VTI Instruments EX1401, Руководство пользователя

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VTI Instruments Corp.
44 EX1401 Basic Operation
A
CQUIRING
D
ATA
In general, the acquisition and retrieval of data on the EX1401 are conducted with discrete
commands that are often separated in time to a large degree. The EX1401 utilizes a 256 MB FIFO
memory storage to buffer acquisition data prior to retrieval. This reading buffer is cleared upon
receipt of the trigger initialize command in preparation for reading storage, and begins storing new
data in a circular buffer immediately. Then, upon fulfillment of the programmed trigger model
conditions, the EX1401 marks one or more records in the circular buffer for user retrieval. It
continues scanning and storing the acquisition data until the trigger and arm count quantities are
reached or the acquisition is aborted. At that point, data acquisition ceases and the trigger model is
returned to the Idle state.
The amount of records that can be buffered within the memory is dependent on the number of
channels enabled and the requested record size. Specifically, the number of records that can be
buffered (Record_Count) is determined by the following formula:
Sample_Size = 4
Record_Bytes = Record_Size * Sample_Size * (Channel DIO_Reporting)
Record_Count = 268435456 / Record_Bytes
where: Channel_Count 0-16 (number of channels enabled)
Record_Size 1-65527 (number of samples per record)
DIO_Reporting 0 or 1 (1 – YES, 0 – NO)
Regardless of the RecordSize, each channel’s circular buffer is then truncated to make its total size
evenly divisible by 4096.
Record_Count sizes for some typical configurations are the following:
Channel_Count DIO_Reporting Record Size Record Count
1 0 1024 65536
0 1 1024 65536
8 0 1024 8192
16 0 1024 4096
16 0 4096 1024
16 0 1 4194304
16 1 1024 3852
16 1 4096 963
T
ABLE
3-6: E
XAMPLE
R
ECORD
C
OUNT
S
IZES
If the circular buffer fills to the point that it would overwrite a record marked for retrieval by a
trigger before it has been read out, the instrument will abort the acquisition and return to the Idle
state.
NOTE The reading buffer memory is volatile and is cleared upon an instrument reset or power cycle.
R
ETRIEVING
D
ATA
Acquisition data can be retrieved from the EX1401 with any of three methods. First, it can be
manually read out of the instrument’s FIFO sometime after it is measured. This method is the
simplest, but runs the risk of aborting the measurement early if the data is not retrieved before the
256MB circular buffer is filled. The data can also be streamed to one or more endpoints. Streaming
endpoints can include one instrument driver session and any number of attached USB storage
devices. When streaming to the instrument driver is enabled, the instrument will send binary
encoded data directly to the driver as soon as it becomes available. The instrument driver can then
either buffer the data in PC memory for retrieval at the user’s convenience, save the data directly to
hard disk, or call a user callback with the new data. When a USB storage device is enabled for