Pinout for SED9421

[Tony Duell]

> I've just spent two days reverse-engineering the Atari Digital Vector
> Generator down to state machine level. Thank %DEITY for schematics!

You're lucky. I normally have to start by producing the schematics (mind 
you, for simple stuff, its quicker _for me_ to draw them out than either 
to  do battle with the manufacturer or to download them and figure out 
how to print them...)

> 
> Only thing I am trying to work out is why the hell the state machine jumps to
> state 9 on startup, which is a DMALD microinstruction. Basically, when it
> starts up, it pulls an address off the stack and jumps to it. Unless I'm

Are you sure it does? In particular, are you sure the output of the stack 
is not tri-stated and the lines driven by something else?

> wrong, that means it jumps to a random address on startup. That is, unless
> the LS670 Register File Memory used for the stack is guaranteed to power up
> with all registers loaded with 0000(bin), but IMO that's a dangerous
> assumption to make without testing.
> 
> Just a shame LS670s are pretty much unobtainium these days, otherwise I would
> have tested it out...

I've used the LS170 quite a bit (open-collector, not tri-state outputs), 
and I am darn sure the power-up state is not guaranteed to be anything 
special.

-tony