of the impression that the problem with parallel cable length
is not specifically to do with voltage drop along the cable, but more to do
with timing issues:

A parallel cable transmits 8 data bits simultaneously. All 8 bits have to
arrive at the other end at "exactly" the same time (or within a
very
specific tolerance of each other), in order for the byte to be correctly
received and understood.

As the cable length increases, any difference in the electrical properties
between any of the 8 data lines causes varying degrees of
"latency" in the
transmission time of each bit, and as the overall cable length increases,
so
this effect is exaggerated, resulting in the 8 bits arriving all at
different times to each other.

This is why serial cables can be driven so much further and faster than
parallel cables.

Also, how could you run a parallel device over CAT5? - CAT5 only has 8
cores, and parallel cables have either 25 or 36. - Not all are used of
course, but I believe you ALWAYS need more than 8, as there are a minimum
of
8 required for the data bits, plus "some more" for timing &
sync signals
etc...

0.02p

Paul G.



>Here's an excerpt from my conversation with Blowback on IRC about
parallel
>cables.
>
><Stubbs> what is the maximum distance for a parralel cable?
><Stubbs> does anyone know?
><blowback> 30 feet
><Stubbs> hmmm, what's that in proper money :-)
><Stubbs> and would it be extended by using cat5 cable?
><Stubbs> I've got an LCD and want to put it in the lounge,
running off my
>PC's parallel port.
><blowback> er, about 9m
><blowback> not sure about the cat5
><Stubbs> I'm guessing that because the device doesn't use as many
of the
>wires as a printer does, the capacitence generated in the cable will be
>less, and therefore I can get a longer run, does that sound sensible?
><blowback> I think voltage drop along the wire is going to be
more of a
>problem than capacatitive coupling tho
><blowback> guess you'll have to try it and see!
><Stubbs> yeah.
>
>Is my guess right? Would using CAT5 make any difference?
>----------------------------------------------
>Stuart Grimshaw
>Schoolsnet Ltd                         Special
>t: 07976 625221                        Projects
>e: stuart@xxxxxxx          Developer
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>                www.schoolsnet.com
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>

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