Ok i found problem and dont know how to solve this. Now I think that there is nothing wrong with multiple tasks on NI USB 6229, but something is deffinitely wrong with multiple for loops in one VI. I made really simple program (and because i dont know how to exactly create task to NI USB 6229 i used DAQ assistant). So in this program i have two for loops. In frst for loop i read data from two serial ports and in second loop i try to write data into NI USB 6229 (daq assistant - 1 sample on demand for digital data). When i start this program this loop which read data on serial port run really fast but loop which send data to NI 6229 make one loop per second (or slower). So ok i tried stop only lopp for reading from serial port and the other loop accelerate at least 100x. This is not normal i think, have you any idea where is problem? And anyway why is sending data to NI USB still so slow even without loop for reading on serial port, the other loop stil makes only 100 loops per second. Picture of my program is attached.