[Coco] Geranium Diode Upgrade

[Gene Heskett]

On Tuesday 08 July 2008, Bruce W. Calkins wrote:
>> Hello everyone.
>> I'm looking at this ebay auction:
>> Tandy Color Computer 3 Complete Operational Set Up
>> http://cgi.ebay.com/Tandy-Color-Computer-3-Complete-Operational-Set-Up_W0Q
>>QitemZ180260384799QQihZ008QQcategoryZ4193QQssPageNameZWDVWQQrdZ1QQcmdZViewI
>>tem
>>
>> I'm wondering what is Geranium Diode Upgrade and why does it need to be
>> upgraded? What does it do?
>>
>> --
>> Charlie
>
>That's at least two of us without a clue as to what they are "bragging"
>about.  Perhaps someone else has a clue?
>
>~BWC~

Well, first off, a Geranium is a flower (or a weed, depending on ones definition 
of a weed) the last I knew.

Diodes however are occasionally made out of germanium though, and have different 
characteristics than the usual silicon diode.  Generally speaking they have 
much higher reverse leakage, and much lower reverse breakdown voltages, and 
about 1/3rd of the drop of a silicon diode in the fwd direction.

About the only thing I can think of that might make them semi-useful in a coco 
would be as a low forward drop diode bypassing the mpi's pin 8's all the way 
back to the irq line in the coco.  I understand that some folks did this as one 
method of avoiding lost characters from the modem.  It will probably work, but 
the much better technical solution is to jumper all sockets of the MPI together 
at each ones pin 8, and then remove 3 of the 4 IRQ pullups along the front edge 
of the MPI pcb.

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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."
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