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Logic Triggering
TDS 620A, 640A, & 644A User Manual
Define a Time Qualified Pattern Trigger
You can also time qualify a pattern logic trigger. That is, you specify a time
that the boolean logic function (AND, NAND, OR, or NOR) must be TRUE
(logic high). You also choose the type of time qualification (greater or less
than the time limit specified) as well as the time limit using the Trigger When
menu selection.
1. Press TRIGGER MENU ➞ Type (main) ➞ Logic (pop-up) ➞
Class (main) ➞ Pattern (pop-up) ➞ Trigger When (main) ➞ True for
less than or True for more than (side).
2. Use the knob and keypad to set the time in the side menu.
When you select True for less than and specify a time using the general
purpose knob, the input conditions you specify must drive the logic function
high (TRUE) for less than the time you specify. Conversely, True for more
than requires the boolean function to be TRUE for longer than the time you
specify.
Note the position of the trigger indicator in Figure 3-44. Triggering occurs at
the point the logic function you specify is determined to be true within the time
you specify. The digitizing oscilloscope determines the trigger point in the
following manner:
It waits for the logic condition to become true
It starts timing and waits for the logic function to become false
It compares the times and, if the time TRUE is longer (for True for more
than) or shorter (for True for less than), then it triggers a waveform
display
at the point the logic condition became false.
This time can be,
and usually is, different from the time set for True for more than or True
for less than.
In Figure 3-44, the delay between the vertical bar cursors is the time the logic
function is TRUE. Since this time is more (216 s) than that set in the True
for more than menu item (150 s), the oscilloscope issues the trigger at that
point, not at the point at which it has been true for 216 s.