VIC-20 Character ROM Adapter
The VIC-20 Character Generator is a 4k 2332 ROM. Bith, in the VIC-20 CR version (ASSY 250403) and the VIC-20 version (ASSY 324003) , it is called UD7. The specialty of this version is, that it uses two chip selects (while the C64 character ROM uses just one chip select signal.
Except from schematics 251027
Both chip selects are low active and are connected to the videao address bits VA13 and VA12. Since both, CS1 and CS2 ate active low, the character ROM is addressed, if VA12 and VA13 are both LOW (0).
The 27Cxxx EPROMs have a /CS and an /E input, which have to be low to access the EPROM. so both can serve as a chip select for the VIC-20 character ROM.
Schematics of the character ROM adapter (click to enlage)
Note: In the C64, the 2nd chip select of the character ROM (Pin 21) is permanently HIGH, so with closing the solder bridge JP3 and opening JP2, the adapter can be configured to work with the C64.
There is another differece:
In the VIC-20CR, the character ROM is very close to the RF shield of the video circuit. Thus, the selection jumopers cannot be at the notch side (Pin1, Pin 28) of the EPROM. they have to be at the other side.
VIC-20 Character ROM adapter in ASSY 250403
The Character ROM adapter was tested both in the VIC-20 (ASSY 324003) and the VIC-20CR (ASSY 250403). It works prefectly in both versions.
Japanese character set
Please find the complete documentation and resources of this project on github: https://github.com/svenpetersen1965/VIC-20_Character_ROM_Adapter