Electronics > Metrology

Read and restore calibration constants from a HP 34401A



As shown here in the forum, for many meters some (undocumented) functions exists to read the calibration data via GPIB.

Does anyone know, if such commands exists for the 34401A? I can't find anything until now.

Best regards

I did it on the hardware way with a test-clip on the eeprom (see the picture), but you have the unit switched on and the processor in "endless" reset, so that its pins are in a high impedance state.

I used different test clips at the processor seen in the picture, that are connected with a 1k resistor.
Then I used a GQ-4X programmer (choose 93C66B-Device with  16bit NOT 8bit).

So you can read and write the eeprom.

I´m also keen on: Is there a different way to read the constants (via HPIB or serial)?

best regards


There are apparently PEEK and POKE commands that might be able to read memory -- however I have never seen any documentation beyond this - https://www.keysight.com/main/editorial.jspx?cc=US&lc=eng&ckey=1000001267:epsg:faq&nid=-31895.536880933.02&id=1000001267:epsg:faq which shows how to enable a 10 mA AC range.

A quick update to this topic - with the help of some other forum members and other old threads I've managed to do some EEPROM hacking to enable some hidden functions on post-07-xx-yy 34401A FW, and one of the discoveries was a method for saving the EEPROM contents (inc. calibration table) without opening the case: https://www.eevblog.com/forum/testgear/hp-agilent-34401a-hidden-menu/msg4436983/#msg4436983

As of yet we haven't figured out how to write arbitrary values to the EEPROM without opening the case and connecting a programmer, but it may still be of interest here as at least you can get a quick backup. Would be best to whip up a script to do the work - I've already got my EEPROM dump from connecting a programmer to it, so haven't gotten around to writing one myself.

As for the hidden functions, tl;dr is that there are 3 EEPROM bits (plus a checksum update) that if set allow for saving and recalling mode settings etc. (inc. on power up), a temperature measurement option (thermistor, RTD or thermocouple) and finally a "SCALE" option in the maths menu that lets you scale and offset readings without involving a computer.


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