>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.