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Appendix C Configuration Aspects for Programmers
NetScan User’s Manual C-11
Pre-Trigger State
The Pre-Trigger state is defined as the period after NetScan has
been configured to collect an acquisition (via the Trigger
Configuration
T
command), and before the actual Trigger event
occurs. While the unit is in this state, the TRIGGER LED will
flash. As in any other acquisition state, the alarms and their
associated outputs will be updated at the fastest rate possible
under the current channel configuration.
It is during this state in which NetScan may be configured to
collect Pre-Trigger data. This may be accomplished by
specifying a non-zero value for the Pre-Trigger
count
parameter
of the Set Count
Y
command. The Pre-Trigger
count
value
coupled with the current channel configuration, however, cannot
exceed the total amount of available memory.
If the Pre-Trigger
count
is set to a non-zero value, NetScan will begin sampling Pre-Trigger data at the rate
specified by the current normal scan interval (via the Set Scan Interval
I
command) immediately after the
Trigger Configuration
T
command is issued. When the Trigger event occurs, the Pre-Trigger data portion of the
Trigger Block will contain the most recent Pre-Trigger scans. The number of Pre-Trigger scans collected will
depend upon when the Trigger event occurs. If the Trigger event occurs before the number of Pre-Trigger scans
collected at least equals the specified Pre-Trigger
count
, the number of Pre-Trigger scans will be the number of
scans collected to that point. Otherwise, the number of Pre-Trigger scans will be the number specified by the
Pre-Trigger
count
parameter of the Set Count
Y
command. If it is not desired to collect any Pre-Trigger data
while in the Pre-Trigger state, the Pre-Trigger
count
parameter of the Set Count
Y
command may be set to zero.
Pre-Trigger data may not be accessed while NetScan is in the Pre-Trigger state. The Pre-Trigger data for a
particular Trigger Block is not made available to be read until the specified Trigger event occurs.
There are two ways to determine if the unit is in the Pre-Trigger state:
1. Visually check the TRIGGER LED. If it is flashing then the unit is in the Pre-Trigger state.
2. Query the Status Byte (
STB
) register and check the Triggered Bit. If the Triggered Bit is not set and the
acquisition has been configured, then the unit is in the Pre-Trigger state. When the specified Trigger event
occurs, NetScan exits the Pre-Trigger state and enters the Post-Trigger state.
The Trigger Configuration
T
command has a synchronization parameter
sync
that allows you to specify if the
Trigger point is to be synchronized to the Pre-Trigger (normal) scan interval. If
sync
is set, the Trigger point
will occur on the next “tick” of the Pre-Trigger (normal) scan interval. Otherwise, the Trigger point will occur
at the point at which it is detected. For example, assume a
sync
setting with a Pre-Trigger of 10 scans and a
sample rate of 1 scan per minute. If a Trigger event takes place 20 seconds after a scan, a recorded Trigger
point will happen on the next scan, 40 seconds later.
Continuous, Gap-Free Acquisition with Two Timebases
NetScan can be configured to have different timebases for its Pre-Trigger and Post-Trigger states. The
system can be configured such that, when the Trigger condition is met, NetScan will switch from the
Pre-Trigger to the Post-Trigger state (changing its timebase). This feature, along with automatic
re-
arm
, can be used to collect continuous, gap-free data at two different timebases.
To ensure that the collected data is gap-free, the Pre-Trigger
count
must be set to -1 (via the Set Count
Y
command). If a value of 0 or higher is used as the Pre-Trigger
count
, only the number of Pre-
Trigger scans is placed in the buffer after the Trigger condition is satisfied. The following graphs show
an acquisition with the Pre-Trigger set to 100, and then an acquisition with the Pre-Trigger set to -1.
Trigger With Synchronization Flag Off
Trigger With Synchronization Flag On