19 Jan
2009
19 Jan
'09
12:50 p.m.
On Monday 19 January 2009 03:02:03 pm Tony Duell wrote:
Alas my expeerience suggests the reverse. Back when computers were
expensive, and chips were expensive, the manufacutrers took the trouble
to put all sorts of protectinon in their machines to protect said
expensive decices in the event of a failure. Now they don't bother.
I would suspect that a younger crop of engineers coming up might have some
bearing on this too. :-)
That depends, a lot, on the PSU design. Certainly
SMPSUs can do some
very odd things if a capacitor goes open-circuit or just high ESR. How
much damage that does to the rest of the PSU or worse the rest of the
machine depends on the design.
Do you think that the designers of this stuff have learned which
approaches they might want to stay away from? :-) A triviel example, but it's almost a classic
computer. As is well-known
when you have a multiplexed LED display, the instantaneous current
through each LED is much higher than the sort of current that you'd pass
continuously to have the display at a suitable rightness. High enough
that if the scanning fails for any reason, the display LEDs will be burnt
out.
Yes, this point is hammered on pretty well in my "8085 Cookbook", which is
one handy reference I have that talks about multiplexing displays a lot.
In the HP98x0 series of machines, there are
monostables triggered from
the display stroe signal so that if the processor side of things
malfucntions, the display will be blanked. It's impossible for one
digit/column to be stuck on.
Right, I remember that being one suggestion as to how to deal with it. There
was something not too dissimilar in my Oki 92 printer, as well. They used
something like 10V for a motor holding voltage, and 30V to kick it -- but
apparently you couldn't run them on 30V continuously, as the little
protection circuit they put in there would integrate the applied voltage and
if it saw too much over too long of a time a crowbar would go into operation
and trip a breaker.
They did a similar thing in the 59309 digital clock,
but that's more
reasonalbe since the scanning cna be derrived from an external
oscillator. If you switch it to 'Ext' with no oscillator connected, then
the display could be damaged without this protection circuit.
But how many modern devices have such prtection circuits fitted? Darn it,
many modern PSUs don't even have crowbar circuits.
I suspect that the answer to that one is probably "too many". :-(
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin