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

[jdaggett at gate.net]

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