[Coco] RS-232 Pak Back to Stock

[Gene Heskett]

On Thursday 04 April 2013 00:21:51 John E. Bielak did opine:

> On 4/3/2013 10:08 PM, Gene Heskett wrote:
> > On Wednesday 03 April 2013 22:01:32 John E. Bielak did opine:
> >> I picked up a deluxe RS-232 pak thats somewhat modified that I'm
> >> trying to put back to stock.
> >> 
> >> The changes:
> >> - The ROM has been cut out
> > 
> > Leave that as it, its far more useful w/o it.
> 
> Yes, that seems to be the overwhelming consensus... though I'll drop a
> socket in there anyway.
> 
> >> - The trace for pin 4 cartridge edge connector has been cut and there
> >> is a wire going out to an external connecter
> > 
> > That is the IRQ line, put it back AND if using an MPI, pull 3 of the 4
> > pullup resistors from the 4 connectors pins 8, and jumper all pin 8's
> > together.  No more missed incoming characters.  And doesn't need an
> > external wire to get the IRQ into the coco when the slot select switch
> > or logic isn't set to the slot the cart is in.
> 
> Thanks, the MPI IRQ, that makes sense of that external wire.
> 
> I haven't really used a MPI much, so forgive the noob question, but will
> jumping all the 8 pins potentially cause any conflicts/problems between
> other paks? (Drive, sound, other RS-232 based devices like the CoCoPak
> drive, etc.)
> 
I can't really offer air tight data on rsbasic usage, but os9 is plenty 
smart enough to find the src of the IRQ when one comes in, absolute worse 
case lag I ever measured was 150 microseconds, but 99.9% of them were 
serviced in 15 microseconds with the resistor removal, and the jumpering I 
described.

The resistor removal is with the pin 8's all tied together, then the source 
of the pulldown to assert the IRQ, often just an output open collector 
stage inside the big chip in whatever cart is plugged in, is going to be 
warmed up and stressed, possibly to the failure point, by having to pull a 
line that may have 4 2.7k ohm resistors effectively in parallel (depends on 
the MPI, most I think had 4.7k pullups, but mine had 2.7k's there) 4 ea 
2.7k pullups is the equ of one 675 ohm resistor, and when that line is 
pulled to ground to assert that IRQ, the current flow is more that 
sufficient to make the video flicker in time with the IRQ's.  TBT, that 
portion of the circuit doesn't need video speed response, and a 27k 
resistor would more than likely be sufficient pullup to do the job.

The GIME, at least mine is, is very sensitive to even a 5 millivolt bump on 
its supply rails.

But I found on the schematic for the coco3, several places where there 
could have been 10 ohm resistors, or wires with ferrite beads for noise 
suppression.  Mine /had/ a 10 ohm resistor, but /now/ has a ferrite bead at 
one location just in front of, and IIRC slightly to the right of the GIME, 
and that noise in the video is now gone.  Be careful, thats teeny stuff to 
muck around with.  And you really need a temp controlled, static grounded 
iron too.  There are lower priced ones that work well for around $130 but 
mine also has a hot air rework kit in it, was about $250.  But thats just 
me, an old (78 now), retired CET, being pickity. :-)

> >> - There's a jumper between E1 and E2
> >> - A jumper appears to have been removed from E3 and E4
> > 
> > That doesn't ring any bells, consult the manual on that.
> 
> Nothing in the manuals that I can find yet.
> 
>  From looking at the board, The E1 pad connects connector pin 4 (IRQ) ,
> through a resister, to pin 15 (VCC) of the 6511.  Jumping E1 & E2 also
> sends the IRQ directly into Pin 26 (IRQ) for the 6551.
> 
> Pads E3 & E4  connect  to cart pins 6 & 8 respectively. (Clock and
> CART).... looking at the solder job it looks like it may just have been
> the E4 pad that was mucked around with.
> 
> 
> Thanks!
> -John-
> 
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Cheers, Gene
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