Guralp Systems CMG-6TD User Manual
Page 58
Configuration with Scream!
5.1.3.3 External triggering
When a digitiser or digital sensor triggers, it sends the trigger itself to
connected devices, as well as any extra data that it has been configured to
record. You can configure other digitisers to respond to this signal by
triggering in turn. This is an option which you can specify at the time of
manufacture.
As an example, to instruct a stand-alone digitiser with digital inputs to
respond to triggers generated by an attached 6TD:
1. Open the Configuration window for the 6TD and tick the Enable
External Trigger Output check-box to make it send triggers to connected
devices.
2. Click
to send the new configuration to the 6TD.
3. Open the Configuration window for the digitiser, and tick the Enable
External Trigger Input check-box to make it listen for triggers coming
from the 6TD. Check (in its Output control configuration) that it is
configured to record data from attached analogue instruments when it
receives a trigger.
4. Click
to send the new configuration to the digitiser,
If a 6TD (or digitiser) has both Enable External Trigger Output and Enable
External Trigger Input selected, it will record data when it receives an
external trigger as if it had triggered itself but it will not propagate that trigger
to other digitisers. It will only send a trigger message if its own triggering
criteria are satisfied.
5.1.3.4 Pre-trigger and post-trigger recording
In order to capture all of a seismic event, it is often useful to be able to record
data immediately preceding the trigger. Güralp digitisers have an internal
buffer of some seconds which allows these data to be added to the triggered
stream. Pre-trigger data is particularly useful for emergent-type signals, where
the system does not trigger until one phase after the first arrival. In addition,
to ensure that the coda of each event is included, some seconds of data are
recorded after the system de-triggers.
The two boxes at bottom right of the Triggering pane allow the user to set the
pre-trigger and post-trigger data intervals, in seconds. These values determine
the minimum length of time during which data will be saved before the
trigger condition occurs, and after it has lapsed. Regardless of the intervals
chosen, the data in the triggered streams will begin on a whole second.
5.1.4 Mux Channels
The CD24 digitiser, as integrated into the 6TD, provides a range of slow-rate
auxiliary channels for reporting the system's state of health and other
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Issue F - February 2014