Many thanks for this explanation.
Good to know that a delay can resolve the issue. My only problem is, that my application is testing thousands of devices a day in production so of course I have to avoid any unnecessary delay. Further I am executing several digital patterns (so performing several writes to the PCI6534) for every single device, so if I'd have to add a delay for each of them that would add up significant test time.
I don't actually need the full 20MHz or even 10MHz performance BUT I need something which runs stable for a production environment. So if the problem depends on the overall load of the PC production might run into the problem any time.
As I am trying to run a finite pattern I would expect the output of data only to start once the tranfer of the whole pattern to the card has been finished. As this apparently is not the case for the code I use, isn't there any way to force such a behaviour (e.g. using properties)?
So I will try to upgrade to DAQmx 8.0 and try to insert some minimal delays.
many thanks for your support