[time-nuts] Lost Calibration on CNT-81/PM-6681

[Magnus Danielson]

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