Chapter 2 Hardware Overview
© National Instruments Corporation 2-13 NI 5911 User Manual
Memory
The NI 5911 stores samples in onboard memory before transferring the
samples to the host computer. The minimum size for abuffer in the onboard
memory is approximately 4,000 8-bit oscilloscope mode samples or 1,000
32-bit decimation mode samples. Software allows you to specify buffers of
less than these minimum sizes. However, the minimum number of points
are still acquired into onboard memory, but only the specified number of
points are retrieved into the host computer’s memory.
The total number of samples that can be stored depends on the size of the
acquisition memory module installed on the NI 5911 and the size of each
acquired sample.
Triggering and Memory Usage
During the acquisition, samples are stored in a circular buffer that is
continually rewritten until a trigger is received. After the trigger is
received, the NI 5911 continues to acquire posttrigger samples if you have
specified a posttrigger sample count. The acquired samples are placed into
onboard memory. The number of posttrigger or pretrigger samples is only
limited by the amount of onboard memory.
Multiple Record Acquisitions
After the trigger has been received and the posttrigger samples have been
stored, the NI 5911 can be configured to begin another acquisition that is
stored in another memory record on the board. This is a multiple record
acquisition. To perform multiple record acquisitions, configure theNI 5911
to the number of records you wantto acquire before starting the acquisition.
The NI 5911 acquires an additional record each time a trigger is accepted
until all the requested records are stored in memory. This process does not
require software intervention after the initial setup has been completed.
Between each record, there is a dead time of approximately 5 µs during
which the trigger is not accepted. During this time, the memory controller
is setting up for the next record. There may also be additional dead time
while the minimum number of pretrigger samples are being acquired.
Figure 2-9 shows a timing diagram of a multiple record acquisition.
