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Modifying the Pageable ROM to work with ColecoVision addressing #12
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More a solution than an issue, but perhaps this could be added to the documentation somewhere. I have verified this modification works with some limited testing.
The standard RC2014 Pageable ROM module canonically uses ports
$30
and$38
, but due to don't-cares all of$3x
and$Bx
are assigned to it. Unfortunately, the$Bx
range conflicts with the ColecoVision ports for the TMS9918A module.A simple modification to the Pageable ROM module allows the ColecoVision ports to be used. To reduce the address range to just
$3x
, the enable pin on the decoder connected to A6 needs to be replaced with A6 OR A7.In hardware, a 74HCT32 (quad OR), with all but pins 1, 2, 3, 7, and 14 removed, is piggy-backed on U1, with the power pins (7 and 14) soldered down. Pins 1, 2, and 3 are straightened horizontally for air soldering. Pins 1 and 2 are connected to A7 and A6 on the RC2014 bus, respectively. Pin 4 of U2 is bent out of the socket, and connected to pin 3 of the new 74HCT32.
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