Memory Streaming; Disk Streaming; Callback Streaming; Sampling Rates - VTI Instruments EX1401 User Manual

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Memory Streaming

Memory streaming is the default mode for these instruments. When the user enables Streaming
without any configuration, the data is streamed into a dynamically allocated buffer on the host PC
and then retrieved via a Read-like interface. This is the fastest streaming interface, since memory is
the fastest storage device on the PC. However, this mode is unsuited for long acquisitions because
it can grow beyond the memory limits of the device and start paging to disk. The internal buffer
grows as needed but does not shrink until streaming is disabled.
Additionally, this mode can be set to watch only select channels of interest. If the
MemoryChannelsList property is set, the data for only the selected channels will be streamed to
memory.
If another type of streaming is enabled while memory streaming is enabled, memory streaming is
disabled unless the MemoryChannelList property is set.

Disk Streaming

Disk streaming is intended for high performance applications and/or long acquisitions. When
enabled by setting a filename, the driver optimizes the data path by not converting the data from the
native VRT data format. The data is written to disk in the HDF5 data format. For more information
on HDF5 file formats and tools, visit the HDF Group's website: https://www.hdfgroup.org.
The Digitizer driver uses an HDF5 file hierarchy that is very similar to the layout used for the Data
Logger feature. See HDF5 File Hierarchy in Section 6 for more details.
The disk streaming file includes several HDF attributes that describe the instrument configuration
at the time that an acquisition was initiated. Additional user-defined attributes can be set by using
the SetCustomAttributes method. The GetCustomAttributes method allows the user to inspect the
attributes that are currently set. RemoveCustomAttribute and RemoveAllCustomAttributes can be
used to remove attributes that are no longer needed.
If Measurement.Initiate is called multiple times without changing the Filename property, the
existing file will be overwritten with the new acquisition data.
Disk streaming has the same limitations as the underlying filesystem. For example, a FAT32
filesystem cannot support files larger than 4GB. If disk streaming is used on a FAT32 filesystem
and the file grows too large, errors and data loss may occur. In addition, disk streaming throughput
may suffer when the underlying filesystem is very slow, such as a USB 2.0 external drive.

Callback Streaming

Callback streaming is enabled when the user registers one or more callback functions, which are
called with available data when it is ready. The user's code may then act on the data. To register a
callback, the user will have to override the COM abstract interface (when using the IVI driver) or
C++ pure-virtual callback class (when using the Linux driver), and then pass this class to the
RegisterStreamingCallback method. The data is returned in the same format as the MemoryRead
method, as can be determined from examining these classes.
Support for registering a callback to receive data only from certain channels, similar to using the
MemoryChannelList, is scheduled for a future driver release.
S R
AMPLING ATES
The properties in the Sampling interface configure the A/D converter and decimation filters to
specify the sampling rate and frequency span of the data to be acquired. Some properties in this
interface are interrelated. Changing one property value can affect the other.
84 EX1401 Instrument Drivers