Source; Types - Tektronix TPS2000 Series User Manual

Understanding Oscilloscope Functions
When you push the RUN/STOP or SINGLE SEQ button to start an
acquisition, the oscilloscope goes through the following steps:
1. Acquires enough data to fill the portion of the waveform record
to the left of the trigger point. This is called the pretrigger.
2. Continues to acquire data while waiting for the trigger condition
to occur.
3. Detects the trigger condition.
4. Continues to acquire data until the waveform record is full.
5. Displays the newly-acquired waveform.
NOTE. For Edge and Pulse triggers, the oscilloscope counts the rate
at which trigger events occur to determine trigger frequency and
displays the frequency in the lower right corner of the screen.

Source

You can use the Trigger Source options to select the signal that the
oscilloscope uses as a trigger. The source can be any signal
connected to a channel BNC, or to the EXT TRIG BNC.

Types

The oscilloscope provides three types of triggers: Edge, Video, and
Pulse Width.
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TPS2000 Series Digital Oscilloscope User Manual