[Coco] Odd behavior

[Gene Heskett]

On Tuesday 17 December 2013 18:04:55 Arthur Flexser did opine:

> But such incompatibility wouldn't cause the CoCo 3 to lock up when
> booted, with nothing on the screen at all, would it, as Bill observed?
> 
> My impression was that the MPI incompatibility would merely cause
> certain OS-9 programs to not work correctly.
> 
> Art
> 
> On Tue, Dec 17, 2013 at 5:02 PM, Gene Heskett <gheskett at wdtv.com> wrote:
> > On Tuesday 17 December 2013 16:56:10 Chad H did opine:
> >> I've never owned or used a MPI.  Were they all CoCo 3 compatible?
> > 
> > Quick answer, No.
> > 
> > Long answer:  The decoding in them for slot selection responds to the
> > data being sent to the gime, function as the memory mapper, by
> > responding to writes to the gime at $FF90-9F.
> > 
> > The MPI's own control register is at $FF7F, so it can switch slots
> > when the gimme is written.  There are conversion kits to isolate
> > that, I believe Mark has then at Cloud9North that you can install if
> > you solder well, or Mark can fix it.  The kit isn't all that much,
> > but labor is still labor.
> > 
> >> Sent from my Transformer Infinity
> >> 
> >> Bill Gordon <cwgordon at carolina.rr.com> wrote:
> >> 
> >> I have been fortunate enough to happen upon another Coco3 that I
> >> could afford, but the ONLY problem with it is that it will NOT work
> >> with an MPI.
> >> 
> >> 
> >> 
> >> When the MPI is hooked up, the only display I get is a green
> >> screen-no garbage, no nothing.
> >> 
> >> 
> >> 
> >> That's even if the MPI is OFF or ON. Otherwise, the MPI works great
> >> with my original Coco3.

Looking back at what you said from Arts perspective, I am inclined to think 
that the coco3 that doesn't work, may have a half blown address bus driver 
in the cpu.  This is something I would confirm by dragging out my dual 
trace scope and checking the logic swings available at the cart socket.  
I'd almost bet on one of them either not pulling down to less than .1 
volts, or up past at least 3 volts. preferably almost 4.

In other words, that coco3 may have been damaged by someone changing game 
carts without shutting it off at least 5-10 seconds before grabbing the 
cart.  A millisecond short between traces is all it would take.  Then, if 
the cpu is damaged its possible to have Mark put a "protector" and a 6309 
in it all in one trip to MN.  Then you would have a super coco3 than when 
running the 6309 built version of nitros9, is nearly twice as fast as the 
NIB version.

Cheers, Gene
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