Reference
Force Trigger
Push the Force Trig button to force an immediate trigger event, even in the
absence of a signal. This function is useful in these situations:
If you do not see a waveform on the screen when using Normal trigger mode,
push Force Trig to acquire the signal baseline to verify that it is on the screen.
After you push the Single Seq button to set up for a single shot acquisition,
you can push the Force Trig button to do a practice acquisition to verify the
control se
ttings.
TDS3000C Series Oscilloscope User Manual 91