NEO-273

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 of the C ROMs and A3  of the S ROM are not part of the multiplexed bus and come straight from the cartridge edge, bypassing this chip.

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

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
See P bus.

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=



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