::: Area #155 (comp.sys.acorn.networking)
Message: #19137 (Read 5 times, has 0 replies, 2764 bytes)
Date   : Wed Jul 22 01:14:39 1998
From   : Jonathan Graham Harston of fidonet#2:254/27
To     : All
Subject: Line jammed - terminators, clocks etc

"Mark Usher" wrote:
MU> OK. The setup is quite simple but a little out of the ordinary. I am
MU> connecting one BBC and one PC with Ecolink card :-)

You have an Ecolink card????   You have an ECOLINK card??!!  YOU HAVE AN
ECOLINK CARD!!??!?!?!?

Gimmiie  Gimmieie  Gimmieiie

Where did you get it?  Have you got full documentation?  Any info really
gratefully received.

MU> I have built a circuit board based on the clock / termination in the
MU> advanced user guide with:

Always suspect anything you've built yourself. :)

MU> I am getting the clock through fine. There seems to be no problem there.
The
MU> BBC though did have the 1770 upgrade fitted, which means removing IC 87 and
MU> fitting a small bridge across two pins. I noticed then that if I

Why...?  Hmmm IC87.... 74LS123... monostable flip/flop; Half for: incoming
data pulse train conditioner to ensure pulses fixed width. Half for:
providing data carrier detect in presence of a network clock.

So, IC87b must be present.

MU> [snip: various Line Jammed situations]

/Something/ is wrong. Suspect:  dodgy drop leads, dodgy sockets, dodgy
cableing, dodgy line drivers, dodgy intefaces, dodgy software.  And in one
particular instance: recent lightning strike and none of the earthing in
the school actually connected to earth anywhere.  Expensive bang!

MU> One thing I am worried about are the terminators as there is no
MU> "distance" involved here, so reflections are going to be pretty
MU> minimal. OK I need to

You can usually get away with no terminators if less than a couple of
meters, and good quality cable.  Be warned: Acorn clock (ie, the one in
the manual) needs Acorn (powered) terminators; SJ clock needs SJ (small
plug) terminators.

MU> One thing I am not sure about is the correct value for termination, ie
MU> what the reading should be across the 1 and 4 lines with a multimeter.
MU> I can send you a schematic if you want to look at the diagram.

You should multimeter your network without anything plugged in, using
known resistances. Once you start plugging in active components such as a
terminator, you don't get static multimeter readings.

--
J.G.Harston (JGH BBC PD Library) 70 Camm Street, Walkley, SHEFFIELD S6 3TR
BBC+Master / Z80+6502 / CoPro+Tubes / Econet+SJ / Devolution / Transport /
Planning / Highways    http://homepages.nildram.co.uk/~amilton/walkley.htm
jgh@arcade.demon.co.uk   (( Anti-UCE address added by Arcade, not by me ))
		- + - * NTAUS#3 24th-26th July 1998 * - + -
		  At my place again - Contact for details
--- ARCbbs RISC OS [1.64á32c]
 * Origin: Via The Arcade BBS Usenet News Gateway +44 181 655 4412 (2:254/27)