Pre-Trigger State; Continuous, Gap-Free Acquisition With Two Timebases - Omega Engineering OMB-NETSCAN 1500 User Manual

Appendix C

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 command), and before the actual Trigger event
T
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
of the Set Count
Y
coupled with the current channel configuration, however, cannot
exceed the total amount of available memory.
If the Pre-Trigger
specified by the current normal scan interval (via the Set Scan Interval
Trigger Configuration
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
scans collected to that point. Otherwise, the number of Pre-Trigger scans will be the number specified by the
Pre-Trigger
count
while in the Pre-Trigger state, the Pre-Trigger
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 (
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
Trigger point is to be synchronized to the Pre-Trigger (normal) scan interval. If
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
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
, can be used to collect continuous, gap-free data at two different timebases.
arm
To ensure that the collected data is gap-free, the Pre-Trigger
command). If a value of 0 or higher is used as the Pre-Trigger
Y
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.
NetScan User's Manual
command. The Pre-Trigger
is set to a non-zero value, NetScan will begin sampling Pre-Trigger data at the rate
count
command is issued. When the Trigger event occurs, the Pre-Trigger data portion of the
T
parameter of the Set Count
) register and check the Triggered Bit. If the Triggered Bit is not set and the
STB
command has a synchronization parameter
T
Trigger With Synchronization Flag Off
parameter
count
value
count
Trigger With Synchronization Flag On
I
, the number of Pre-Trigger scans will be the number of
count
command. If it is not desired to collect any Pre-Trigger data
Y
parameter of the Set Count
count
setting with a Pre-Trigger of 10 scans and a
sync
must be set to -1 (via the Set Count
count
, only the number of Pre-
count
Configuration Aspects for Programmers
command) immediately after the
command may be set to zero.
Y
that allows you to specify if the
sync
is set, the Trigger point
sync
re-
C-11