Hi Stefan,
VFD is a new thing for me.I srudied about a lot in these days.The main thing I was wrong about Filament voltage.The Filament Voltage is Fix -20v one side & you should give pulses of certain freq. on the other side.The pulse width I couldn't decide is the main problem in the last program I put here.I studied different ICs which driving VFD & concluded that the filament freq. should be such that you should give the low voltage of less than 1 ms.I generate the timer of 5ms in the last program.So the minimum time of low pulse is 5 ms.That would flicker the VFD display.I then make the timer lower & lower & then getting the good result.So ultimately I put it in the infinate loop.I can also put that thing with the timer of 0.1 msec but it will make my one timer dedicatedly to control this volatage.
The thing about the schematic is I have physical copy of it.The VFD is a very small part of big project.Our customer provide us whole Hardware & physical Schematic for programming purpose.So I can't share with you.But if you plan to use VFD I'll definately try to guide you.

Warm Regards
from
Maunik Choksi