Setting up a VAXstation
ard at p850ug1.demon.co.uk
Fri Oct 5 17:14:56 CDT 2007
> > A logic analyser is a bit lile a 'scope in that it displays a grpah of
> > signals against time. The differnces are (a) it records the signals and
> > displays the recorded version (some 'scopes do that too -- storage
> > 'scopes), (b) it only works with digital signals (it doesn't display th=
> > voltage, only whether they are high or low), and (c) it has many more
> > input channels (even a good 'scope rarely has more than 4 channels, a
> > logic analyser will have 16 or more).
> Fascinating. The snag is, I know very little about electronics below
> the level of a broad knowledge of TTL, the rudiments of circuits and
OK, a higher-level description would be 'something that records a number
of digitla signals and displays their relative timing'.
The sorts of things you might use one for would be to trace machine code
(or microcode!) instructions/addresses on a processor bus (but cache
memory and even a pre-fetch buffer makes that a little hard!), checking
things like handshake sequencies on an interface, recording bit-seiral
signals so you can decode them later, checking state machine sequences,
and so on.
What it is _not_ is a majic box that finds all digital electronic faults.
It's a tool, which if used intellegently will help, though.
> gates and so on. I have a bit of theory, no practice. I could not
> diagnose a faulty chip or anything; my troubleshooting consists of
> swap bits until it works. And at this, I know more than most people I
You mean oyu've not been on the receiving end of my flames about this
:-). Suffice it to say I've just written a presentation where I describe
that as a 'ridiculous method'.
I tried it twice when I was young-and-foolish and both times it not only
didn't find the fualt, it actually left me more confused. I ended up
tracing the fault using test gear in the traditional way, and it took me
a lot less time.
The main prolems are :
1) You have to know the module you're swapping in works. This is not
always certain, particulalry on classic machines. Whre would you find
known-good boards for an obcure 30-year-old system?
2) E very careful if the old and new boards are not the same revision.
This caught me once, I swapped in what I thought was the same board (same
part number, etc), only to find (after _much_ testing) it was a later
version that needed a backplane modification to work.
3) A fault elsewhere in the machine could have damaged the origianl
board, and might damage the replacemetn too. An obvious example of this
is a defective power supply that's damaging chips on the logic boards
4) Even if you replace a board and the machine works, you may not have
found the fault. One example of this (which happened to me) can be
simiplified into a system of 2 modules linked by some interface. Let's
call them 'A' and 'B'. You replace 'A' and the machine works.
Unfortunately, the old 'A' was working fine _and was withing tolerance
for things like interface timing (but towards the edge of that
tolerance_, module 'B' was failing and couldn't handle the timing of the
old 'A'. The new 'A' was on the other side of that tolerance, so it
worked with the fialing 'B', but in time 'B' got worse and worse and the
machine failed again.
> I'm interested in things like the Sun and the DEC - and my couple of
> IBM PS/2s and even the 9" mono compact Macs - because I hugely admire
> the engineering and design of these machines from before the rise of
> the mass-market PC. Things were /different/ then.
Things were even more different before that IMHO. In many of my machines
(and those of others on the list), theres isn't a processor chip. The
processor is several boards of fairly simple chips -- simple gates,
flip-flops maybe some small RAMs or PROMs.
> I am, a bit like Chuck, mainly a software person, but one who's
> competent with hardware to a basic level. I know bugger-all about
> electronics and while I regret that, I'm not inclined to fix it now. I
I actually wonder how you can be 'competent with hardware' and 'know
bugger-all about electronics' To me those are contradictory statements.
We also ssem to have different attidudes about learning new things. I
tend to spend the time to learn whatever I need to fix the problem. I
don't claim to be a programmer, I;d never write an OS or a compiler or an
emulator, or.. from scratch. But when I had a problem which was clearly
due to a device driver not correctly hadnling the somewhat odd hardwre
in my PC, I learnt enough C to understand how said driver worked, and
then editied the sources to get it to work.
> Well, I would offer it here, but my "vintage" kit is old ISA boards,
> dead MC680x0 Macs and suchlike. Probably not of much interest to
> classic collectors, I fear.
Actually, there are some ISA boards I am still looking for. Top of the
list is an origianl IB< PGC.
I don't suppose your Apple bits include Laserwriter spares, do they? I am
looking for partially-dead boards to raid for custom chips (PALs,
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