[Coco] 6809 to 6309 switcher

[richec]

On Sat, 21 Nov 2020 12:05:49 -0600
RETRO Innovations <go4retro at go4retro.com> wrote:

> On 11/21/2020 9:23 AM, Joe Schutts via Coco wrote:
> > Hi Everyone,
> > The problem was that it would NOT WORK with the 2 Meg memory
> > upgrade that is available for the CoCo 3. I contacted the person
> > and asked him IF the circuit board was STILL AVAILABLE and he told
> > me that it was available (for a cheap price). I also asked him
> > about the problems with the 2 Meg memory upgrades and he told me
> > that (as far as he knew) there were no problems. He then asked me
> > WHO it was who told me that there was a problem with the 2 Meg
> > upgrade and I told him. He than told me that he would check into it
> > (I guess contact whomever had told me that there was a problem) to
> > see what the problem was. So far he has NOT gotten back in touch
> > with me and I DO NOT know what (or even if) the problem still
> > exists...  
> 
> Hmm, I thought I responded, but in case it went to spam:
> 
> Yes, I did figure out the issue, and there is a problem (well,
> calling it a problem is overdoing it.  I'd probably call it it a lack
> of functionality).  The initial design was engineered to support 2
> CPU ICs, any combination of 6809E and 6309E DIP ICs.
> 
> But,
> 
> The way it does that is to take advantage of 2 things about the CoCo
> and it's use of the 6809E:
> 
> 1) The 6X09E can be effectively stopped by bringing the TSC line high 
> (Tri State Control).  Bringing the line high causes the CPU to 
> essentially go into a holding pattern.  As well, many of the signals 
> lines on the 6X09E at (as the name implies) tristated, which means
> they are effectively hidden.
> 
> 2) There are, though, some lines are are not hidden, various control 
> lines called BA, BS, etc. (Bus This and Bus That, so to speak).  
> *HOWEVER*, the CoCo does not sue any of those lines, so they can be 
> effectively disconnected from the CoCo without any ill effect.
> 
> It's this second item that is the cause of the issue for some memory 
> expansions.  While the CoCo does not need those bus control lines,
> the memory expansion does.
> 
> So, where's the problem?  Well, there is no problem if you consider
> the MMU/DAT portion of these memory expansions as part of the CPU
> assembly. If the CPU is attached directly to the DAT/MMU portion of
> the memory expansion, and then that combination is then installed in
> the Dual 6X09 adapter I sell, there is no issue.  The CPU provides
> the control signals to the DAT/MMU board, and then DAT/MMU board acts
> on them, and all is well.
> 
> *EXCEPT*
> 
> Using things this way would require the owner buy *2* DAT/MMU boards, 
> one for each CPU, which is neither financially advantageous nor easy
> to technically implement (You'd have 2 DAT/MMU boards trying to
> control the memory, which I doubt will work).
> 
> SO,
> 
> The way people want to use the switcher is to:
> 
> Install the DAT/MMU board into the CoCo CPU socket
> 
> Install the switcher board in the DAT/MMU CPU socket
> 
> Install the 2 CPUs in the switcher CPU sockets.
> 
> But, as noted above, that means that the CPU's control signals will
> be sent to the switcher board, which.... does not connect them to the 
> combined CPU header.
> 
> And thus the failure.
> 
> The good news is, the solution is technically simple: tristate those 
> signals manually and pass a set of them onto the CPU header,
> depending on which CPU is turned "on".
> 
> The bad news is, that required a redo of the PCB and some testing,
> which has not completed yet.
> 
> Now everyone knows more about this than they ever wanted to :-)
> 
> > Sorry that I cannot be more helpful about this.
> > P.S. The gentleman did tell me that (as far as he knew) the circuit
> > board DID WORK on the 512 K upgrade. Also, the circuit board comes
> > as a plain board that you solder the sockets (or the chips) to the
> > board...  
> I can confirm the "passive" switcher works in this situation.
> > Hope this helps...
> > Joe...
> >  
> Jim
> 

Thank you for the explanation and for doing what you do.