VTI Instruments EX1401, User Manual

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EX1401 Instrument Drivers 79
R
ETRIEVING
D
ATA
The Digitizer driver provides two ways to retrieve data from EX1401. By default, the acquired data
is stored in the instrument’s FIFO buffer. The data in the FIFO can then be read using the standard
Read method. An alternative option is to use data streaming. The streaming mechanism makes it
possible to transfer data with less latency and overhead than the standard FIFO Read method;
complete records will be pushed to the driver by the firmware as soon as they become available.
FIFO Read
Once the measurement is triggered and data becomes available in the instrument’s FIFO buffer, data
can be retrieved using the
Read method in the Measurement interface. The Read method returns the
specified number of data records from all enabled input channels in a channel order in the Channels
array. The NumFIFORecords property in Measurement interface returns the number of data records
currently available in the instrument’s FIFO buffer. This value decreases when the data is retrieved
by the host, and increases when new trigger events are processed. FIFO buffer overflow may happen
when the trigger events arrive faster than the host can retrieve data. See the
FIFO description in the
Data Acquisition section for more information.
Streaming
Streaming data is an alternative method for retrieving data from the EX1401. Unlike the FIFO read
function, instruments send new data records to the host PC as soon as it becomes available when
streaming data. The data is kept in the host memory buffer managed by Digitizer driver. The data
in this memory buffer is then retrieved to the user’s application through the
MemoryRead method.
The streamed data can be directly written into disk files or passed to a user’s callback method. In
order to use this streaming mechanism, it must be enabled by calling the
EnableStreaming method
in the StreamingData interface.
The key advantage of the streaming method over the FIFO Read method is data transfer speed and
latency.
User Application
M
e
m
o
ry
R
e
a
d
FIFO
Buffer
Host
Memory
Disk
Stream
Streaming
Stream
Files
Fi
le
Re
ad
Sampled
Data
FIFO
Rea
d
Host Disk
EX1401
F
IGURE
5-5: FIFO R
EAD AND
S
TREAMING