Date   : Tue, 04 Dec 2007 17:23:00 +0000
From   : pete@... (Pete Turnbull)
Subject: Tube splitter

Rob wrote:
> I'm not aware of any commercial offerings, however I've seen instances
> where two co-processors have been crimped onto the same ribbon cable,
> with power to each individually switched - just turn on the one you
> want to use.  I'm not convinced at how good for the tube ula this
> might be, though.

The ULA that's unpowered would be in just the same state as in the 
common case where the copro is connected but unpowered.  It won't mind. 
  It won't present a significant load to the other one either (high 
impedance) so it's no problem.  I ran a 6502 + Z80 pair like that for 
several years and never had any problem.

It's fairly easy to do.  I crimped a 40-way IDC male header onto the 
6502 Tube cable and plugged the Z80 cable into that.  I fitted plastic 
PCB pillars to the 6502 board and then mounted the Z80 on top (the 
mounting screw holes line up, of course).  The other thing I did was to 
solder 0.1" Molex pins to the link positions for the power, and 
connected them to a DPST centre-off switch so I could switch them on and 
off; they got +5V power from the Tube on the Beeb, and weren't mounted 
in an expansion box.  If you look at the circuit diagrams which I think 
are on the web somewhere you'll see the link positions for this on the 
schematics.

> Something a bit more sophisticated that provides electrical isolation
> between the devices shouldn't be too hard to run up with a handful of
> TTL buffer chips, and that then lends itself to introducing software
> controlled switching between them too.

It would be a fair amount of work to buffer all the bidirectional 
signals that have moderately critical timing, and you still need to do a 
reset if you switch between copros, or switch one on.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York