Transmission lines Was: UNIBUS extension card/cable sets

[Paul Koning]

> On Jun 25, 2015, at 12:02 PM, Noel Chiappa <jnc at mercury.lcs.mit.edu> wrote:
> 
>> From: John Wilson
> 
>> I chose the digital version of EE as my major precisely because I knew
>> I'd flunk Fields and Waves. Transmission lines are black magic as far
>> as I'm concerned!
> 
> I too have a hard time with analog in general, but transmission lines I seem
> to be OK with.
> 
> The way I think about them is to model them as pipes, and the signal as a
> sound (single pulse) sent down the pipe. Proper termination is like a piece
> of cotton at the end of the pipe, it sucks up the sound and you don't get a
> reflection. If you just cap off the end of the pipe (i.e. no termination),
> the sound bounces, and you get an echo.
> 
> So if you have a small un-terminated branch, part of the pulse bounces off
> the end, and comes back out, and then propogates both ways, so the original
> pulse gets a messy trailer tacked on the back of it. Etc, etc.
> 
> I dunno how accurate this model of mine is, but it seems to work OK! :-)

Nice analogy.  Those “bounces” occur at any irregularity, and in fact you can locate impedance fluctuations by the echoes they produce; this is called TDR (time domain reflectometry).

I still have a photo copied out of the 1980s magazine RSTS Professional, which claimed to show how to convert thick to thin Ethernet.  The simple answer is “with a coax connector adapter” since both are 50 ohm coax.  The article instead used a thinwire T connector, with the terminator still on it.  As Tony points out, terminators go at the end, not the middle (that’s what the word means).  So that configuration would be somewhere between marginal and broken.

	paul