Date   : Tue, 30 Mar 2004 17:07:22 +0200 (BST)
From   : Johan Heuseveldt <johan@...>
Subject: Re: MDFS incomplete startup

Hi Pete,

Thanks for your clear answer

On Mon 29 Mar, Pete Turnbull wrote:
> On Mar 28, 13:52, Johan Heuseveldt wrote:
> 
> > Actually there are terminators on board: All lines have a serial
> > resistor of 100K and pull up of 10K.
> 
> No, those are bias for the comparators, not terminators.  The
> characteristic impedance of the Econet bus is 110 ohms.  The serial
> resistors are there to make the whole thing high impedance, and to
> *prevent* them having any effect on the line, like termination.  Look
> at any data sheet or example circuit which shows how to use opamps or
> comparators and you'll see what they're for.

I'll take your word for it, and accept I'm totally wrong.

I was in some way looking for the question, knowing the answer.
That is, I know termination is not needed while using short connections,
in fact there's no network cable at all - sort of. Just the usual
connection leads functioning as such.
Trying to find the reason - for no need to terminate, I completely walked
the wrong way.

Trying to learn something from magazines - like Elector - I thought to
gradually understand the basics. Oh dear, how wrong a man can be!

> In fact, Issue 2 and Issue 3 BBC B boards *do* have places to fit
> termination components (and a clock) but Acorn removed them from later
> boards because they caused a lot of trouble (think what happens if you
> have extra terminators, or two clocks; or if you add an extra machine
> to the end of the net; or even have the right number in the right
> places but one of the machines is switched off).

I did know about the clock circuitry, terminatiom is new to me.
I wished I knew.

> > Compare the connection to the net with Ethernet Coax, using T-pieces. 
> > [...]

> No, you're wrong.  You're not allowed/supposed to have stubs more than
> 1m long on an Econet network.  That's comparable to thin Ethernet,
> which allows stubs of 15cm (6") -- almost all of which is taken up by
> the "equivalent length" of the interface.  You can get away with more
> on Econet because the speed (pulse width, actually) is an order of
> magnitude slower.

Indeed it is slow compared to other net thechniques.

> As for termination on the machine side, if you understood what
> termination does, and why it is necessary, you'd know it must match the
> characteristic impedance of the cable -- 50 ohms for Ethernet, 110 ohms
> for Econet.  Those resistors you see have nothing at all to do with
> termination.

You're right about the values. I just 'conveniently' forget about it, as it
doesn't fit my story. I should have been warned about, that I was going to
talk nonsence. Sorry Pete.
And an apology to you all. I can only say I was far to enthusiastic. :-(

> The Econet or Ethernet cable is a transmission line.  As with any
> transmission line, a signal passed along it will be reflected when it
> reaches the end.  This reflection is A Bad Thing; to put it
> simplistically, the point of the termination is to "soak up" the signal
> so it doesn't get reflected.  To do this it must be the same impedance
> as the impedance of the line.

Again, you're so right. I've read about - not so long ago - for RS485
connections. And still I choosed to neglect that. :-(

> > It is small enough to prevent too much load on the network cable with
> > many machines connected. So termination in the machine is as small as
> > possible to allow for some length of the connection lead.
> 
> The length has nothing to do with it!  The resistors you mention are,
> however, there to minimise the load.  They're designed so the interface
> places as little load as possible on the line; they are there to do
> exactly the opposite of providing any "termination".

Whoops. 

[snip: pseudo telephone cable]



In the mean time I was lucky to find the statement that no termination is
needed when connecting over short leads. I've seen the T-pieces mentioned,
but I have used Y-cables: two short lines with female 5-pins DIN connector
ending up in a single male DIN.
Total overall length for each of the two lines male<->female is 18 cm.


It's in the "FileStore Network Manager's Guide"
            Part No 0454,010
            Issue No 1
            19 February 1987

At the start of 'Appendix A. INSTALLING FILESTORE
             on page 74

before paragraph '1. UNPACKING', the remark there states:


  You can connect Filestore to up to ten microcomputers, using Econet
  leads and DIN connector "T" pieces. For a network with a larger number
  of stations, or one that extends more than ten meters in length, you
  will need to have cabling installed, with an Econet clock, socket boxes
  and terminators. [...]


I hope I've made something good with finding this, after my blunders,
and everyone was not too much confused about my 'story out of thumb'. :-|


Of course a big thank you to you all for thinking with me, and all the
responses so far. Nice to have your input.



-- 
Johan Heuseveldt <johan@...              >
  aka  waarland

  The best place is a Riscy place
 
Quia Costodiet Ipsos Custodes.
(Who will watch the Guardians)