ADM-3A Lower case ROM issue

[Eric Smith]

>From the ADM3A Maintenance Manual, page 6-11:

The two character generator ROMs are rather straightforward. The upper case
> ROM is a standard masked part (2513) but the lower case ROM is a custom
> masked part. The one unusual thing about this is that all of the address
> lines into the lower case character generator are inverted.
>

That refers to the character code address inputs to the ROMs; the line
address is common between the two.

The character addresses for the ROMs come from two 74LS175 four-bit latches
at K13 and L13, which have both inverting and non-inverting outputs.  The
non-inverting outputs go to the UC 2513 ROM (L15), while inverting outputs
go to the LC ROM (L14).

>From the schematic, the only TTL chip that appears to be needed solely for
the LC option is the 74LS157 at K12, but I'm not sure whether a UC-only
model would work without that installed. The manual doesn't show that part
as optional. My UC-only ADM3A does have it.

Upgrading from UC to UC/LC does require adding the H11 and J11 RAM chips
for the seventh bit of the refresh memory. Only one of the RAMs has to be
installed for a 12-row-ony model.