[Coco] How to mod MPI for ganged interrupts

[Gene Heskett]

On Sunday 17 November 2019 09:14:00 Brett Gordon wrote:

> Gene,
>
> As us youngins say: "You da man!"
>
> Thanks so much.  I didnt even see the pull-ups.  This all jibes with
> what I was guessing.  Its a nice feeling going into a project without
> uncertainty.
>
> I'll probably desolder the resistors carefully and pull out a leg; 
> Throw a scrap of paper explaining the mod in the case of the MPI, and
> tape a label on her outside to properly warn the next owner.
>
> Thanks again!
>
Most welcome. As I very rarely run rsdos, I've not tested for bad effects 
on it, so I can't testify that it breaks anything for rsdos.  Here, its 
Just Worked for nearly 30 years.

If your rs232 pack reports a framing error, check the priority byte in 
the descriptor, it should be a higher value than anything else. IRQ's 
are searched for the src by reading this byte, then reading the 
associated cart, in high to low value order. I don't recall which byte 
in the descriptor is the priority byte now, but it should be documented 
in the nitros9 stuff.

> Sincerely,
> brett
>
> On Sat, Nov 16, 2019, 10:39 PM Gene Heskett <gheskett at shentel.net> 
wrote:
> > 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
> > --
> > "There are four boxes to be used in defense of liberty:
> >  soap, ballot, jury, and ammo. Please use in that order."
> > -Ed Howdershelt (Author)
> > If we desire respect for the law, we must first make the law
> > respectable. - Louis D. Brandeis
> > Genes Web page <http://geneslinuxbox.net:6309/gene>
> >
> > --
> > Coco mailing list
> > Coco at maltedmedia.com
> > https://pairlist5.pair.net/mailman/listinfo/coco


Cheers, Gene Heskett
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
If we desire respect for the law, we must first make the law respectable.
 - Louis D. Brandeis
Genes Web page <http://geneslinuxbox.net:6309/gene>