Chapter 2
Using Your NI 653X
©
National Instruments Corporation
2-31
Start and Stop Trigger
When using a start and stop trigger, transfer starts upon receiving a trigger
on the start trigger line (ACK/STARTTRIG pin) and ends upon receiving
a trigger on the stop trigger line (STOPTRIG pin). A predetermined amount
of pretrigger and posttrigger data is saved in the buffer. If a stop trigger is
received before a start trigger, it is ignored. If the stop trigger arrives before
all the pretrigger data is acquired, NI-DAQ returns an error.
Figure 2-18.
Using a Start and Stop Trigger
Pattern-Matching Trigger
Instead of using an external signal on the start/stop trigger pins on the
I/O connector, you may start or stop (not both) an operation once a
user-specified digital pattern is matched.
Specify four parameters to set a pattern-matching trigger:
•
Whether it is a start or stop trigger
•
The data pattern to be detected/matched
•
The mask, which selects the bits of interest for pattern detection
Note
The mask for the pattern-matching trigger is the same as the one used for change
detection. In other words, input lines significant for the pattern-matching trigger are also
significant for change detection.
•
Polarity (whether to detect data that matches or mismatches the
specified pattern)
The NI 653
X
immediately detects any occurrence of a specific pattern as
the data arrives. When a match occurs, the NI 653
X
starts acquiring data.
For example, if you want to start an acquisition when the two least
significant bits of your data are 1 and 0, you would specify your trigger
parameters to match those in Figure 2-19.
REQ
STOPTRIG
ACK (STARTTRIG)
Pretrigger Data
Posttrigger Data