Re: AFP protocol

David Empson <dempson@xxxxxxxxxxxxx> wrote:

Rowland McDonnell <real-address-in-sig@xxxxxxxxxxxxxxx> wrote:

David Empson <dempson@xxxxxxxxxxxxx> wrote:

Rowland McDonnell <real-address-in-sig@xxxxxxxxxxxxxxx> wrote:

David Empson <dempson@xxxxxxxxxxxxx> wrote:

Rowland McDonnell <real-address-in-sig@xxxxxxxxxxxxxxx> wrote:

Tim Streater <tim.streater@xxxxxxxxxxxx> wrote:
The original ethernet spec called for various voltages to be sent
down from the computer to the transceiver,

???

Er?

That is quite correct, if you are using an AUI connector.

But what does it mean?

I think Tim meant that the AUI connector supplies several power supply
voltages to the transceiver, so it needs quite a thick cable to carry
all those signals. If some of them have high current then it might need
extra thick wires and/or internal insulation.

According to Wikipedia, it only has one power supply signal, which is
12V.

Now I'm confused - are you saying that you think Tim was wrong?

Yes. I wrote the first paragraph before checking Wikipedia, then added
the note that it only uses 12V, without commenting that Tim appeared to
have misremembered or misunderstood that detail.

Righto - ta.

btw, you don't need extra insulation for high current, only high voltages.
And I find it hard to credit that the spec might have been such that it
was meant to supply more than a handful of amps, if that. D-sub connector
terminals are usually spec'ed at 5A, FWIW.

Agreed.

What's the typical resistance of wire?

What material and gauge?

Perhaps the cable had to be thick
because it was using all fifteen signals

Erm, no? Whatever the reason, that ain't it. I don't think they
thought of the cable being particularly thick if you ask me. And I
don't know what `all 15 signals' you might be talking about. Looking at
the AUI connector spec, I can see eight used signal wires - and of them,
`data in' at one end turns into `data out' at the other.

And who knows if `circuit B' was used? I don't, and I don't see
Wikipaedia telling me. And what the *hell* can `Control in circuit
shield' possibly be?

and could get a fair amount of
signal loss over 50 metres if thin wire was used.

Wire resistance on its own has not a lot to do with signal loss. You
want to think about the impedance/s/: source, load, transmission medium.
It's the relationship bewtween them that tells you how much signal
you'll loose.

which uses a "vampire tap" where you
push a sharp pointed plug directly through the outer insulation and it
pierces the inner core. These taps have to be a minimum of 1 metre
apart,

Actually 2.5 metres. Wow. That would have been a problem in my office.
:-)

Loops, that's what you put in, loops.

How flexible was the thick Ethernet cable?

9.5mm isn't particularly thick, and being unarmoured, the Ethernet
cables were/are plenty flexible enough to make loops. The first time I
ever saw the stuff was a loop pulled out of a wall (or was in down from
the ceiling? Either way, a distinctly non-recommended installation, I'm
sure).

I'm having mental images of a
half meter diameter loop of cable, which would lose about 1.5 metres
from the maximum linear distance the cable could travel, and still
require a one metre linear gap between taps.

So you use a bigger loop. Who says you have to use a circular loop? Or
a single loop? Two spins round a 40cm diameter circle is 2.5m; one spin
round a circle twice that diameter. Or you could make a figure of 8 if
you liked, or similar.

Rowland.

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