[Coco] Upgrade MPI?

[Gene Heskett]

On Thursday 07 May 2015 10:15:30 Salvador Garcia wrote:
> The uncertainty with this type of issue is whether it will cause
> actual damage. Fact is, it is not 500mA going through those lines so
> the CoCo3 won't go out in a blaze of glory. Since the current is
> minimal there is that chance hat the short will not fry the GIME.
> But... of course there is a but... if a perfect storm is created then
> damage can occur. What this means is that CoCo 3 #12999 might live a
> long and fruitful life alongside an un-upgraded MPI and never feel the
> heat while CoCo 3 #14778 might get jolted into a parallel universe
> right after the MPI is connected to it. The question here is: Which of
> these two CoCo 3s do I own? As a wise man once asked: Do you feel
> lucky punk? :-)
>
>
> Salvador
>
As a CET, let me clairify this too, at least for the MPI I am using.  
There IS a read/write shortcircuit in the logic for something less than 
10 nanoseconds on each cpu cycle.  Enough to cause the ground plane in 
the MPI to bounce as high as 2.5 volts.  So MY MPI has additional heavy 
jumpers connecting the various parts of the rather circuitous ground 
plane together, and has additional .1 uf bypass capacitors added between 
the ground and power rails, not to isolate the short, but to supply the 
currents it draws for that sub 10 nanosecond time frame.  The average 
draw because of this is probably less than 5 milliamps, not enough to 
heat anything and make it wave a big red flag in front of an IR 
thermometer.

But during that overlap, it could easily exceed an amp or more.  So its 
not much time, and its not much average current, but the ground plane 
voltage bounce CAN make the logic make mistakes.  My 2 scopes, one 
analog, one digital are nominally 100 mhz scopes, and to nail it down 
well would probably take a 5Ghz sampler.  I suspect it is not gime 
related because thats back in the coco, not in the MPI, but if I ever 
really ran it down, I suspect the guilty parties are the 74ls245's.  The 
additional power rail bypassing reduces the 2.5 volt bounce to about 150 
millivolts and no logic errors are generated because of it.

Cheers, Gene Heskett
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