Re: 911 Fatal Error on TransWarp GS?

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

Rubywand replied:

>"Michael J. Mahon" wrote:
>> 
> ....
>
>> Actually, there isn't much to an 18-gauge wire except resistance,
>inductance,
>> and capacitance, and all those are addressed in my reply.  Assuming only
>> that most of the strands are not broken, the analysis stands without
>change.
>> Note also that a break in half the strands is bridged by the fact that
>strands
>> are not insulated from one another, so that the incremental increase in
>> resistance is very small (though in extreme cases, local heating could be
>> a problem).
>
>     Yet, measurements indicated a significant drop across the +5V line at 3
>Amps. Perhaps the lne is not truly 18 gauge. Perhaps a flaw in PS sensing and
>regulation is a factor. Whatever, going to heavier lines eliminated the
>apparent drop. 

What was the significant voltage drop?  How was it measured?  Did it
include the connector?  Did it include any main board traces?  How
much voltage drop was induced by inserting the current meter?

Have you looked at the +5 with a scope?  If the noise is present at
the power supply terminals, then fatter leads won't help.  If power is
clean at the power supply terminals and dirty at the main board
connector, then, and only then, bigger leads could help.  (Again,
proper gounding of the scope is essential.)

Have you tried simply replacing the wires with known-good wires of the
_same_ guage?  Ohm's Law is not just a good idea--it works!  ;-)
 
>> Nothing in my analysis depends on how "ideal" the power supply is--only
>> that it meet the voltage, current, and filtering requirements of the Apple
>II.
>
>     Which is not always a valid assumption. For instance, plugging in stuff
>like accelerator boards often leads to a rash of "mysterious" system crashes,
>increased noise in audio outputs, etc..

As would be expected on any system in which the power supply had
become marginal relative to the load.

>     If swapping in heavier leads did not so often solve such problems, then,
>there would be no particular reason to suspect that the original leads are too
>skimpy. Since heavier leads reduce or eliminate the problems, it looks like
>swapping them in is a good idea.
>
>> 
>> Except for ripple, which should be in the millivolt range, the power supply
>> voltages are, indeed, DC.
>
>     The voltage through a line is always, to some extent, modulated by
changes
>in the load. Besides the PS leads, the IIgs has many power lines (traces)
which
>act as resistances across which voltage is modulated.

Yes, but these traces are factored into the design of the IIgs, and should
operate properly if not improperly loaded.

>>  My analysis assumes that the poser supply is
>> properly decoupled for high frequencies by the main board, and for low
>> frequencies by the power supply filter capacitors.  If this is not true,
then
>> the problem is not in the wires, but in the decoupling components.
>
>     Most likely, in the IIgs, it is both. For instance, it is widely known
>that the ROM-01 board Slot card power traces are too skimpy and/or not
>adequately bypassed.

Slot power decoupling is specified to be on the plug-in cards, not on
the main board.  You would not  want the decoupling capacitors across
a connector from the load--in this case, the logic on the card.

>     Regarding the rest of the motherboard, 'adequate bypassing' will depend
>upon what is supposed to be bypassed. Adding an accelerator, PC transporter,
>Second Sight board, etc. increases high-frequency load modulation and, thus,
>line noise. If we reduce an important noise source-- e.g. replace the chief
>power supply leads-- then,  motherboard bypassing may be roughly adequate.
>Jumpering the main power lines on the Slot plane-- e.g. from the PS connector
>to Slots 3 and 7-- further reduces noise generation.

Again, this indicates that either the cards are not adequately bypassed, or
the transient loads are outside of spec, or both.  I suspect that some
accelerators
(particularly the TransWarp) are considerably outside the slot power spec, and
direct-wiring the power may be compensating for this design problem.

-michael

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