Weird Lear-Siegler ADM-3A board

Eric Smith spacewar at
Tue May 29 02:04:51 CDT 2018

On Sun, May 27, 2018 at 3:10 PM, Noel Chiappa via cctalk <
cctalk at> wrote:

> I decided I'd do an article about the Term-Mite for the CHWiki; I found
> Ciarcia's long article about the Term-Mite (in his book, which Google books
> has); it talks throughout the article about the NS455 - but it also says
> "[t]he Term-Mite actually uses a NS405 chip which is a specialized version
> of
> the generic NS455".

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.

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