And here is some info from a post in 2018... from user crasyboye at:
http://bbs.38hot.net/thread-258164-1-1.htmlSome information found abroad, but did not mention anything too substantial.
The original PM6681 calibration setup includes a PC, an
ancient Philips ISA-bus GPIB interface and an ancient Philips driver and a DOS program.
Collecting these and put them together to work will be an interesting
challenge.
A more modern variant of the calibration routines run can run on more
modern HW and LabView software. Philips PM5781 and Agilent 81112A pulse
generators is supported by those routines.
By now the production of the counter has stopped, as the counter core
ASIC ran out of stock.
The folks who designed and maintained them does not work there anymore.
Turns out the core setup requires a generator that creates a skewed
Sweep over that you SUCH Frequency at The interpolation Phases. TWEAK by You
at The Calibration value (3.86-4.50 ns 0.02 ns in Steps) you that the SET over
GPIB at The * PUD a using the Command. by You CAN IT Yourself a using the Check * PUD? .
It Sweeps over the this range and checks for the value giving lowest RMS
value (SDEV result) and then sets this as the final value.
You can set the value using the command :SYST:UNPR;*PUD %s where %s is
the string, looking somewhat like this:
FACTORY CALIBRATED: %s%s, CALPLS 3.98 ns
The two %s in there is for some string and date, I just don't bother to
dig up a correct example.
Anyway, you can actually read-out the data and write it back without the
software. It should be possible to do something with the existing pieces
and it might be possible to actually get the calibration software
operational again.
I just don't have a CNT81 anymore to try this out on.
With the hints from the former Pendulum service guy, I havestarted to
write my own code in order to restore calibration on a CNT-81/PM-6681.
This have been a discussion on and off for a couple of years, so I ended
up buying a PM-6681 which had the Loss of Calibration message " CALLOSS".
First things is to replace the CMOS battery, which is a trivial thing to
replace the CR2032 battery, had one laying around.
Then, I've been digging into the NI VISA files, which have snippets of
actual code in it. As I don't have NI VISA and not running with my
Prologix, I was a bit out of luck there. So I had to start from the
ground up, taking a serial interface hack I already have, write some
minimalistic Prologix support for it (TvB hp59309.c provided some needed
clues on how to get it working stable).
Then more and more bits and pieces have been falling together, like
being able to build and write the *PUD string. Also, triggering the
calibration itself and using an external source.
Now my counter does not display the error anymore and seems to behave
more coherently. I'm not completely trusting it, as I am not doing the
full sweep over calibration pulse calibration values and measuring their
effect, that will be part of the complete solution, but at least I get
sufficient part of the way.
Far from bullet-proof, it is however worth celebrating these baby steps
in the right direction.
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