Selects the source interface to use as timing reference for
output alignment. The source can be any other interface on
the same board except itself. The value is the relative
interface number on the board, where the first interface on
the board is number 1, the second is number 2 and so on.
This object is applicable only if the interface supports
reference locking, i.e. if the `ref' bit is set in
itsIfCapabilities.