Hi mlloyd,
The procedure for stopping and starting an acquisition or generation session on an SMC-based digital board (656x, 655x, 654x) typically will involve a couple of software calls. Let's say that you have setup a continuous generation by setting the generation repeat mode to continuous. Once the generation is running, you will need to call the niHSDIO Abort function to stop the generation, and you will need to call the niHSDIO Initiate function to start the generation after you change the sample clock setting (discussed below). On the other hand, if you are performing a finite generation or acquisition, the session will stop automatically after the specified number of samples have been generated or acquired. In this case, you will not need to call the Abort function since the session is already stopped.
I want to make a distinction between starting and stopping the data versus starting and stopping the session. Using a pause trigger, you can actually stop the data, but the session itself is still running. It is the session that must be stopped before you can change your settings. Since this is done with software calls, I cannot say how many clock cycles will pass. The sequence of calls should take on the order of a few milliseconds, but that will depend on the speed of your system.
The setting that I think you will need in this case is whether to export the sample clock from the 6561. We have a shipping example that shows how to export the sample clock, but it is a simple matter of calling the niHSDIO Export Signal function. You specify that you want to export the sample clock, then choose which output pin. To stop exporting the clock, you would again call the Export Signal function, telling it to not export the sample clock. Internally, the sample clock is running regardless of whether it is exported. That's a long way of saying "yes", you can continue to send data while the sample clock is not exported.
Hope that helps,
Allen
