Here is the update after couple days more attempt. I did a factory image recovery to not working Dell computer and reinstall the 2015 LV. There is no more question mark on PCI-6221 in the device manager, but error 200279 still happens in couple minutes. I even try to update the firmware and software in that Dell computer, but the problem still can be solved.
Here is what I found. there are 4 same Dell computers I'm working on. The labview in two of them were updated to LV2020 and then both of them have this issue. The one I used to report issue here belongs to them. I've reinstalled the windows and the NI LV2015 package, but it still doesn't solve this issue. As for the other two computers, I just installed NI LV2015 package recently and they are not updated ever, however they work without this issue. I'm not sure if, during the update, NI software updates some parameter in Dell motherboard information which can't be restored even I re-install the windows and NI software.
As not all of the computers in lab need the PCI DAQ to be used, I will have to give up more attempts now and still come to update if there is any progress later. Thanks for all the help here especially to Kevin and ZYong.
