|Re: HP-48GX RAM Cards - Better with or without built-in battery?|
Message #6 Posted by robert rozee on 14 Feb 2011, 8:24 a.m.,
in response to message #5 by aj04062
way back people were using memory cards for smith-carona typewriters. however, it was marginal if a given card would work or not - as far as i can recall the smith-corona cards were designed to operate at a slightly higher voltage than the HP ones, and could latch up due to input protection circuitry.
my recollection is that the cards are essentially just an sram chip (62256 or similar) with some glue logic. these days (with surface mount device readily available) it should be quite simple to produce a card with one _large_ sram chip, no glue logic needed, and a switch on the top edge of the card to select from a number of banks. alternative, a cpld could be used to implement calculator-controlled bank switching.
the input protection on the original cards is largely superfluous, especially if a large card is left permanently in the calculator.
addendum: CY62177EV30LL-55ZXI-ND from digikey, us$36.00 in quantities of one. "TSOP I Configurable as 2M x 16 or as 4M x 8 SRAM". this is a package that is hand-solderable, about 2mm high profile, with all the needed address and data lines brought out. 2.2-3.7v operation. with a bit of luck just this device, a soldered battery, 2 diodes and a suitable connector would do the trick, all assembled onto a small pcb. battery drain in standby is 3uA - technology has come a _long_ way in 20 years since the 48 was designed!!
Edited: 14 Feb 2011, 8:44 a.m.