I have a 27S, which may or not be similar, but does take three button cells and has markings like what you described. Mine are MS76. I think Energizer 357 is equivalent, also SR44. --Patrick

The 28C and 28S have the same case, and take the same cells, but the logic boards are very different (i.e. a 28S is _not_ a 28C with extra/different RAM chips added). The 28C has 3 direct-on-board chips under the display (CPU in the middle, 2 RAM/display drivers), and 2 saturn-bus ROMs on the back side. The 28S is more like a 19B -- it has 2 TAB chips under the display (which I think are CPU/ROM/display drivers, with the CPU disabled on one of them) and a 32K JEDEC-pinout RAM on the back side. Actually, the 28C is the more expandable one. You have access to the Saturn bus (which can address 512K bytes), and the machine _will_ recognise extra RAM (e.g. taken from an HP71). There are no extra address lines available on the 28S CPU AFAIK, so you're pretty much stuck with the 32K. I've often wondered if it would be possible to get a 28C to talk to an HP71 HPIL module (the hardware interface should be trivial). Of course writing the necessary software could prove 'interesting'... -tony

At 09:42 PM 4/7/03 -0400, David wrote: Good point! The batteries have a tendency to press againt the cover and push it sideways out of it's slot. Then when you really press on it to try and close it it jams and you break it. Press the batteries into the calculator and try to keep them from pressing against the battery cover as you install it. Make sure that you get the cover lined up properly in it's slot before you press it closed. Joe