Memory card with "stacked" dram chips: I don't understand how this works \ VOGONS

Memory card with "stacked" dram chips: I don't understand how this works

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First post, by keenerb

Posted on 2021-02-09, 00:29

keenerb Offline

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I've come across a decently rare combo parallel port and memory upgrade card, and I'm confused.

The main board has 128kb of memory on it, using "stacked" dram chips, soldered directly on top of one another:

The attachment ram_pins_2.JPG is no longer available

The attachment ram_pins.JPG is no longer available

I'm not well versed in the sorcery of DRAM address decoding and whatnot, but it seems to me that BOTH chips would receive identical signals. How would the top chip know to behave differently from the bottom chip?

Reply 1 of 4, by imi

Posted on 2021-02-09, 00:52

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they're probably two slightly different chips

Reply 2 of 4, by Jo22

Posted on 2021-02-09, 01:02

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Oh, that used to be called piggy-back style, I believe.
In simple words, all chip lines are in parallel, except for two address decoding pins - these two are in series (the cards memory address logic connects to the input of RAM a and the output of RAM a goes into input of RAM b; while output of RAM b in turn goes to input of RAM c and so on).
That way, the addressing goes through all the RAM chips..

Edit: Small edit.

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Reply 3 of 4, by keenerb

Posted on 2021-02-09, 02:13

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imi wrote on 2021-02-09, 00:52:

they're probably two slightly different chips

It is a different part number on the top chip vs. the expansion daughtercard. Maybe you're right.

Reply 4 of 4, by mkarcher

Posted on 2021-02-09, 17:43

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keenerb wrote on 2021-02-09, 02:13:

imi wrote on 2021-02-09, 00:52:

they're probably two slightly different chips

It is a different part number on the top chip vs. the expansion daughtercard. Maybe you're right.

Most DRAM chips have the bits in a square matrix of rows and columns, where both the row count and the column count is a power of two. So you can get 4k, 16k, 64k or 256k addresses inside one
chip. The piggy-back chips are the missing steps of 8k, 32k or 128k. In this case, the capacity is distributed over two chips of the conventional size. Usually, the lower and the upper chip use different pins for "/RAS" or "/CAS", which work similar to a chip select signal. In fact, the setup is very similar to double-sided PS/2 SIMMs. Double-sided PS/2 SIMMs (technically: dual-rank SIMMs) also have all the data and address lines of the chips from both sides in parallel, they only have /RAS or /CAS split for both sides.

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