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

[Gene Heskett]

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
>>>>>
>>>>> --
>>>>> Coco mailing list
>>>>> 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"