UCD9222: No connection via PMBus

Hi all,

we have a problem with UCD9222. We designed a DSP board with nearly identical
UCD9222/7242 architecture as on the EVM6678, and now we want to program
the UCD9222 via TI's USB adapter. This works fine on five of our boards, so I think
our circuitry should be ok.
On one board, however, the Fusion software fails to connect the UCD9222.
Maybe the UCD is damaged, but before replacing I want to ensure there is
no other bug.
So my question: what is minimally required to access the UCD registers via
PMBus? We have checked that V33DIO=V33A=3.3V available, AGND2=AGND3=DGND=0V,
/reset=1 and en1=en2=1. Are there more requirements to access the registers
via PMBus? Do you have any further suggestions where to start searching?

We have only a small number of prototype boards, so it would be fine to get
them all working.

Thanks in advance for your help.

Regards,
Marcus