Do I have to use an external clock with a cDAQ-9188 and NI 9401 to read a linear encoder? I'm seeing error message that implies that but the example code doesn't show it.

We can set up a task that works on demand but when we go to continuous sampleing we see Error -200303, which claims an external sample clock source must be specified for this application.  However, some of the NI linear encoder example code doesn't show that (uses the cDAQ 80 Mhz clock) and the documentation didn't mention that.