M20 Olivetti M20

Modification of the ram cards, by Dwight Elvey

Dwight has modified 32kB RAM expansion, in order to accept 4164 chips. Many thanks also to Bruce Damer and the DigiBarn Computer Museum .
This is what Dwight wrote about his modification:
"The mod isn't too complicated. I was a little frustrated because the board outer trace are identical with the 128K boards. The internal power plane connections are not compatable with the 128K boards. This required some cuts and the jumper you see."

There are two types of expansion cards: corlor and B/W. It is possible to mix the two different types, but it is important that the pair is composed by expansion having exactly the same size.
The existence of two types of memory expansions is explained by Dwight:
The B/W or 2 color use DMA directly from the memory, over the main memory bus. The video is in the main RAM rather than a separate RAM, like a PC video card. The mapping of this area is one of the things that the memory management ROM controls. For 4 and 16 color, you need to supply up to three different colors simultaneously. They handle this by having each memory card, that is used for color, include a shift register. That way, when the DMA looks at a location, all the color cards transfer the information into the shift registers. The video chip then reads the bits serially as it needs them for the video output. On the B/W, the serializing is done on the motherboard after the transfer through the memory bus. Each card is selected be a decoded bit from the ROM. When it is the video accessing for color, all the color RAM cards are accessed at the same time and read their image data into the shift registers as a parallel action. Mapping of the RAM boards is kept looking continuous to the processor by the ROM, even though, there are holes in the physical map for the video sections.

Front view Component view of the 128kB color expansion

Front viewSolder view of the 128kB color expansion

Front viewComponent view of the 128kB B/W expansion

Front viewSolder view of the 128kB B/W expansion


Jumper settings

To make your system work with the bigger cards, you should configure correctly jumpers on the motherboard. This configuration differs slightly with the motherboard revisions. You can find detailed instructions and photos in this page.


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