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Chapter 4 Digital Input/Output and PFI
Digital Output
To generate digital output, insert a digital output C Series I/O module in any slot on the cDAQ
chassis. The generation specifications, such as the number of channels, channel configuration,
update rate, and output range, are determined by the type of C Series I/O module used. For more
information, refer to the documentation included with your C Series I/O module(s).
On a single digital output C Series module, you can assign any number of channels to either a
hardware-timed task or a software-timed (single-point) task. However, you cannot assign some
channels to a hardware-timed task and other channels (on the same module) to a software-timed
task. Furthermore, you cannot run multiple tasks of any type on a single digital output module.
(NI cDAQ-9174/9178) With parallel digital output modules (formerly known as hardware-timed
modules), you can do multiple software-timed tasks on a single module, as well as mix
hardware-timed and software-timed digital output tasks on a single module. On serial digital
output modules (formerly known as static digital output modules), you cannot mix
hardware-timed and software-timed tasks, but you can run multiple software-timed tasks.
Digital Output Data Generation Methods
When performing a digital 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 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 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 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 DO operations on the cDAQ chassis must be buffered.
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