Tektronix TDS3012C User Manual page 145

Reference
Logic Triggers
Edge triggering can trigger on most signals, and is the default trigger
type. Edge triggering sets the oscilloscope to trigger (acquire signal
data) when a signal meets a specified signal slope and a single
voltage-threshold condition.
However, there are times when you need to trigger the oscilloscope
on a more complex signal, or when two signals meet a condition to
troubleshoot a problem. These problems include a pulse that is too
narrow or wide, and situations in which one signal is true when a second
signal transitions from low to high.
Logic and Pulse triggering lets you further qualify the trigger
conditions by adding parameters such as pulse width, delta time, logical
comparisons of two signals, and dual threshold levels.
Thresholds. Both Pulse and Logic types trigger the oscilloscope when
one or two signals are logically true. To determine whether a signal
is true or false, you must set a signal reference point that determines
whether a signal is in one of two states. You set this reference point by
specifying a threshold voltage level for each trigger signal. Crossing the
threshold level toggles the state value of that signal.
TDS3000C Series Oscilloscope User Manual 117