Re: Cable Extension

On Wed, 9 Jul 1997, Hanno Mueller wrote:

> I doubt that 100 feet is possible with any kind of cable extension.
> Connectix says that 3 feet is allowed, I was able to use a 6 feet

<Dredging up old knowledge>

This is what I remember for Circuits 1 in college, so... [By +/- 1, I
mean 1 * the max voltage, not 1v]

The problem with using long cables (especially with a digital signal)
is that the lengths of wire develop capacitance, and the switching
time degrades. So your signal which should look like:

+1       ______    ______
         |    |    |    |
         |    |    |    |
-1 ______|    |____|    |_____

Looks like:
+1         __        __
          /  \      /  \
         /    \    /    \
-1 _____/      \__/      \____

If your system receiving reads +1 and -1, then the periods at which
the voltage is high enough to register is much shorter. Then longer
the cable, the greater the capacitance, and the shorter the time
period. If the cable gets long enough, you won't even be getting +1 or
-1, rather +0.9, +0.5, or whatever. If your receiver registers at +/-
0.1, you'll be ok with a longer cable, but most hardware won't
register that low.

This is the same reason that your SCSI, 10BaseT, serial, monitor, etc.
cables shouldn't be too long.


If there are any facts wrong, lemme know. I haven't dealt with this
for years.

Brandon High                          http://users.ccnet.com/~bjhigh
A power so great, it can only be used for Good or Evil!