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Triggering
TDS 620A, 640A, & 644A User Manual
To Set to 50%
You can quickly obtain an edge or pulse trigger (except for the Runt class) by
pressing SET LEVEL TO 50%. The oscilloscope sets the trigger level to the
halfway point between the peaks of the trigger signal.
You can also set the level to 50% in the Trigger menu under the main menu
item Level if Edge or Pulse (except for Runt class) is selected.
Note that the MAIN LEVEL knob and menu items apply only to the main
trigger level. To modify the delayed trigger level, use the Level item in the
Delayed Trigger menu.
Force Trigger
By pressing the FORCE TRIG front panel button, you can force the oscillo-
scope to immediately start acquiring a waveform record even without a trigger
event. Forcing a trigger is useful when in normal trigger mode and the input
signal is not supplying a valid trigger. By pressing FORCE TRIG, you can
quickly confirm that there is a signal present for the oscilloscope to acquire.
Once that is established, you can determine how to trigger on it (press SET
LEVEL TO 50%, check trigger source setting, etc.).
The oscilloscope recognizes and acts upon FORCE TRIG even when you
press it before the end of pretrigger holdoff. However, the button has no effect
if the acquisition system is stopped.
Single Trigger
If your goal is to act on the next valid trigger event and then stop, press
SHIFT FORCE TRIG. Now you can initiate the single sequence of acquisi-
tions by pressing the RUN/STOP button.
To leave Single Trig mode, press SHIFT ACQUIRE MENU ➞ Stop Af-
ter (main) ➞ RUN/STOP Button Only (side).
See the description under “Stop After” on page 3-7 for further discussion of
single sequence acquisitions.
The digitizing oscilloscope has display readouts and status lights dedicated to
monitoring the trigger circuitry.
Trigger Status Lights
There are three status lights in the Trigger control area (Figure 3-76) indicat-
ing the state of the trigger circuitry. The lights are labeled TRIG’D, READY,
and ARM.
When TRIG’D is lighted, it means the digitizing oscilloscope has recog-
nized a valid trigger and is filling the posttrigger portion of the waveform.
Readouts