Timed sequence doesn't work right with external timing source

[george seifert]

Mayb I should start up front and tell you what I want to accomplish and then any brave souls can read further and suggest why the attached VI doesn't work. I have a SMU (PXI 4130) that takes differing amounts of time to set a current depending on if the setting spans a range boundary. I want to be able to make that SMU setting and then wait a specific amount of time (sometimes 1 msec). If the SMU takes a little longer to return then I want that extra time to come out of the wait time. To do this I figured I would need an external time source to control a timed sequence to give better than the normal 1 msec resolution. So now onto the problems I'm having.

I've got two problems here that I would really appreciate some insight on. The attached VI is the subject of both problems (as is the VI is worthless, but it illustrates the problem). Note that you need some type of DAQ board with a counter on it to run it.

Download File:post-2786-1213281601.vi

I can't figure out why this timed sequence doesn't work with an external timing source. It works mostly as expected (see next problem) with the internal source, but fails miserably with the external source. Run the VI as is to get a baseline. Then connect the DAQmx Create Timing Source VI to the Source Name input of the timed structure. It seems like it should run exactly the same both ways.

Problem 2 (run with the internal timing source). The VI should take the same amount of time to execute regardless of the input to the Wait VI (within reason of course). That's actually the main impetus for this whole exercise. But according to the Actual End indicator it runs faster with a longer wait time.

Thanks for any input. I've been beating my head on this for a few days and my head's getting sore.

George