Fellow time-nuts, With the hints from the former Pendulum service guy, I have started 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 "CAL LOSS". 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. Cheers, Magnus