25 Jun
2015
25 Jun
'15
12:02 p.m.
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! :-)
Noel
25 Jun
25 Jun
12:14 p.m.
On 2015-06-25 12:02 PM, Noel Chiappa wrote:
For the YouTube generation - there's Dr John Shive's 1959 video:
https://www.youtube.com/watch?v=DovunOxlY1k
Note, there's an intro of 1 min 20 seconds.
--Toby
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! :-)
Noel
12:38 p.m.
On Jun 25, 2015, at 12:02 PM, Noel Chiappa <jnc at
mercury.lcs.mit.edu> wrote:
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
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! :-) 
1:49 p.m.
>> 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
working.
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.
-tony
2:05 p.m.
On Jun 25, 2015, at 1:49 PM, tony duell <ard at
p850ug1.demon.co.uk> wrote:
I see a slight difference in the input current spec of the receiver part of the
transceiver. 250 uA instead of 25 uA. That makes some small amount of sense given the
smaller station count.
>> ...
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.
3315
days inactive
3315
days old
4 comments
4 participants
participants (4)
-
ard＠p850ug1.demon.co.uk -
jnc＠mercury.lcs.mit.edu -
paulkoning＠comcast.net -
toby＠telegraphics.com.au