Hi Steve did you go with a NV 512K sram chip or regular sram?
Just a plain 512Kx8 70nsec. SRAM. Not sure if an NV part would work in
that circuit.
The part spec'd in Alex's schematic is DS1245AB-85
The SRAM I used was selected for speed (55 nS - good for 20 MHz
operation)
and DIP case easy to hand - assemble. Also available at Jameco for a
very
reasonable price.
As an added benefit it has an almost zero power shut down mode and
rertains
data down to 1.8 V or so.
The 128K version is particularly easy to use with a battery back-up as
it has
an active high second chip select. Most likely just adding a diode and
a coin
cell will make it NV.
-Alex.