Transmission lines Was: UNIBUS extension card/cable sets

tony duell ard at
Thu Jun 25 12:49:52 CDT 2015

> >> 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 really do fail to see how you can possibly understand or design digital systems
without understanding analogue electronics, in particular transmission lines. 

FWIW the Motorola ECL databooks were pretty good at giving an introduction
to this, simpy because with ECL you have to design just about every interconnection
as a transmission line. Do it right and the design works, and unlike some other
logic families where you hope for the best with the interconnections, ECL stays

> 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 

Err, yes :-). The BNC-N adapter is very useful :-). More seriously, I've seen thickwire transceivers that
had a pair of N connectors (not a beesting tap) fitted with BNC-N adapters and ues on thinwire. Technically
that is wrong, there is a minor difference in the transceiver spec (I forget what, but the data sheet for at 
least one of the transceiver ICs pointed it out), but in will work.

> article instead used a thinwire T connector, with the terminator still on it.  As Tony points out, terminators go at 

ARGH! Ethernet is more touchhy than most as IIRC the transmitter is a current source, the receiver effectively 
senses the voltage across the terminator. A collision is too high a voltage. So ethernet can't work with
incorrect termination.

That's why DEC had you put 2 terminators on a T piece on the ethernet BNC connector of a VAXstation (or
whatever) to get it to pass diagnostics. WIth a network that short you are not going to get detectable 
reflection problems, but if you only had one terminator, every transmission would be a collision.


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