22 Dec
2016
22 Dec
'16
10:44 p.m.
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.