[Coco] *SLENB behavior?
mechacoco at gmail.com
Thu Nov 10 02:24:26 EST 2016
On Wed, Nov 9, 2016 at 8:45 PM, RETRO Innovations wrote:
> Though I *think* I know the answer to this question, I'd like to get more
> opinions on the behavior of *SLENB. I assumed *SLENB is a signal to be
> treated like RESETB. Namely, the signal is weakly pulled high and any
> number of devices can pull *SLENB low if needed to offer functionality.
> The internal wiring of the 'G' line of the '138 to *SLENB with the 4.7K
> pullup suggests this usage, but I've found a cartridge that appears to hold
> *SLENB at an active high state, preventing other cartridges from pulling
> the line low.
Cartridges which use the SLENB* signal should do so in a Wired-OR fashion
(an open-collector or open-drain output). You should do the same for the
CART* and NMI* interrupt lines as well. A cartridge which actively drives
the line high would create a conflict with another cartridge using SLENB*
when both are plugged into an MPI.
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