[lug] testing for straight versus cross-over patch cables?

J. Wayde Allen wallen at lug.boulder.co.us
Tue Aug 20 13:31:04 MDT 2002

On Mon, 12 Aug 2002, Mr Viggy wrote:

> Heh.  What annoys me is the IT guy that told me I could easily tap into 
> the thinnet (RG-58) in the cieling to connect up an HP plotter.  Funny 
> thing, after connecting it up, half the machines in the marketing dept 
> go offline.  Seems that the 3 extra feet of cable I added was just too much!

How exactly did you go about doing this?  The reason I'm asking is that
the coax based (RG-58) thin net can't simply be tapped.  It has to loop
through your computer and then back to the other machines downstream from
you.  Basically, you took the downstream computers off-line when you cut
the cable.  If you tried simply putting a "T" in the line and running a
three foot extension to your network card, you effectively created a
coaxial tuning stub on the line that modified the impedance of the line.  
This can cause standing waves that will have rather unpredictable results
(some machines keep working while those in the RF null fail).

The tap would need to be something like:

   Thin net <--------------| |------------------> Thin net
                           | |
                           | |
                           | |
                    | Your computer |

- Wayde
  (wallen at lug.boulder.co.us)

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