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Digital Output Data Generation Methods
When performing a digital output operation, you either can perform software-timed
generations or hardware-timed generations.
Software-Timed Generations
With a software-timed generation, software controls the rate at which data is generated.
Software sends a separate command to the hardware to initiate each digital generation. In NI-
DAQmx, software-timed generations are referred to as on-demand timing. Software-timed
generations are also referred to as immediate or static operations. They are typically used for
writing out a single value.
For software-timed generations, if any DO channel on a serial digital module is used in a
hardware-timed task, no channels on that module can be used in a software-timed task.
Hardware-Timed Generations
With a hardware-timed generation, a digital hardware signal controls the rate of the generation.
This signal can be generated internally on the controller or provided externally.
Hardware-timed generations 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 Single Point (HWTSP) Mode
In HWTSP mode, samples are acquired or generated continuously using hardware timing and
no buffer. You must use the sample clock or change detection timing types. No other timing
types are supported.
Use HWTSP mode if you need to know if a loop executes in a given amount of time, such as
in a control application. Because there is no buffer, if you use HWTSP mode, ensure that reads
or writes execute fast enough to keep up with hardware timing. If a read or write executes late,
it returns a warning.
Buffered Digital Output
A buffer is a temporary storage in computer memory for generated samples. In a buffered
generation, data is moved from a host buffer to the cRIO controller onboard FIFO before it is
written to the C Series module(s).
One property of buffered I/O operations is sample mode. The sample mode can be either finite
or continuous:
•
Finite—Finite sample mode generation refers to the generation of a specific,
predetermined number of data samples. After the specified number of samples is written
out, the generation stops.
•
Continuous—Continuous generation refers to the generation of an unspecified number of
samples. Instead of generating a set number of data samples and stopping, a continuous
cRIO-904x User Manual | © National Instruments | 67
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