Date   : Wed, 14 Jul 1993 13:18:33 +1200
From   : David Andrew Sainty <David.Sainty@...>
Subject: Re: Tube connections (was RE: colour Graphics)

Date: Tue, 13 Jul 93 10:28:36 +0100 (BST)
From: "I Stephenson" <ian@...>

>>I assume it suffices to isolate the Ntube on a second processor to disable
>>it.... (Hmmm, and NIRQ I guess).
> 
> More or less - NIRQ doesn't go through the tube (otherwise everything would
get  
> VERY confused). IRQ's are generated independanly on the 2P when data is
written  
> to the tube hardware(ish). The circuit (in addition to zapping NTube) also  
> holds the reset line low on the disabled processor. All the lines are buffered,
 
> as they now need to drive a greater load.

But what about the other direction? Are the tube registers polled on the IO
processor? I always assumed that they were tested on IRQ, in fact, it
seems strange that Tube IRQ is an output, as the IRQ line is used for all
the hardware that the 2nd P shouldn't care about. Are you sure about that??

> You probably want to check out the Master circuit diagrams if you can, as it  
> can switch between internal and external processors. It probably works almost  
> identically (this circuit is described as the ABM tube splitter, so it
predates  
> the Master).

Wish I could.... (though with all those custom Acorn chips on the
motherboard, tracing the circuit would be real tricky!