[Coco] How SLENB works ?

[Darren A]

On Fri, Jul 18, 2014 at 7:05 AM, Luis Antoniosi (CoCoDemus) wrote:

> Hi all,
>
> I decided to play with arduino + coco but I need to know exactly how the
> SLENB works. As I can see in the schematics it's connected to the 74SL138
> pin 6. This means when SLENB is LOW all the 138 outputs will be HIGH. But
> what is the practical effect of this, will the coco wait for the cartridge
> before doing anything ?
>


The 74LS138 provides the device select signals for the internal ROM,
external ROM (CTS), both of the PIAs and the cartridge spare select (SCS).
On the CoCo 1 and 2 it also provides the RAM select signal.

If you assert SLENB low while one of these devices is being addressed by
the CPU, it will prevent that device from responding. This allows you to
override the normal use of those addresses and use them for something else.

Darren