I am considering this, as I don't care to be stuck with NT for the life of
my instrument, which is currently only 3 years old. Has anyone tried it?
What exactly does 'not stable' mean? A glitch every run or every 100 runs?
Every 100 would put it on par with NT ver 1.2 in my experience, and I lived
with for almost a year.

Is there any way of upgrading a Delta Plus to the optical interface? I know
the bulk of the electronics and controls are different between a Plus and a
XP and I don't want all that (well, I do, but I know that's not possible),
but It would be nice to have a simple optical interface for my Plus that
does what my GPIB does now.

I spoke to Frank Trensch on this matter also and I asked the very same
question. He said the only way to get the optical interface is to purchase a
Delta XP. It seems the electronics are completely different in the two and an
upgrade is not possible.