NEO-273

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 which 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