[Coco] Geranium Diode Upgrade

[Mike 0rtloff]

Earlier today I had noticed

Dennis Báthory-Kitsz =is= maltedmedia... I 
promptly dove into my ancient, dusty boxes 
searching for the magazines he once published. 
Didn't find 'em (they're there, somewhere) but I 
stumbled across the following document, from a kit I purchased in '91...

--- begin author unknown

INTERRUPT MODIFICATION INSTALLATION INSTRUCTIONS

The purpose of this modification is to correct a 
problem with the flow of the cartridge interrupt 
to the CPU in a Color Computer 3. The cartridge 
interrupt is used primarily in OS/9 with the 
RS232 Pak to inform the CPU when a character has been received.

The ACIAPAK driver requires this interrupt signal 
to operate properly, although the MODPAK driver 
does not. The signal in the CoC03 is routed 
througi1 the GIME chip, which every so often does 
not pass it to the CPU. When this occurs, the 
ACIAPAK driver (and thus the /T2 or similar port) 
locks up. Terminal programs and other software 
utilizing an ACIAPAK type port show a tendency to 
lock up because of this problem. The modification 
consists of a jumper with a diode wired into it, 
which connects the cartridge interrupt directly 
to the CPU, bypassing the GIME. The diode keeps 
interrupt signals from going backwards.

[and as anyone can tell you, the forward voltage 
drop of a germanium diode is lower than a similar 
silicon diode, so more of the signal gets 
through, something must have been out of spec for it to matter]

Note that this modification only affects the 
CoCo3 motherboard and that if a Multi-Pak 
Interface is being used that it can also cause 
the same problem. The cartridge interrupt is 
routed through the Multi-Pak controller chip, 
which also may loose interrupt signals 
occasionally. If this is the case, simply wiring 
pin 8 ( CART) from the slot for the RS232 pak to 
pin 8 of the card edge will solve it. No traces 
need be cut and any number or all of the slots can be wired together.

INSTALLATION

Open the case by removing the screws in the 
bottom, exposing the motherboard. Locate resistor 
R7, which is right next to the metal grounding 
tab for the keyboard on the left side. Clip 
the  BLACK lead onto the right side of R7 
(closest to the big 6821 chip). Next, locate R2, 
which is directly behind the reset button. Clip 
the RED lead onto the side of R2 away from the 
reset button. The unit is now installed, and the 
only difference that should be noticed is 
elimination of the ACIA lock-up syndrome.

----- end document

[there's an ASCII graphic at the bottom of this 
document, but experience with ASCII is it only looks good on paper ;)]

As the engineers & software authors in here can 
tell ya, the ACIA driver was modified sometime in 
the early 90s and only a stone-stock OS/9 L2 (like mine ;) would have a problem

=M0=


>This is apparently a mis-word used.   "Geranium" is in fact a flower as many
>know, but some people actually refer to some diodes as "geranium diodes",
>and are using the wrong word.   Two of the most common elements used to make
>diodes are Silicon and Germanium, hence the "Germanium Diodes", which is
>what this really is.   As to its function on the CoCo?  Well I don't have a
>clue.  Anyone?
>- Chad