I couldn't find any more detailled instructions on google (I spend many hours searching)... I also think it doesn't work since the only chip added is a 74LS138 in order to manage 2 more columns (CAS). In all the third party upgrade I saw (in photo) there is much more chips added!
Why would it need more than a single 'LS138? The 74LS138 is a 1-of-8 demultiplexer, IE, based on the input of three "address" lines (basically an octal data input) in activates one of 8 output lines. Comparing the circuit description in the PDF to the datasheet I see what they're doing here is they're using 4 outputs on the '138: each row of 8 chips has its own CAS probe (I kind of wonder why it's wired that way instead of all 16 together, but maybe it has to be because the original circuit supports byte-size operations?), so there's the 2 original lines *plus* the two added ones. So that's only half the capacity of the chip. (Reading the datasheet for the RAM CAS basically doubles as chip select, so the way the circuit works is it putting the '138 in a location where it can watch A19 and act as a switch to change whether the first block of RAM or the one you added is enabled when the CPU is looking at the given address range.)
As to why the third-party boards you're looking at have more chips on them, well, you'd have to ask the designers. I googled up a picture of a Dove 1MB card and it only has three chips on it, that being a mutiplexer, a demultiplexor, and a dual NOR gate. I'm *guessing* the reason it has the (slight) amount of extra circuitry is because of how it installs by pressing over the existing chips without requiring a trace cut means it has to do some additional chip select work you get for free with the piggyback design.
Yes but I want it to work with stock 64k rom too
There's nothing about the Dove boards that depends on the ROM version, unless the 64K ROM doesn't bother trying to test for the presence of RAM above the 512k mark. If that's the case then no mod will work.
