Re: Second Sight

[mjmahon@aol.com (Michael J. Mahon)]

Sheldon Simms wrote:

>On Fri, 30 May 2003 20:55:35 +0000, Michael J. Mahon wrote:
>
>> Sheldon Simms wrote:
>> 
>>>For people
>>>who are a little more knowledgeable about the electrical side of
>>>things than I am, the 80200 at 600Mhz has a maximum current
>>>requirement of about 600mA and dissipates 1W with a core
>>>voltage of 1.3V and a periphery voltage of 3.3V.
>> 
>> One watt isn't much of a problem, but I'm guessing that all the
>> rest adds up to at least another watt.  Voltage levels are not a
>> problem either, since very compact DC-DC converters can be
>> used.  I haven't checked ZipGS power consumption, but I'll
>> bet that it alone is more than two watts!
>
>I was more worried about the current requirements. I don't have
>my IIgs hardware reference any more, but according to the IIe
>technical reference manual all cards share:
>
> +5V : 500mA
> -5V : 200mA
>+12V : 200mA
>-12V : 200mA
>
>But, off the top of my head, I don't know how that translates
>to 585mA @ 1.3V + 165mA @ 3.3V (I forgot that second part in
>the first post)

Well, 585mA @ 1.3v is 760.5mW, and 165mA @ 3.3v is 544.5mW,
so they total 1.305W.  Only power is relevant, since DC-to-DC
converters would be used to "transform" voltages with good
efficiency.

Assuming that the DC-to-DC converters are, say, 80% efficient
(easy), the power required from some combination of +5v and +12v
is 1.305/.8, or about 1.63W.  You can "load share" between
the +5 and +12 supplies, as, for example, the Disk ][ Controller
does (though inefficiently).  When Woz designed this card he
actually tried to adhere to the per slot specs--he even goes to
the trouble to power down the ROMs when not in use!  (Ever
wonder what those power transistors on the card are doing?)

I agree that this seems to use all the power allocated for all the
cards, but fortunately, the Apple power distribution and supply
are more robust than the specs suggest.  There are several
cards (accelerators come immediately to mind) that consume
several watts, and, if a system's power supply is not tired, they
work fine.  In fact, I think that as long as the card is under 5W,
you're cool (so to speak ;-).

Supplemental cooling could be an issue, but considering that
the Apple power supply is cabable of pumping 50W or more
into the case, what's another couple of watts?  ;-)

-michael

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