An open architecture tester allows a third party to develop its own instrument. Such a tester must be open in the sense that it needs to be able to integrate this instrument with minimal or no impact on the overall performance and cost. It is therefore fundamental that the data flow and synchronization among all the instruments in an open architecture tester be efficient. One of these two key topics, the data flow, is discussed in this paper. Some possible solutions that optimize the data flow are presented and one among them is given in full detail. Synchronization is a very wide topic whose discussion could be the subject for a separate paper.