Some time ago I found pages on making homebrewed HP48 cards (typically battery-backed RAM), but I can't find them anymore. Does anyone here know anything helpful on that subject?
There are a couple of designs available at hpcalc.org under HP48 -> Documentation -> Miscellaneous.
http://www.hpcalc.org/hp48/docs/misc/
Edited: 8 Jan 2012, 9:18 p.m.