[Coco] An 8-Slot Multi-Pak Design for my Color Computer Friends
jdaggett at gate.net
jdaggett at gate.net
Mon Jul 5 14:46:14 EDT 2010
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.
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.
More information about the Coco