[Coco] *SLENB behavior?

[Barry Nelson]

What might be useful for the CART line is a register that would allow you to specify some or all of the CART lines to be always connected. The default would be the current switched behavior, but for a given slot if a certain bit were on, then that CART line would always be connected regardless of what MPI slot is currently selected.

> RETRO Innovations go4retro at go4retro.com  <mailto:coco%40maltedmedia.com?Subject=Re%3A%20%5BCoco%5D%20%2ASLENB%20behavior%3F&In-Reply-To=%3C6296d7c5-aeae-e515-bd5b-997bb0420ca7%40go4retro.com%3E>
> Thu Nov 10 12:45:00 EST 2016
> On 11/10/2016 10:28 AM, Darren A wrote:
> > On Thu, Nov 10, 2016 at 8:29 AM, Mark J. Blair wrote:
> >
> >> Program Paks simply connected CART* to Q, and the Multi-Pak MUXed the
> >> CART* signals from each slot. The MPI also provided a 4.7k pull-up for each
> >> slot's CART* signal. Either a totem-pole or wired-or driver should be fine
> >> for CART*.
> >>
> >> NMI*, HALT* and SLENB* should be treated as wired-or signals, though, even
> >> if some contemporary designs got that wrong.
> >>
> >>
> > Yes, the HALT* line is another I forgot to mention.
> >
> > Although not strictly required in an MPI, I would still recommend any new
> > design treat CART* as wired-or.  There are those who have long advocated
> > modifying the MPI to tie all the CART* pins together and remove 3 of the
> > pull-ups so that interrupts from a serial pak or other device are not lost
> > when the slot is not active.  Of course you then have to be careful not to
> > use any auto-starting program paks in the MPI, or put some tape over their
> > Q or CART* pins.
> >
> >
> > - Darren
> >
> 
> It's a good callout.  I had the following for the CART line:
> 
> 
> assign cart =                    (autostart ? q : 'bz);
> 
> so, I changed it to:
> 
> 
> assign cart =                    (autostart & !q ? 0 : 'bz);
> 
> To cover the use case you're describing.
> 
> Jim