[sdiy] ota's as gates.

[Shokwave]

----- Original Message ----- 
From: "Tim Parkhurst" <tparkhurst at siliconbandwidth.com>

> I want to see what the EEs have to say on this, but from what I've seen
it's
> generally a bad idea to feed a chip inputs that are outside of its supply
> rails.

Hehehe. I just today was talking with an EE who was reminiscing about the
old days, making a point to a bunch of my game design students. Seems he
worked on the first TTL logic module for telephony switching in the
province, and the very first module was an card puncher driver that tracked
the "error cards" and dynamically adjusted what level of error got printed;
if the shite hit the fan, it didn't print all the little stuff, just the big
errors etc.

They get this into place and fire it up for the first time...and IT starts
throwing errors like crazy, and smoke is pouring out of certain chips etc.
Now, none of these guys are dumb bunnies, and their leader & grand vizier
has a brain the size of a train; they are all floored though! IN THEORY, it
all should be working great...they painstakingly checked the schematics, and
inspected the boards...what gives?

Hint: the equipment it is connected to is a room full of relays.

When they throw a scope on the TTL gates, they get a shocking surprise;
spikes in their 5V logic that reach 195V!!!! The power supply had SOME
filtering applied (I said they weren't dumb), but nowhere near what they
actually needed. No one had ever noticed just exactlly how badly those
relays really treated the power supply when many thunked over at once; it
was truly murderous.

The point he was trying to make? No matter how prepared you are with theory,
you WILL learn something the first time you try out something new and big.
You probably will make a hash of it. No big deal, just be prepared for it,
and don't let it stop you from going on. No matter how old you get, you
never have all the experience you need, unless you like being stuck in a
rut.

-Darren