Re: Dynamic RAM / memory expansion

From: Ruud Baltissen (
Date: 2001-01-19 15:43:35

Hallo Nick,

> Now...  30pin 256kB SIMMS unfortunately generally use two 44256 chips (and
sometimes one 41256 for parity). The 1MB SIMMS use either 8 or 9 1MegaBIT
chips.  The problem in both cases is refresh will not work on all cells.

I do have some tech specs of the 44256 and also came to the conclusion that
this chip needed 9 refreshlines instead of 8 like the 41256 :( But I wasn't
sure until now. An other source pointed me to the fact that some 72 pin
SIMM's operated on lower voltages: so even more problems.

Designing the circuit is not the problem; you only need two 74LS393's (which
supply you with 16 lines) and three 74LS257's. These you need to switch
between the original lines and the ones for the refresh. Then you need a
circuit to generate a RAS-pulsen during the low halve of CLK2 which does the
trick. The only problem of the circuit is that it isn't small. If you want
to devellop a complete RAM-expansion, you'll need at least another three
74LS257's to do the multiplexing of the original addresslines.

For those interested to see the above in working order: I've seen some
VIC-20 RAM expansions using the above principle.

Groetjes, Ruud

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