When performing digital input measurements, you either can perform software-timed or hardware-timed acquisitions.

Software-Timed Acquisitions

With a software-timed acquisition, software controls the rate of the acquisition.

Software sends a separate command to the hardware to initiate each acquisition. In NI-DAQmx, software-timed acquisitions are referred to as having on-demand timing. Software-timed acquisitions are also referred to as immediate or static acquisitions. They are typically used for reading a single sample of data.

Each of the USB-6453 DIO lines can be used as a static DI or DO line. You can use static DIO lines to monitor or control digital signals. Each DIO can be individually configured as a digital input (DI) or digital output (DO).

All samples of static DI lines and updates of static DO lines are software-timed.

Hardware-Timed Acquisitions

With hardware-timed acquisitions, a digital hardware signal controls the rate of the acquisition.

This signal can be generated internally on the USB-6453 or provided externally.

Hardware-timed acquisitions have several advantages over software-timed acquisitions.

  • The time between samples can be much shorter.
  • The timing between samples is deterministic.
  • Hardware-timed acquisitions can use hardware triggering.

Hardware-timed operations transfer data to the USB-6453 more efficiently by transferring a larger block of data rather than one point at a time, allowing for much higher sample rates. The sample mode can be either finite or continuous.

  • Finite sample mode—Acquires a specific, predetermined number of data samples. Once the specified number of samples has been read in, the acquisition stops. If you use a reference trigger, you must use finite sample mode.
  • Continuous sample mode—Acquires an unspecified number of samples. Instead of acquiring a set number of data samples and stopping, a continuous acquisition continues until you stop the operation. Continuous acquisition is also referred to as double-buffered or circular-buffered acquisition.

    • If data cannot be transferred across the bus fast enough, the FIFO becomes full. New acquisitions overwrites data in the FIFO before it can be transferred to host memory, which causes the USB-6453 to generate an error. With continuous operations, if the user program does not read data out of the PC buffer fast enough to keep up with the data transfer, the buffer could reach an overflow condition, causing an error to be generated.