[sdiy] Xpander

[Rainer Buchty]

On Wed, 21 Oct 2020, Roman Sowa wrote:

> both boards need different data - different notes are played, and it 
> can be set up so each voice plays different patch. Well I can imagine 
> a scenario when at every even cycle the master CPU can mingle one 
> slave's RAM, and on every odd cycle - the other RAM. Still that seems 
> unnecessary as data transfer is needed very rarely compared to bus 
> speed.

Aaah, now I understand the confusion.

There's no difference of phase between slaves.

If this is the host clock

----____

this would be *all* slave's clocks

____----

So the master can write to any slave board without colliding with that 
slave's own local accesses; it could even write to all slaves in one go, 
e.g. in terms of patch data transfers.

But what of course cannot take place is slaves accessing any other 
slave's memory (collision with local accesses) or the master's memory 
(possible collision of remote accesses).

> There is no arbitration logic, only buffers. Just set the Halt line 
> and bus is yours to write those few bytes and go back. That seems 
> simpler than anything else.

That would be if it behaved like BUSRQ# on the Z80 which indeed 
interrupts a running instruction.

HALT# on the 6809 would however become effective *after* the 
currently running instruction, so you would need to also monitor BA 
and BS for becoming 11. If you would just assign HALT# and access the 
bus, you're likely to create bus collisions.

Why going the extra lenghts of triggering a bus-free interrupt or 
E/Q-clock delaying on the accessing CPU when it's not needed? It's an 
(IMO) wonderful feature of the 6800 derivatives that you can rather 
simply set up multiprocessing environments without the need for any bus 
arbitration.

That's all I was saying.

> There are many thing we can argue over Matrix12 or Xpander. It's clear 
> to anyone they are not perfectly designed. May compromises and bad 
> decisions all the way starting from the worst power supply ever 
> designed. Tell it to Tom Oberheim he did bad. Actually last time I 
> asked him about Matrix12, he told me he wasn't involved in the design 
> on digital/CPU stuff.

Again: I was neither criticizing him, his team, nor the design of M12 or 
Xpander, which I think are quite exciting machines.

Rainer