After more than 1 week trying to solve the issue, I'm starting to get a bit desperate, so I hope somebody can help ...
My System
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ASUS P5B
Conroe E6600
2 GB RAM
Pulsar II
Scope SRB 15 DSP (2nd hand, probably type I, but I'm not sure)
Scope v.4.5
Cubase SX3
What I would like to do
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To use VDAT to record both live and sequenced tracks.
How I have configured things
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SFP is master clock at 48 KHz. Cubase is slave.
VRC-S controls VDAT. VRC-S clock output goes into ASIO clock, and cubase is configured to use ASIO sync. I use 30 fps.
What works well
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No problem recording live tracks (e.g. electric guitar), for which cubase is not involved.
No problem recording sequenced tracks in cubase which drive a VST instrument and which are sent via ASIO to SFP.
What doesn't work
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Sequenced tracks in cubase sent via MIDI to SFP in order to drive a SFP instrument get out of synch.
This happens either using the Sequencer MIDI Source/Dest modules or the HW MIDI ports (i.e. Cubase -> USB MIDI interface -> MIDI Cable -> Scope MIDI Port). I have also tried with and without the "system timestamp" option in cubase, but it made no difference.
As a test, I sequenced a couple of measures in cubase, sent them to SFP and back into cubase, where I recorded what was received through the loop. The result is that the MIDI notes received are randomly displaced with respect to the original ones: some are right, some are ahead, some behind, some get shorter...
Now, if I make cubase to use internal sync, the test is successful. That is, the notes recorded through the loop are identical to the original ones.
So, the problem seems to be related to cubase synching to SFP, but it only affects to MIDI. As I pointed out above, sequenced notes driving a VST instrument do not get affected by this problem.