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Why Is the Analog Output Performance On the USB DAQPad-6015/6016 Slower Than Expected?

Hardware: Multifunction DAQ (MIO)>>Basic>>DAQPad-6015 for USB, Multifunction DAQ (MIO)>>Basic>>DAQPad-6016 for USB w/Screw Term

Problem: I am not able to achieve the analog output rates specified for the DAQPad-6015 or DAQPad-6016. What factors affect these specifications, and how can I improve my performance?

Solution: The achievable analog output rates for the USB DAQPads are relatively slow compared to their PCI counterparts. This is a result of 3 factors.
  1. These DAQPads do not have an analog output hardware FIFO. This means that only a single point of data can be sent to the hardware at a time. Analog output boards with hardware FIFO's can achieve much higher update rates because several samples can be sent to the device at a time.

  2. These DAQPads do not support DMA for analog output or any other tasks. For analog output, data can only be transferred via programmed I/O (software timed) or interrupts (hardware timed). Both methods are slower and require more overhead than transferring data via DMA.

  3. These DAQPads communicate over USB. USB communication is inherently slower than equivalent communication over a PCI bus.

Overall, you will be able to achieve a higher rate if you perform a software timed operation. While the update rate will not be as deterministic as a hardware timed operation, this lack of determinism helps make up for the slow communication. Attached to this KnowledgeBase are two VIs created in LabVIEW 7.1 that allows you to benchmark your update rate for both a hardware-timed and software-timed operation. When these VIs were run on a 3 GHz, Pentium 4 with 1 GB of RAM, the following values were measured:

Benchmarks:
Software-timed: 333 Samples/second
Hardware-timed: 75 Samples/second

Note: Benchmarks for both software-timed and hardware-timed operations are system dependent.

Related Links:
NI DAQPad-6015 (for USB): Product Page
NI DAQPad-6016 (for USB): Product Page

Attachments:


software timed.llbhardware timed.llb

Report Date: 04/04/2005
Last Updated: 03/24/2006
Document ID: 3K3BITB9

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