National Instruments NI 5911 - Manual (Page 21)

Chapter 2 Hardware Overview
NI 5911 User Manual 2-10 ni.com
Trigger Hold-Off
Trigger hold-off is the minimum length of time (in seconds) from an
accepted trigger to the start of the next record. In other words, when a
trigger is 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
runs out, the next record begins and a trigger may be accepted. Setting a
hold-off time shorter than posttrigger acquisition time has no effect, as
triggers are always rejected during an acquisition.
Note Time to acquire posttrigger samples is (posttrigger samples)/(sample rate
(megahertz)).
Trigger hold-off is provided in hardware using a 32-bit counter clocked
by a 25 MHz internal timebase. With this configuration, you can select
a hardware hold-off value of 40 ns to 171.8 s in increments of 40 ns. For
more information regarding trigger hold-off, see the Common Trigger
Parameters section in Chapter 3, Common Functions and Examples, of
your NI-SCOPE Software User Manual.
Memory
The 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
flexible resolution 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 transferred into the memory of the host computer.
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.