8 Oct
2007
8 Oct
'07
3:02 p.m.
Tony Duell wrote:
Looked some more and it seems that power switching is done to stop a
PROM output being shorted by a 74ls05 output !
i.e. An input ot the 74ls323 is either driven by the Stateprom, or
pulled low by the ls05 . That is indeed two outputs being shorted.
FOrutnaely it's not quite as bad as that. That '05 (section d if my
schematic is right) holds the S0 input of that '323 in a defined state
when the state machine isn't powered. Why, I am not sure :-). But that
'05 is driven by the power switching signal, there's no possility of a
contention. It's pulling low an output pin on an IC that's grounded but
not powered.
When
backengineering it I found that both Proms and the 74174 have their
power switched. Why would that be ? seems very hairy to me.
Yes. This was a typicval Apple method of saving power. No matter that
it's a made idea to paply logic signals to an unpowered chip. And people
wonder why I dislike Apple hardware designs...
And this is supposed to be a clever design....
Many times I'ev crtiicised the Apple ][ design and been flamed for it. At
least you realise why I think it's a right kludge from start to finish!.
-tony