Re: Single ended termination?.

On 11/2/21 8:30 am, Al Thomason wrote:
> Heinz,
>=20
> The answer is we have a cabling system with short adapters to allow
> for click-n-go daisy-chaining connecting up the nodes.  (And to
> Heikki's point, it is built around DTM connectors in this case, using
> the terminators and Y's which are available).  The reality at hand
> is:  If all this is short, even with reflections from one
> un-terminated end it does often work.
>=20
> But (And I want to be clear on this) WE do not support customers who
> want to do this, for all the reasons Heikki mentioned.
>=20
> Which is why I wanted to check with this group to see if there was
> indeed some legitimate paper, or study that had been done around
> single ended termination of short / low node count installs...   I
> have never seen one, but that does not mean there isn't one out there
> <smile>
>=20
> I can make measurements in the lab, but am not keen on proceeding
> based on only a 1x sample 'In the Lab'.  Much different between that
> and when we get 1,000's installed in the field - with age impacting
> things, wires ran with cross coupling, Who knows what can and will be
> done by installers.  Which is why I was seeking if there was some
> study, or reference that existed on this specific topic.
The thing that matters is the longest round-trip delay time from a node=20
to the unterminated end of the cable. There may be many unterminated=20
ends in a star.

If the fastest rise/fall time of a node is close or longer than any=20
delay time involving reflections from unterminated ends, then waveform=20
overshoot and distortion will be small. The system will work ok. A good=20
looking waveform on a cro with smooth non-notchy edges is what is needed.

The only drawback is a fast rise-time node might be added one day, or=20
the bit rate increased.

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