🚀 go-pugleaf

Are you certain that to use short runs of 75 ohm cables terminated with 75
ohms is better than using short runs of 75 ohm cable terminated with what
ethernet expects to see 50 ohms?

It's just that the current drive for the ethernet will put out and then
look for a collision.  The current drive will go out to the 75 ohm load
and produce 50% higher voltage than expected (half way to a collision)
then all the ringing will get added to that.  

Versus, taking your lumps at the cable mismatch but making certain the
ethernet drive only drives 50 ohms.  

It seems qualititatively that 50 ohms would buy you more margin for error
since at short run lengths there would be absolutely no ringing from
mismatch and the system would work perfectly.  

                                                  - Robert -

PS  of course if the runs were into the 100 ft + this might all change,
but then I'd just model the system's components and run it through PSpice
demo to see which way works best.



On 29 Aug 1998 danhicks@millcomm.com wrote:

> In <35e6319c.53011606@news.lakwod1.co.home.com>, blosser@home.com (Aaron Blosser) writes:
> >On Fri, 21 Aug 1998 11:07:04 -0500, Daniel Erskine
> ><netsup75@edwardjones.com> said:
> >
> >>> Using existing 75ohm coax as if
> >>> it was 50ohm coax (with F to BNC adapters) is asking for trouble.
> >
> >>  I'll inquire with my network staff for more research material, as
> >>I'm curious as to why 75ohm cable would be that much of a problem?!?
> >
> >I wonder as well.
> >
> >For short runs of cable, I highly doubt that using 50ohm vs. 75ohm would
> >make a difference.  That measurement is a measure of the cable's
> >impedance over some distance (1000 ft?), so for a short bit of maybe
> >50-100 ft, the difference *may* be neglible.
> >
> >Of course, this all depends on how senstive the receiving equipment is,
> >and I can't make any claims to know how it would really affect the
> >performance.
> >
> >The reason you *should* use properly rated cable is because with
> >impedance matching you can achieve maximum power output.
> 
> With 10Base2 proper impedance matching is critical to prevent reflections 
> -- which cause errors and false collisions.  However, for reasonably small
> nets you should be able to use 75 ohm cable **IF** you use 75 ohm 
> terminators.  The drivers and receivers should be able to tolerate the 
> difference in current and voltage.
> 
> Dan Hicks
> Hey!!  My advice is free -- take it for what it's worth!
> http://www.millcomm.com/~danhicks
> 
> 
> 
>