Hi mazhar,
The likely reason why you are able to run one DAQmx task at a time is because each cDAQ 9172 has only one analog input sample clock. This means that each chassis can only have one AI task going at one time because the sample clock is reserved by that task. Because you have three chassis, you have three sample clocks. Hence the reason why you can have three tasks running at once - one on each chassis.
However, you can still acquire data from multiple channels on multiple modules within the same chassis, you will just have to include those additional channels in the same task. When you are selecting the physical channels in your DAQmx task, just select multiple channels by selecting browse and ctrl + clicking each channel. This will cause each of those channels to be read in the same task. Then, change your DAQmx Read to the polymorphic instance that reads from multiple channels.
Another thing to consider is that the cDAQ 9172 is an older cDAQ chassis. The newer cDAQ-9178 has three AI sample clocks, meaning you can have three separate analog input tasks running at the same time. Check the cDAQ 9172 User Guide for more information.
Hopefully this helps,
Chris G
Applications Engineer
National Instruments
