User Guide

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-9172 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-9172 onboard FIFO before it is written to the C Series I/O

modules.