[Coco] Grr!!! Bad MPI! No biscuit!

[Mark Marlette]

Robert,

The LS245 has hystersis which helps in the noise arena. F245 doesn't.

I wouldn't switch them.

Regards,

Mark

Quoting coco at yourdvd.net:

> I believe Marty Goodman once recommended that the ls245 in the CoCo AND
> the MPI be replaced with a faster version (the ''F'' version, I think
> he mentioned). -r
>
>> -------- Original Message --------
>> Subject: Re: [Coco] Grr!!!  Bad MPI!  No biscuit!
>> From: Gene Heskett <gene.heskett at verizon.net>
>> Date: Sat, May 05, 2007 1:48 pm
>> To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
>>
>> On Saturday 05 May 2007, Joel Ewy wrote:
>> >Gene Heskett wrote:
>> >> On Saturday 05 May 2007, Joel Ewy wrote:
>> >>> Jim Cox wrote:
>> >>>> I have a MPI board that someone gave me.  I am not sure if it
>> works or
>> >>>> not, but it's yours.
>> >>>>
>> >>>> Jim
>> >>>> PS: We need a new MPI for the CoCo
>> >>>
>> >>> Here's a humble suggestion for anyone contemplating such a thing:
>> >>> Orient the board perpendicular to the desktop so that any cartridges
>> >>> plugged into it will be parallel with the desktop.  Having the disk
>> >>> controller stick up 8.5" from the surface of the desk is just a bit
>> >>> silly.
>> >>>
>> >>> As much as I loathe trying to find places to plug them all in,
>> maybe an
>> >>> external transformer would be a good idea here...
>> >>>
>> >>> Would it be feasible to put bi-directional bus drivers in a short
>> >>> cartridge and connect the CoCo to the MPI via a short length of
>> ribbon
>> >>> cable?  That would provide much greater flexibility in placement, and
>> >>> probably reduce the chances of bumping things out of place.
>> >>>
>> >>> JCE
>> >>
>> >> This idea falls over due to a bus contention in the mpi's internal
>> timing,
>> >> which in turn causes a ground bounce that is only barely tolerable
>> if the
>> >> grounding ear clips in the side port have a good clean grip on the
>> mpi's
>> >> ground plane.  Additional jumpers of heavy wire to tie the mpi's
>> rather
>> >> surpentine ground plane together also help, as does additional
>> bypassing
>> >> near the ls245.  You can make it stable, as shipped, its a house of
>> cards.
>> >
>> >Are you saying that this problem is intrinsic to any CoCo multi-pak
>> >design, or just Rad Shak's?  FWIW, I'm musing about a possible future
>> >product, not modifications to the existing design.
>>
>> I'd say it would be pretty well builtin, Joel, unless someone actually
>> went to
>> the trouble of fixing the timing problems, which are in actual fact
>> quite
>> narrow, existing for not more than 10ns, but during that time its an
>> absolute
>> crowbar across the ls245 in the mpi, and whatever is driving the buss
>> on the
>> coco's side of the edge connector.  The ground bounce, measured from the
>> coco's ground plane, say at the contact ears, over to a ground plane
>> connection around the ls245 and its environs, can exceed a volt for 8
>> to 10
>> ns in the shacks first version.  The mods I made to mine pull it down to
>> around 150mv, but its still there.  They also lowered the operating
>> temperature of that ls245 by a noticeable with the back of your finger
>> amount.
>>
>> I think, but won't swear, that its the turn around delay in the ls245
>> itself,
>> and that one of the faster chip families version of it might alleviate
>> it at
>> least as well as the extra grounds and bypassing I added to mine did.  I
>> should have taken the time to socket it and experiment but didn't. :(
>>
>> There needs to be a few ns of dead time, where the ls245 is truly open
>> in both
>> directions on a switch in directions, but I'm not sure how one would
>> go about
>> separating the read and write functions in order to achieve a buss
>> that's
>> tri-stated on both sides of it.  That is whats needed to eliminate it
>> though.
>> I'd guess that 5ns of dead time would be sufficient, based on what my
>> measly
>> 100mhz dual trace scope says.  5ns is of course faster than my scope,
>> but it
>> is useable to around 200 mhz as I've used it to look at a channel 8 tv
>> transmitter (175mhz), just not anywhere near full height traces when
>> stuff
>> that fast goes by. 10ns should cover it all, and that still leaves the
>> coco
>> with a couple of eons to do stable reads and writes through it.
>>
>> >JCE
>> >
>> >>>> On Sat, 5 May 2007 03:12:01 -0600
>> >>>>
>> >>>>  Willard Goosey <goosey at virgo.sdc.org> wrote:
>> >>>>> My old-style MPI is apparently toast.  My CoCo has therefore lost
>> >>>>> access to its hard drive.  :-(
>> >>>>>
>> >>>>> I suspect power supply problems.  Either that or the upgrade PAL is
>> >>>>> loosing its mind.
>> >>>>>
>> >>>>> Just what I need, something else broken.
>> >>>>> Willard
>> >>>>> --
>> >>>>> Willard Goosey  goosey at sdc.org
>> >>>>> Socorro, New Mexico, USA
>> >>>>> "I've never been to Contempt!  Isn't that somewhere in New Mexico?"
>> >>>>>   --- Yacko
>> >>>>>
>> >>>>> --
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>> >>>>> Coco at maltedmedia.com
>> >>>>> http://five.pairlist.net/mailman/listinfo/coco
>> >>>>
>> >>>> --
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>> >>>
>> >>> --
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>> >
>> >--
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>>
>>
>>
>> --
>> Cheers, Gene
>> "There are four boxes to be used in defense of liberty:
>>  soap, ballot, jury, and ammo. Please use in that order."
>> -Ed Howdershelt (Author)
>> Inara: "It wasn't entirely a disaster."
>>
>> Mal: "I got stabbed! Right here!"
>>
>> Inara: "You also lined up exciting new crime."
>> 				--Episode #4, "Shindig"
>>
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