Chapter 2
Hardware Overview
2-10
ni.com
Trigger Holdoff
Trigger holdoff 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 holdoff
time. After completing its current record, the digitizer records no data and
accepts no triggers until the holdoff counter runs out. When the counter
runs out, the next record begins and a trigger may be accepted. Setting a
holdoff time shorter than posttrigger acquisition time has no effect, as
triggers are always rejected during an acquisition.
Note
Time to acquire posttrigger samples is calculated by the following formula:
(
posttrigger
samples
)/(
sample
rate
).
Trigger holdoff is provided in hardware using a 32-bit counter clocked by a
25 MHz internal timebase. With this configuration, you can select
a hardware holdoff value of 5
µ
s to 171.79 s in increments of 40 ns. For
more information on trigger holdoff, refer to Chapter 3,
Common Functions
and Examples
, of the
NI-SCOPE Software User Manual
.
Memory
The NI 5911 allocates at least 4 kB of onboard memory for each record in
a single multi-record 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 conventional mode samples
or 1,000 32-bit flexible resolution mode samples. Software allows you to
specify buffers of less than these minimum buffer sizes because only the
specified number of points is transferred from onboard memory 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. The maximum number of records in a single multi-record
acquisition is equal to the size of the memory module divided by 4 kB.
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.
