View source for NEO-273

[[File:crt_273.jpg|right|thumb|NEO-273 chip in a MVS [[cartridges|cartridge]]. Picture courtesy of [[http://www.mvs-scans.com MVS-Scans]].]]

This chip is used in cartridges to replace 74LS273 and 74LS174 latches that were used in early boards (Like on [[CHA board|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 ROM]]s) in one package. Note that A4 ({{Sig|CA4}}) of the C ROMs and A3 ({{Sig|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==

{|class="wikitable"
|
|colspan=4|external
|
|colspan=24|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]]

[[Category:Chips]]