National Instruments 653 Series User Manual page 30

Chapter 2 Using Your NI 653X
To prevent a transient data value during line switching from falsely causing a match,
Tip
set a valid pattern for at least 60 ns to guarantee detection. In addition, keep glitches to less
than 20 ns to guarantee rejection.
NI 653X User Manual
ACK (STARTTRIG)
STOPTRIG
REQ
Figure 2-5. Using a Start and Stop Trigger
Pattern-Matching Trigger (Input Only)
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 or not matched.
Specify four parameters to set up 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 comparison
(0 for bits not of interest)
• The polarity (whether to trigger on data that matches or mismatches
the specified pattern)
For example, if you want to start 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 2-6.
Pattern to Detect
Mask
Polarity
Figure 2-6. Pattern-Matching Trigger Example
Pretrigger Data
X X X
0 0 0
Postive: Search for Match
2-10
Posttrigger Data
X X X 1 0
0 0 0 1 1
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