
Chapter2 HardwareOverview
NI5911 User Manual 2-10 ni.com
TriggerHold-Off
Trigger hold-off is the minimum length of time (in seconds) from an
accepted trigger to thestart of the next record. In other words, when a
triggeris accepted, the trigger counter is loaded with the desired hold-off
time.After completing its current record, the digitizer records no data and
accepts no triggers until the hold-off counter runs out. When the counter
runsout, the next record begins and a trigger may be accepted. Setting a
hold-off time shorter than posttrigger acquisition time has no effect, as
triggersare always rejected during an acquisition.
Note Timeto acquire posttrigger samples is (postt rigger samples)/(samplerate
(megahertz)).
Triggerhold-off is provided in hardware using a 32-bit counter clocked
bya 25 MHz internal tim ebase. Withthis configuration, you can select
ah ardwareho ld-offvalue of 40 ns to 171.8 s in increments of 40 ns. For
more information regardingt rigger hold-off,see the Common Trigger
Parameterssect ion in Chapter 3, Common Functions and Examples,of
your NI-SCOPE Software User Manual.
MemoryThe NI 5911 allocates at least 4 kB of onboard memory for every
acquisition. Samples are stored in this buffer before transfer to the host
computer. Thus the minimum size for a buffer in the onboard memory is
approximately 4,000 8-bit oscilloscope mode samples or 1,000 32-bit
flexibleresolution mode samples. Software allows you to specify buffers
ofless than these minimum sizes. However, the minimum number of points
are still acquired into onboard memory, but only the specified number of
points are transferred into the memory of the host computer.
The total number of samples that can be stored depends on the size of the
acquisitionmemory module installed on the NI 5911 and the s ize ofeach
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 5911continues to acquire posttrigger samples if you have
specifieda posttrigger sample count. The acquired samples are placedinto
onboard memory. The number of posttrigger or pretrigger samples is only
limited by the amount of onboard memory.