You should have a look at the Traditional NI-DAQ Function Reference Help, and in particular the DIG_DB_Config and DIG_DB_Transfer functions.
At the bottom of teh DIG_DB_Transfer function help it reads,
Using This Function
If you have set partialTransfer for an output group, the ptsTfr field allows Traditional NI-DAQ to make transfers of less than half the buffer size to an output buffer. This is useful when Traditional NI-DAQ must output a long stream of data and the amount of data is not evenly divisible by half the buffer size. If ptsTfr is equal to half the buffer size, the transfer is identical to a transfer without partialTransfer set. If ptsTfr is less than half the buffer size, however, Traditional NI-DAQ makes the transfer to the circular output buffer and alters the DMA reprogramming information so the digital output operation halts after the new data is output.
Refer to Chapter 5, NI-DAQ Double Buffering , of the Traditional NI-DAQ User Manual for PC Compatibles for an explanation of double buffering and possible error and warning conditions.
Best of Luck,
Jared A