3421A CAL RAM

[Dave Frederickson]

(04-04-2017 04:08 PM)BobVA Wrote:  I managed to get the cal information out of my 3468A, but only after this hilarious goose chase:

- Dig out 82160A HP-IL interface, only to realize HP-41 has packed up.
- Fix HP-41, connect to 3468A then realize that 41 only accepts 24 characters in an HPIL response.
- Dig out PILBox and set that up on bench computer. Side trip on failed quest looking for second HP-IL cable. Bodge something.
- Realize ILCtrl can't issue a "Talk" command, so even though I can send the "B2" command, there's no way to read it.
- Consider learning C++ or VB so I can modify it to do this, but then realize I'm not smart enough.
- Re-read 82160A manual and realize that the entire string *can* be read by the HP-41 using multiple "INA" commands. Bang head on bench.
- Connect printer to HP-41 and use 5 line program to read and print cal data.

Done! :-)

(Still not sure how I'd write it back if I had to, but perhaps the B3 command followed by the data would work from ILCtrl, since a response isn't needed.)

Good, glad to see you followed the standard procedure.

Here's what I've learned about the cal data for the 3468. First I loaded the CAL RAM with zeros (all @ characters) using the B3 command, then dumped the data after each step in the calibration procedure. This is what changed after each step.

Code:

.3V Zero,   Line 2: 999998500000E5FB
.3V Gain,   Line 2: 999998902B04EBCF
3V  Zero,   Line 3: 999999900000F9FF
3V  Gain,   Line 3: 99999991FCE0FAB7
30V Zero,   Line 4: 000000600000F6FF
30V Gain,   Line 4: 00000061F2B4FAA3
300V        Line 5:
ACV?        Line 6:
300 Zero,   Line 7: 00069500000050FF
3K  Zero,   Line 8: 000067500000E5FB
30K Zero,   Line 9: 000006700000977C
300K Zero, Line 10: 000000700000F77F
3M Zero,   Line 11: 000000200000F27F
30M Zero,  Line 12: 000000100000F17F
3A Zero,   Line 13: 000000500000F5FC

It looks like the first 7 nibbles in each row are the Zero, the next 5 the Gain, and the last 4 the checksums.

From the HP Journal:
Besides ESD, there is also a small chance that an alpha particle or a cosmic ray may alter a single bit of the memory. To protect against this rare occurrence, the two constants for each function and range have parity bits associated with them so that the 3468A's microprocessor can locate singlebit errors and detect double-bit errors. The processor can correct a single-bit error in a calibration constant and the instrument will remain calibrated. If the processor detects more than a single-bit error, the CAL annunciator is displayed and the error bits are set to warn the user that the readings are uncalibrated. When this type of error is detected, the processor uses default values of zero for Y and one for G.

What parity/checksum/CRC method can correct single-bit errors and detect double-bit errors?

I believe the 3468 and 3421 report CAL RAM errors on a per/range basis. I need to investigate.

Dave