Chapter2 HardwareOverview
©NationalInstruments Corporation 2-13 NI5911 User Manual
MemoryThe NI 5911 stores samples in onboard memory before transferring the
samplesto the host computer. The minimum size for a buffer inthe onboard
memoryis approximat ely 4,000 8-bit oscilloscope mode samples or 1,000
32-bitdecimation mode samples. Software allows you t o specify buffers of
lesst han 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
acquisitionmemory module installed on the NI 5911 and the size of each
acquired sample.
Triggeringand Memor y 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
specifieda 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 AcquisitionsAfterthe trigger has been received and the posttrigger samples have been
stored,t heN I 5911 can be configured to begin another acquisition that is
stored in another memory record on the board. This is a multiple record
acquisition.Toperform multiple record acquisitions, configure the NI 5911
tothe number of records you want to acquire before starting the acquisition.
The NI 5911acquires an additional record each time a trigger is accepted
untilall the requested records are stored in memory. This process does not
requiresoftware i ntervention after the initial setup has been completed.
Between each record,there is a dead time of approximately 5 µs during
whichthe trigger i s 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 showsa timing di agram of a multiple record acquisition.