M-Series read + Write, threading or async , performance issues

Question

I have a slightly unusual issue: Using 2 USB-6251 and 2 USB-6015 within the same application.
1 of the USB-6251s reads data on a 50 msec look (Driven by an async timer).
Depending on the input the code decides to output a 40 Hz, 300 usec  waveform  of varying amplitude  - the output goes one until there is  a user event that might make it stop.
Now , here is the problem.  I have implemented the output function using a sepereate high priority thread and the delay( delay_in_sec) command.
I find that while the data acquisition timer is NOT running the frequency of the output is very close to 40 Hz. When the data acquisition IS running , the frequency of the output varies considerably - most likely reflecting USB delays.
I was able to see the output 40Hz, 300 usec waveform using a seperate app using a single async timer w/o any data acq and the timing was immaculate.
My questions are:
1. Could I use 2 different async timers in the same app - I tried to do this and the output was still wacky when I turned data acquisition on
2. Is there any way outside the delay() command that could improve the timing accuracy of the analog output thread?
3. I measured the time to create a 300 usec pulse of a certain amplitude, write it to the USB-6251 and start the task and it was ~ 13 msec. This is way too slow for 300 samples at 1 MHz - is it a USB bus limitation, or I should look at my design to see what is going on?
4. Any NI hardware that would allow me to handle 4 analog outputs more gracefuly? (FYI, 2 of my outputs are at 1 MHz and 2 of them at 50 Hz).
Thanks
AP

Answer 1

THanks for taking the time to look at my message.
I am using 4 DAQs. First one, USB-6251 is used to collect data (4 analog inputs @ 20Hz) and also occasionally provide analog output @ 1 MHz.
Second DAQ is a USB-6251 occasionally provide analog output @ 1 MHz.
Third and Fourth DAQ  areUSB-6015 occasionally providing analog output @ 50 Hz.
All devices are connected via USB to a Dell Dual Core notebook.
THis is how I create & configure the analog output channels - using 1 task per device.
                    DAQmxErrChk(DAQmxCreateAOVoltageChan(TaskAnalogX,channelstring , "Ramp 1",0 , 10, DAQmx_Val_Volts, ""));
                    DAQmxErrChk(DAQmxCfgSampClkTiming(TaskAnalogX, "",1MHz OR 50Hz, DAQmx_Val_Rising, DAQmx_Val_FiniteSamps     , SECONDS*1MHz));
                    DAQmxErrChk(DAQmxWriteAnalogF64 (TaskAnalog1, ( int32 )SECONDS*1MHz OR 50Hz ), 0, DAQmx_Val_WaitInfinitely, DAQmx_Val_GroupByChannel , PHAOarr, &sampsPerChanWritten, NULL));
THe design/architecture is pretty complex but on a very high level whenever I need to deliver analog output I configure the output waveforms and then write it to the DAQs and then use some timers/logic to deliver the waveform from the DAQ as needed.
Now the challenge has been that in one specific case I need to deliver analog output of uknown duration (whereas everything else is deterministic). In that case i  created a seperate thread and write each 300 usec pulse individually to the analog output (using USB 6251)using the following tASK:
                    DAQmxErrChk(DAQmxCreateTask("Tonic Output Task", &TaskTonic));  
                    DAQmxErrChk(DAQmxCreateAOVoltageChan(TaskTonic, channelstring , "PulseToPulse",0 , 10, DAQmx_Val_Volts, ""));    
                    DAQmxErrChk(DAQmxCfgSampClkTiming(TaskTonic, "",1.0E+06, DAQmx_Val_Rising, DAQmx_Val_FiniteSamps , 301));
As described in my first message this is very sensitive to timing and it takes 13 msec to stop the task , write the array to the DAQcard and then start the task again.
On the hardware timing recommendation. I tried to configure the DAQ  using the DAQmx_Val_HWTimedSinglePoint samplemode but it seems not to be supported by the hardware I am using.  Is there a CVI example of using hardware timing that I can take a look at?
THanks
AP
Hi AP,
My first impression is that you would want to use hardware timing
rather than software timing.  Try using a DAQmx Timing command and
specify the rate there.  Aside from that, could you be more specific
about your application?  How do you have your hardware set up and which
devices are handling each part of the task?  How are you generating the
output?
Regards,
Joe S.