PCM

Picture courtesy of [MVS-Scans].

Early cartridges had separate ADPCM-A and ADPCM-B sound ROMs. Since one was often emptier than the other, SNK devised a way to mix both types in the same ROM chip to save space and reduce cost.

PCM (not "NEO-PCM" for some reason) is a banking and demultiplexing chip, found in AES and MVS cartridges, and on ROM-only arcade boards.

can also be found in late cartridges and with.

On some Cartridge boards, VA20~VA22 can be used to select which of the 4 possible V ROMs to use (decoded by a LS139).

Uses several latches for address and data, as well as a 4-bit counter clocked by for synchronisation.

=Internal logic=

The trick takes advantage of the asynchronous and predictable access to the ADPCM-A and APDCM-B ROMs by the. See YM2610 bus timing.

Verilog: to do.

=Pinout=



OpenOffice Draw file:

Has an inverter on pins 26 and 27.


 * SDPOE, SDROE, SDPMPX, SDRMPX: from the YM2610.


 * SDRAD0~SDRAD7: ADPCM-A multiplexed bus part
 * SDPAD0~SDPAD7: ADPCM-B multiplexed bus part


 * SDRA8, SDRA9, SDRA20~SDRA23: ADPCM-A address bus
 * SDPA8~SDPA11: ADPCM-B address bus


 * V D0~V D7: V ROM(s) data bus
 * V A-1~V A22: V ROM(s) address bus