[Coco] An 8-Slot Multi-Pak Design for my Color Computer Friends

[Little John (GIMEchip.com)]

James,
That is why I didn't put the 245 on the buffer board since the CoCo has one 
right at the cart connector - I figured it would be okay. I did however, 
once fry a CoCo3 when I was only 3 years old - I bumped the computer which 
wiggled the MPI in the slot and poof - nothing but a green screen. Dad fixed 
it in like 15 minutes - it fried the 74LS245 on the CoCo3 mobo - thankfully 
the CPU survived (dad didn't have a spare back then).

I am going to redo this whole thing this evening from scratch and fix 
everything that you pointed out (I did accidentally create a GND1 signal 
when I net named the GND pin of the cartridge connector - it was a typo that 
ended up creating a secondary GND - I would have missed it had you not 
pointed it out).
That is why I post this stuff - not just as gifts to the CoCo community, but 
so you guys can catch my screw-ups.

Once I (and the rest of you guys) are satsified with the design, I may put 
it into a single CPLD. The only problem is that my poor eyesight keeps me 
from soldering surface mount easily - dad can do it though - he uses a 
hoof-tip and vertical drag soldering with excellent results.
-John

----- Original Message ----- 
From: <jdaggett at gate.net>
To: "CoCoList for Color Computer Enthusiasts" <coco at maltedmedia.com>
Sent: Monday, July 05, 2010 1:46 PM
Subject: Re: [Coco] An 8-Slot Multi-Pak Design for my Color Computer Friends


> On 5 Jul 2010 at 16:06, afra at aurigae.demon.co.uk wrote:
>
>> Quoting "Little John (GIMEchip.com)" <sales at gimechip.com>:
>>
>> > Had to have a fourth of July Holiday, so now I'm back. Let's have a 
>> > look see:
>> >>
>> >> A huge mistake in not buffering the data bus on the main board. If
>> >> you are running a stock
>> >> Coco3, you do run the risk of damaging the MC6809E and the HD6309E
>> >> if the board
>> >> becomes detached while powered up.
>> >>
>> > That's easy enough to fix. However, even with the databuffer on the
>> > main board, detaching while powered up could cause damage,
>> > especially if it were knocked sideways - zap.
>>
>> I keep hearing this over and over again, but have not found the 6809E
>> any more suceptable to damage in this way compared to say a 6502 or a
>> Z80, and believe me I've done enough (un)intentional
>> plugging/unplugging things with the power on :( :( I don't recall
>> killing a 6809 yet.....perhaps I have been lucky.
>>
>> Mind this is in a CoCo 1/2 or a Dragon 32/64, so it may be different
>> in the CoCo 3.
>>
>> Cheers.
>>
>> Phill.
>>
>
> A lot depends on the load that is being plugged in or unpluged. NMOS 
> depletion loads are ot
> designed for hot swapping. So are CMOS outputs. When at a logic one 
> output, all the current
> is passing through a NMOS diode that has rather small geometry. Geometry 
> determines the
> amount of current that can pass throug the diode. Drawing much more than o 
> TTL load (~2.4
> mA) is potentially dangerous. Once that diode load burns out, the port 
> will no longer function
> properly. In any case you have been lucky.
>
> I went back and rechecked the Coco3 schematic and the databus is buffered 
> with a LS245.
> So that concern is not as critical. The address bus is not. It is pulled 
> high through some
> resistors that the COco1/2 fail to use. This does help some in transients. 
> The resistors along
> with the internal NMOS depletion load act like a current divider. Thus 
> enough of the
> damaging current could pass throough the resistors instead of the IC. 
> Transients can cause
> latent defects that build up over time to induce a permanent failure.
>
> james
>
>
>
>
>
>
>
> --
> Coco mailing list
> Coco at maltedmedia.com
> http://five.pairlist.net/mailman/listinfo/coco


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