Reverse engineering the Digital KC 4MB mapper

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Van Grauw

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afbeelding van Grauw

07-06-2019, 01:50

Digital KC 2MB mapper to 4MB upgrade instructions:


I added four 1M×4 70ns DRAM ICs which I harvested from a 4MB SIMM that I got from Omega. This adds the 2 MB. These need to be added in the empty positions on the board that I marked with orange above. I also added 100nF decoupling capacitors for them at the spots marked in red.

Lastly the mapper registers need to be extended to store 8 bits rather than 7. Add a little jumper wire connecting the two traces at the spot marked in yellow. This connects the D7 bus pin to pin 15 of the 74HCT670 IC that’s above it, extending the register to store all 8 bits. Also make sure pin 15 is not connected to pin 16 (VCC +5V, high), otherwise cut the connection (for me it was not connected). The multimeter readings from pin 1, 2, 3 and 15 to pin 16, or from D0-D7 to 5V on the cartridge connector, should all be the same.

Van Grauw

Enlighted (8085)

afbeelding van Grauw

07-06-2019, 01:38

This Digital KC mapper is described extensively by Kees Folst himself in MCCM 65, page 33 (Dutch). Including schematics and many in depth details about how the Sony HBM-512 circuitry was simplified and optimised for 7 MHz and large DRAMs. A really nice read.

It also explains the differences between the 1M, 2M and 4M versions of the mapper. The spot marked in yellow where I added the jumper wire is called “L2” by Digital KC. Next to the big capacitor on the front side of the board you can see another small jumper, this is “L1”, this one will be absent in the 1M version. To upgrade an 1M mapper to 2M you need to place the jump wire at L1 (connects D6 to pin 1), and make sure it has no continuity to 5V.

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