Wireless communication is not deterministic so there is no guarantee that starting two tasks on two different 9191 devices will be synced in any way over a wifi connection. Also the devices' clocks will have some drift from each other also hindering syncing the measurments. However if you have an analog input module that supports a PFI line you can export a clock signal from one device to the other and then they can be synced via that clock signal on the PFI line. This is the only way to sync the measurements between two 9191 chassis.
Brian A
R & D Product Support Engineer | WSN/Network DAQ/Academic
National Instruments
