NEO-273

From NeoGeo Development Wiki
Revision as of 18:27, 15 January 2013 by Furrtek (talk | contribs)
Jump to navigation Jump to search
NEO-273 chip in a MVS cartridge. Picture courtesy of [MVS-Scans].

This chip is used in cartridges to replace 74LS273 and 74LS174 latches that were used in early boards (Like on CHA-32). It demultiplexes the P bus to C ROM and S ROM addresses.

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

This chip was later replaced by NEO-CMC which performs the same function along with quite a few other things.

For cartridges that use this chip but have more than 8MiB of C ROM, extra bits were added using 7400 chips. An added LS74 (2 bits) in CHA256 boards for example increases the C ROM capacity from 8MiB to 32MiB (256M).

Demultiplexing

external internal
P BUS P23 P22 P21 P20 P19 P18 P17 P16 P15 P14 P13 P12 P11 P10 P9 P8 P7 P6 P5 P4 P3 P2 P1 P0
C address A24 A23 A22 A21 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 A16 A15 A14 A13 A12 A11 A10 A9 A8 A7 A6 A5

A /C_A20 signal is available on pin 47 to allow cartridges who use 2MiB C Roms to switch between 2 pairs without an additionnal inverter chip.

Pinout

File:Neo-273 pinout.png

OpenOffice Draw file: File:Neo-273.odg