© National Instruments
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3-9
Figure 3-7.
Comparison of Step Response in Frequency Domain and Time Domain Modes
Data Throughput
The amount of data generated by a single PXIe-4480/4481 with all six channels acquiring at
20 MS/s can cause buffer overflow errors if the system is unable to transfer sample data from
the device quickly enough. If this occurs, NI-DAQmx software will stop the acquisition and
return error -200361, “Onboard device memory overflow…” Buffer overflow errors can be
prevented by using data compression and by ensuring the PXI Express system has sufficient data
bandwidth.
Data Compression
In normal operation, the PXIe-4480/4481 sends the 24-bit sample data to the host system using
32-bit data words. This means that an additional byte of unused data is transferred for every
sample. To improve efficiency, the PXIe-4480/4481 supports packing the sample data so that
this unused byte is eliminated. This can be controlled by setting NI-DAQmx Channel Property
Node (
Analog Input»General Properties»Advanced»Data Transfer and Memory»
Compression»Raw Data Compression Type
) to use lossless packing. The use of lossless
packing is required when acquiring 20 MS/s on all six channels continuously, and it is
recommended for other configurations to maximize efficiency of system bandwidth resources.
There is, however, a caveat to using lossless packing. Lossless packing requires the total number
of samples to be transferred in a finite acquisition to be a multiple of four. The total number of
samples to be transferred is equal to the number of acquired samples multiplied by the number
of channels used.
Volt
s
11
–1
–0
1
4
5
6
9
10
Time (µ
s
)
2
8
12
1
3
14
15
16
20
21
22
2
3
27
3
2
8
7
17
1
8
19
24
25
26
Fre
qu
ency Dom
a
in Mode – 1.25 M
S
/
s
Time Dom
a
in Mode – 20 M
S
/
s