Date   : Tue, 13 Jul 1993 12:50:06 +0200
From   : stjohn@...
Subject: Re: Tube connections (was RE: colour Graphics) 

>I assume it suffices to isolate the Ntube on a second processor to disable
>it.... (Hmmm, and NIRQ I guess).

Yes. I believe at least the 32016 2nd processor does exactly this.
On my ACW there's a little toggle switch behind the keyboard that
switches in the tube.

>So, this seems to indicate only the low 8 locations are required by TUBE
>hardware. Does the splitter ensure splitter switching writes don't make it
>to the second processor? (What I'm really asking is, if a write is make
>to a second processor above those bottom 8 locations, does it ignore it,
>or might it panic?)

Pandora won't. I've disassembled all the importants parts of it so
I can guarantee that in general it won't panic. If you want to make
extra sure, see the new AUG which details the Tube I/O and protocol.

Be careful with NMI because Pandora at least uses the NMI as a 
sort of software DMA. After a disk I/O the beeb sends NMIs to the
32016 with data in the Tube ULA. This is how data is transfered
transparently into the second processor. If you start fiddling with
them, then expect some BIG problems!

  ,--,             Chris StJohn, LIX, Ecole Polytechnique, 91128
 /    /_  __' _    Palaiseau, Cedex, FRANCE. (+33) 1.69.33.45.93
(____/ (_/ (_/_) - actively seeking employment in the UK (ha! :)

PS: On my board there's a big 16MHz crystal hooked up to a NAND
gate chip... why don't you try following where the output of that
clock goes and trapping it somewhere on the board instead of messing
with the Tube? Knowing Acorn, there's probably a pin somewhere on 
the board that lets you plug straight in to the system clock ;-)