How do I configure a counter to generate pulses using DAQmx?

Question

How do I configure a counter to generate pulses using DAQmx?
Is says in the DAQmx C reference help
"CtrnInternalOutput—The signal at this internal terminal is where the pulsed or toggled output of the counter appears. The output of a counter pulses or toggles when the counter reaches terminal count. When counting down, the counter reaches terminal count when the count reaches zero. When counting up, the counter reaches terminal count when the counter rolls over. To configure the counter to toggle or generate pulses, use the Export Signal function/VI with Counter Output Event as the signal name."
I've tried this but can't get it to work, I may have the parameters wrong or something. The DAQmxExportSignal() function is very unintuitive to me. Here is my counter config code...
int ret = 0;
ret = DAQmxCreateTask("",&task_);
errorMsg(ret);
if (ret != 0)
throw ret;
// Configure the counter
ret = DAQmxCreateCOPulseChanTicks( task_, "Dev1/ctr0", "", "/Dev1/PFI8", DAQmx_Val_Low, 0, divider, divider );
errorMsg(ret);
ret = DAQmxCfgImplicitTiming( task_, DAQmx_Val_ContSamps, 1000 );
errorMsg(ret);
// Change to pulse mode
ret = DAQmxExportSignal( task_, DAQmx_Val_CounterOutputEvent, "/Dev1/Ctr0InternalOutput" );
errorMsg(ret);
// Start the counter
ret = DAQmxStartTask(task_);
errorMsg(ret);
The above code works fine, in toggle mode, if I just comment out the DAQmxExportSignal() part.
HELP!!!
Stefan

Answer 1

The problem is due to the DAQmxExportSignal call.  By default, when you create a counter output pulse train task, the driver will automatically set the output of the counter to toggle when the count for each high ticks and low ticks expire.  The driver also automatically routes the signal present at Ctr0InternalOutput terminal (the output of the counter internal to the device) to the Ctr0Out terminal (the I/O pin available externally).  Using the line
 ret = DAQmxExportSignal( task_, DAQmx_Val_CounterOutputEvent, "/Dev1/Ctr0InternalOutput" );
will tell the driver to tristate the Ctr0Out terminal and not output the signal to the external I/O pin.  Using the DAQmxExportSignal function is generally only useful if you want to route the signal to some other terminal internal to the board without having the output show up on the external connector, route the signal to some other external pin other than the default pin, or if you want to have the output show up at multiple locations. 
If you want to change the output behavior from toggle to pulse, you need to use the DAQmxSetExportedCtrOutEventOutputBehavior function.  However, this is generally only useful if all you care about are edges (not the duty cycle of the pulse train), and you want to generate higher frequency signals.  For example, with a 20 MHz timebase as the source of your counter, you can only generate a 5 MHz pulse train by default.  This is because the minimum value for the low and high ticks parameter is 2 (20 MHz / 4 = 5 MHz).  By changing the output behavior form toggle to pulse, you can generate a pulse train at 10 MHz since the output is now pulsing instead of toggling.  However, the width of each pulse is not programmable so you will no longer have a 50% duty cycle signal.  I don't remember exactly what the width of each pulse is, but I believe it's in the neighborhood of 50 - 100 nanoseconds in width.  I hope this information helps.