Hello and thank you for a great repair site.

I have extracted a few firmwares from HP/Agilent 34401a (one of which AFAIK is the latest) together with each instrument's calibration EEPROM content.
You will also find the firmware of Keysight 2010.

Keep up the good work!

/Zerblatt


Shipping versions and distinguishing features:

11-05-02
Agilent
Gray Shift button
Black and red terminals
Serial MY4xxxxxxx
Manufactured 2005
Main board 34401-66511 REV 003
Blue Meder Reed relay

10-05-02
HP
Gray Shift button
Red terminals
Serial US3xxxxxxx
Manufactured 1998
Main board 34401-66501 REV D
Red Coto Reed relay

07-05-02
HP
Blue Shift button
Red terminals
Serial 3xxxAxxxxx
Manufactured 1995
Main board 88809F 34401-66501 REV B
Red Coto Reed relay

05-01-01
HP
Blue Shift button
Red terminals
Serial unknown
Manufactured 1995
Main board 88809F 34401-66501 REV B
Red Coto Reed relay

Fuses (not specified in the service or user manual):
F7A Littelfuse 314
F3A Littelfuse 235
T250mA Schurter FSD (early units might be installed with T125mA for line
voltages >=220V AC (as are Keithley 20x0 units for these voltages))

The 34401a CPU FW can be written to the same FLASH EEPROMs that also work for
Keithley 2000 and 2010: SST39SF020A-70. An upgrade generally requires
desoldering of the CPU FW ROM and (if you are a risk-averter) mounting of
a 32-pin PLCC socket. Some FW upgrades benefit from an EEPROM content
upgrade as well (or it might be necessary) - see below.

According to HP Service Note 34401A-06, the front panel assembly
34401-66502 works with firmware 06-04-01 and lower. 34401-66512 works with
06-04-02 and higher.

The latest code AFAIK is 11-05-03. XX-ZZ-03 is for the third generation of
front panel assembly that is a direct replacement to -02. It is unclear if
it also is compatible to units that were using -01 (but at least from
05-01-01 it can be upgraded to XX-05-YY).

As main board 88809F 34401-66501 REV B is used for both 05-01-01 and
07-05-02, the communications processor can be upgraded to version
XX-01/02/03/04-YY to XX-05-YY. (This code can AFAIK not be downloaded.)

The EEPROM structure looks the same from 07-05-02 to 11-05-02, so no
calibration seems to be necessary when upgrading FW from 07-05-02 to
11-05-02.

The EEPROM structure of 05-01-01 is different from 07-05-02, but it seems
that overwriting EEPROM of 05-01-01 with EEPROM of 07+-05-02 works
(although calibration is definitely needed afterwards).

Service note 34401A-07 says that communications processor XX-03-YY and
XX-04-YY suffer from RS232 error and an upgrade to XX-05-YY is
recommended.

Service note 34401A-08 says that communication processor XX-02-YY,
XX-03-YY, and XX-04-YY suffer from a GPIB error and an upgrade to XX-05-YY
is recommended.

Service note 34401A-09 says that CPU FW 06-AA-BB in some units suffers
from an "erroneous overload" error and an upgrade is recommended in this
case.