[Coco] How to mod MPI for ganged interrupts

[Gene Heskett]

On Saturday 16 November 2019 22:02:50 Brett Gordon wrote:

> Robert,
>
> I'm trying to run a rs232 pak and my SDC together simultaneously.  My
> rs232 handling needs to use the CART(FIRQ) interrupt.  The SDC, in
> harddrive mode, doesn't use/need interrupts, but does use the MPI
> switched $ff40 addresses.  This means i do have to have it's slot
> selected in order to talk to it, which has the unfortunate side effect
> of turning off the CART line from the rs232 pak.  My theory is: the
> thread that is accessing the SDC should be fully interruptable (and
> justly so) by the more time sensitive rs232 pak.
>
> I suspect that I can affect this mod by simply jumpering the some of
> the interrupt lines coming into the interrupt mux chip from the
> different slots.
>
> I remember hearing of old-timers doing this to make a better
> performing os9.

You rang?  I'm one of those old-timers. :)

There are 2 versions of its pcb that I know of, and the instructions, 
entirely from memory, are to bridge all 4 interrupts (pin 8 on the cart 
sockets IIRC) together.

But this also parallels all 4 of the irq pullup resistors, which on mine 
are in a row in front of the cart sockets, so remove 3 of them else the 
combined pullup may be too much for just one cart to pull down.  So get 
out your schematics and just cut 3 of the 4 in two. Done. Leaving all 4 
active will cause a mild video flicker with incoming rs232 data due to 
the excess current drawn.  And It may lead to a cart failure due to the 
excess currents drawn.

> And I thought I would ask here, before I spend an hour or two thunking
> over schematics, and risk possibly blowing up my MPI...
>
> (Looking at you Gene Hesket) :)
>
> Cheers, brett


Cheers, Gene Heskett
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