20 Nov
2005
20 Nov
'05
6:29 p.m.
On Nov 20 2005, 0:55, Tony Duell wrote:
Ah, does the TX output change on one edge of the clock and the Rx input
sample on the opposite edge, or something? Time to get out the 6854
datasheet....
What is the benefit of an asymmetic clock? Or
more precisely, why
does it help?
I forget whether it's the mark that's longer than the space, or the
other way round, but in essence the longer interval gives the hardware I believe so. However, Field change Order No. 2020
says "Design error:
Bi-phase network clock is 180 degrees out of phase with clocks
generated by Econet Clock Boxes and E01S Filestores. Consequently the
system fails to recognise the presence of an external clock when these
are installed in large networks. This can cause unreliable data
transfer." The fix given is to cut the tracks on the component side of
the PCB, between IC26 pin 14 and SK10 pin 4, and between IC26 pin 15
and SK10 pin 5; then on the solder side link IC26-14 to SK10-5 and
IC26-15 to SK10-4.
I'm not quite sure what this implies for clock detection. Since the
clock is free-running it won't remain phase-locked to anything else,
let alone an external clock, so why that should affect the detection I
don't know. Presumably something to do with the uneven mark-space
ratio.
I think, without having the board in front of me, that IC26 is the 26LS30
chip that drives the clock lines, and doing that mod would, indeed,
invert the phase of the clock. The only possible reason for doing that
would be to invert the mark and space times.
Well, the normal ACB4000 bridgeboard is actually SCSI, not SASI, but
it's an early implementation and doesn't use the Common Command Set.
All I said was "Yes" -- *you* wrote SASI :-) The winchester host
Ah, right. Well, SASI and SCSI are pretty much the same at the hardware
level, particularly if you only have on initiator (as here).
adaptor board doesn't have the ATN line though.
I haev now seen the manuals on The BBC Lives (great site, should have
thought to look there first!). It appears I have an E01 Filestore (not an
E01S, which would have the host adpater logic on the mainboard). The 34
pin connector, as you say, is a subset of the 1MHz bus signals on the
same pins, but the clock is 2MHz.
The E20 hard disk unit contains the standard Host Adapter card linked to
a 20NByte SCSI hard disk. That was the normal thing to link to the 34 pin
connector on the E01. It sohuld be relatively easy to put one of those
together... Is there any restriction on the hard disk size? What would
happen if I linked up a unit of several hundred Mbyte capacity? Would it
just not work, would it only see it as a 20Mhyte unit, or would it see
the whole thing?
Accordign to the manual, there is a utility disk for this unit, and maybe
a service disk too (it's not clear whether this is one disk or two). The
former contained the program to format the winchester, and is therefore
somewhat importent. The latter ran on a BBC Master and contained various
test programs. There was a special test box for Econet testing that
plugged into the econet ports on the Master and on the unit-under-test
(here, the Filestore), and could source a clock and simulate a
poor-quality line, both contrlled by lines on the user port. Amazingly I
have this test box, I don't have the software. I've not looked for it
seriously in the archives on the web, but does anyone have a link for it
to hand?
More pressing
is the lack of drives and cables. I assume the floppy
drives are plain, 300 rpm, 80 cylinder, ones. Anyhting I should know
about them?
Pretty ordinary SA400-interface double-sided 80-track single/double
density half-height 3.5" drives, normally Sony F6 3W drives as fitted to Master Compact and similar machines including some
early Archimedes
or Citizen drives as fitted to other Archimedes machines. Some of the
Citizen drives need a small mod (remove R61). Drives intended for PCs
may or may not work because they may not respond correctly to DS0 and
DS1.
Yes, but presumably a bit of work with wire-swapping in the cable will
sort that out if necessary.
There's more at the BBC Documentation Project,
http://members.aon.at/~musher/bbc/econet.htm, and at The BBC Lives!
site, http://www.nvg.ntnu.no/bbc/dir.php3?dir=3Ddoc
Thanks, that's where I found the manuals.
-tony