26 Jun
2018
26 Jun
'18
5:02 p.m.
On Jun 26, 2018, at 7:20 PM, Eric Smith via cctalk
<cctalk at classiccmp.org> wrote:
On Tue, Jun 26, 2018 at 4:37 PM, Chuck Guzis via cctalk <
cctalk at classiccmp.org> wrote:
Exactly. And the specs for the Ethernet terminator are quite tight for that reason. The
connectors themselves have non-zero impact but very small; they are high quality microwave
grade connectors.
You got the definition precisely correct: a terminator is a device that is electrically
equivalent to an infinite length cable. You can cut the unused part of a coax anywhere
you want and put a terminator at that point instead, and as far as the rest of the cable
is concerned nothing has changed (apart from very small effects because the components are
not perfect).
There clearly is confusion about what terminators are and how they work. It's all
perfectly straightforward elementary classic E & M, and any halfway decent RF theory
textbook will make things clear. Even a source as elementary as the ARRL Radio Amateur
Handbook will serve.
paul
On 06/26/2018 03:15 PM, Grant Taylor via cctalk
wrote:
If the termination resistance matches the characteristic impedance of the
cable, there should be no difference. When terminated properly, there is no
reflection from the terminator, so it looks equivalent to an infinitely
long cable, though in practical terms with less leakage than an "actual"
infinitely long cable would have.
Of course, in reality it will never be terminated perfectly, so there will
always be a small reflection, which can be seen with a TDR. If the
termination resistance is pretty close, the reflection will be small enough
not to matter at all for Ethernet. I can only guess that having a terminator too
close interferes with or
weakens the signal too much in some way.
Exactly what would the effect be? I recall putting terminators on
10base2 coax just hanging off one leg of a BNC tee. Really, no distance
at all. Didn't seem to affect speed or distance.