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Package QFP64R
First use 1992 ?
Used on MVS cartridges

The NEO-273 chip is used in cartridges to demultiplex the P bus to C ROM and S ROM addresses (graphics). It replaces the discrete components that were used in early boards such as CHA-32.

This chip was later superseded by NEO-CMC which performs the same functions along with a lot of other things.

For cartridges that use this chip but have more than 8MiB of sprite graphics data, extra bits were added using discrete chips. An added 74LS74 (2 bits) in CHA256 boards for example increases the C ROM capacity from 8MiB to 32MiB (256Mbits).


  • The C ROM address is latched on the rising edge of PCK1B.
  • The S ROM address is latched on the rising edge of PCK2B.

Mapping of the P bus to the ROM address lines:

P BUS P19 P18 P17 P16 P15 P14 P13 P12 P11 P10 P9 P8 P7 P6 P5 P4 P3 P2 P1 P0
C address A3 A2 A1 A0 A20 A19 A18 A17 A16 A15 A14 A13 A12 A11 A10 A9 A8 A7 A6 A5
S address A4 A2 A1 A0

Note that A4 (CA4) of the C ROMs and A3 (2H1) of the S ROM are not part of the P bus and come straight from the cartridge edge, bypassing this chip.

P20~23 are left out or routed to additional latches if needed.

An inverted C_A20 signal is available on pin 47 to allow cartridges which use 2MiB C ROMs to switch between 2 pairs without the need for an additional inverter chip.


It includes one 16-bit latch (to address a 128KiB S ROM) and one 20-bit latch (to address 8MiB of 2*16bits C ROMs).


Edit this pinout


The NEO-273 doesn't appear to fail often. If some outputs are dead, their pins can be lifted and wired to a 74LS74 (2 outputs), a 74LS171 (4 outputs) or 74LS273 (8 outputs).

  • Wire the data input to the appropriate P bus line (see table above)
  • Wire the output to the address pad
  • Wire the clock to either PCK1B or PCK2B depending on the affected address bus.
  • Wire power and don't forget to ground the unused inputs if using a CMOS part.