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Chapter 6
Digital I/O
The voltage input and output levels and the current drive levels of the DIO lines are listed in the
device specifications.
Digital Input Data Acquisition Methods
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 and are typically used for reading a single sample
of data.
Each of the X Series 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 (di/SampleClock) controls the rate
of the acquisition. This signal can be generated internally on your device 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.
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Hardware-timed acquisitions can use hardware triggering.
Hardware-timed operations can be buffered or hardware-timed single point. A buffer is a
temporary storage in computer memory for to-be-transferred samples.
•
Buffered—Data is moved from the DAQ device's onboard FIFO memory to a PC buffer
using DMA before it is transferred to application memory. Buffered acquisitions typically
allow for much faster transfer rates than non-buffered acquisitions because data is moved
in large blocks, rather than one point at a time.
One property of buffered I/O operations is the sample mode. The sample mode can be either
finite or continuous:
–
Finite sample mode acquisition refers to the acquisition of 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.
6-2 | ni.com
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