[volt-nuts] 3458A - calibration constants coding

Frank Stellmach frank.stellmach at freenet.de
Sat Nov 5 10:27:40 UTC 2011


I have analysed the content of the calibration NVRAM (U132, DS1220Y).
I assume that the 253 cal constants are stored as 8 Bytes of Double 
Real, because that perfectly fits into the 2k size, and because the 
instruments already defines DReal as an output format.

The content of the first two constants by MEMREAD:

might represent the actual Ohm and Volt calibration values, together 
with their corresponding DReal translations according to IEEE-754:

40003.0920    40E38862F1A9FBE7
7.20509364    401CD2041131AB9F

One can easily see from the first 5 bytes of each DReal number, that the 
first bit of each Byte of the DReal number read by MEMREAD is inverted. 
The last three bytes deviate by rounding to 9 decimal places.

So I assume, that HP'encrypted' the constants, either directly in the 
RAM, or during readout by MEMREAD.

This is my question: Is the content of the NVRAM, if read out on a 
programmer, also encrypted as described above, or does this occur only 
by the MEMREAD function?

This would make a difference for a future replacement and reprogramming 
of the cal. NVRAM.


More information about the volt-nuts mailing list