Use bit0 (or ext SMB if XGO and TGO) to trigger start. The rising edge of this signal should be coincident with the data clock rising edge of the first sample of interest. If XSOE is enabled, the output signal will go high on the rising edge of the 2nd sample. To slave another channel or card to this signal, make sure to add both SGO and TGO to tell the slave that the enable signal is one sample late.