@0xfede:
There must be a way to unlock the CALRAM already in firmware, because you can calibrate the meter from the front panel and through GPIB. There might be a way to use that built-in code to accomplish what you want to do, but it is going to take some effort to "hack" the machine.
Another way is to read the contents of the CALRAM, then pull the RAM chip and replace it with a ("no battery needed") non-volatile chip (NVRAM) that is programmed with the calibration data. The type of chip you use depends on whether the CS line goes low (after the address becomes valid) with every access-- (or not). Old-school CMOS RAM chips are just fine with tying the CS line low, and only changing the address lines-- newer NVRAM chips (FRAM, and some others) latch the address lines when CS goes low (becomes active).
I haven't looked at the schematic, but I suspect that if they have some kind of write-protect scheme, then the CS line will indeed toggle low with each access. You could check this in the schematic and verify with a 'scope. If the CS line does toggle with every access, then you can replace the CMOS RAM with a FRAM chip (also now from Cypress). If the CS line does NOT toggle with every access, something like a Cypress NVSRAM might work (the type that writes the RAM to EEPROM during power-down, and reads the EEPROM into RAM after power-up-- all done quickly in hardware).
If you can replace the old-school battery backed CMOS RAM with one of the above newer technologies, you will never have to worry about the battery in your DMM ever again (in fact you can remove the battery permanently I think, unless it is needed for something else).
Also, thank you for sharing your discovery of how to read the CALRAM on this forum-- that will help a lot of people!
*** EDIT *** ::
After looking at the schematic, and some data sheets, it appears that the FM16W08 will work just fine, but it will need a special DIP adapter to be made, and there must be a way (somehow) to get the CAL data into the chip before it is installed. Only 1/4 of the chip is actually used (the top 2 address bits are tied low). It is likely that most chip programmers will not be able to program this device properly, because it needs a lot of time after power-up before you start accessing the chip (probably internal charge-pumps at work here). So, maybe a DIY programmer made of a 40-pin PIC processor (or something similar) to read and write the chip via a serial port; this should be really easy to do. You should add an RC filter (very small R, very big C) on the VDD line, as the FM16W08 does not like a very fast rise-time on it's VDD line. This same chip (and adapter) can also be used for the 3458A. It appears that once this new NVRAM is installed, the battery becomes redundant, and may should be removed.
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