NI cDAQ-9178 - Analog Output Data Generation Methods

NI cDAQ-9178/9174 User Guide and Specifications 18 ni.com
For each analog output module, you can either:
• Assign all of the channels on the module to the hardware-timed task.
• Assign all of the channels on the module to one or more software-timed tasks.
On a single AO module, you cannot assign some channels to a hardware-timed task and other channels
(on the same module) to a software-timed task.
Analog Output Data Generation Methods
When performing an analog output operation, you either can perform software-timed or hardware-timed
generations. Hardware-timed generations must be buffered.
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 DAC conversion. 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, such as a constant
DC voltage.
The following considerations apply to software-timed generations:
• If any AO channel on a module is used in a hardware-timed (waveform) task, no channels on that
module can be used in a software-timed task.
• You can configure software-timed generation to simultaneously update.
• Only one simultaneous update task can run at a time.
• Simultaneous update is not restricted to 16 channels.
• A hardware-timed AO task and a simultaneous update AO task cannot run at the same time.
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 chassis 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 AO operations on the NI cDAQ-9178/9174 chassis must be buffered.
Buffered Analog 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 NI cDAQ-9178/9174 onboard FIFO before it is written to the
C Series I/O modules.
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.