On Sun, May 27, 2018 at 3:10 PM, Noel Chiappa via cctalk < cctalk at classiccmp.org> wrote: The NS455 has firmware in masked ROM, which it uses if the EA pin is low. If EA is high (min spec 3.8V, normally tied directly to Vdd), external memory (usually ROM or EPROM) must be used instead. The NS405 has the masked ROM disabled, so external firmware must be used, and EA must be tied high. It is actually the same die with unspecified ROM content, so it probably would execute some sort of internal firmware if EA was low, but that might or might not be the same as the standard NS455 firmware; it could be firmware from a custom version of the NS455, or test firmware, or (less likely) any of the above but with some errors due to mask defects. I've been searching for the NS405 manual (not the datasheet) for a very long time.