HP 16534A User Reference page 72

The Trigger Menu
Level Field
If the trigger point voltage level is set above or below the waveform
amplitude, the trigger point cannot be found. If Auto-Trig is set to On, this
causes the waveform display to become unsynchronized and to "float" on the
display. If Auto-Trig is set to Off, the message "Waiting for trigger" is
displayed.
The trigger point voltage can be set, either by the autoscale function, or by a
voltage value set into the Level field. When you touch the Level field and
turn it light blue, you can change the value of the trigger level voltage by
turning the knob.
The trigger level can be set to any voltage value contained within the
waveform display window, in increments of 0.05% of full scale vertical
voltage range (V/Div × 4 divisions = full scale). For example, if full scale
voltage range were 400 mV, trigger level would be set in increments of 2 mV
(V/Div = 100 mV × 4 × 0.005 = 2 mV).
Trigger level values can also be entered with the pop-up keypad. The keypad
appears when you select the Level field when the field is white (already
selected once). You can enter any value, in 0.05% increments of full scale
vertical voltage range. Values entered that are not in this range will be
rounded to the nearest 0.05% increment.
Since the trigger level range is limited by the voltage values displayed in the
waveform window, the voltage window limits can be easily determined.
Turn the knob in both directions until the Level field reads minimum and
maximum voltage. These voltage values are the limits of the waveform
window. However, if the level is set at the edge of a window and offset for
that channel is changed, the trigger level will track that change, thereby
changing the window limits.
If the trigger source channel (except EXT) is displayed in the waveform area
of the screen, the trigger level marker will appear on the screen as a
horizontal dashed red line. The trigger level marker will move up and down
on the screen as the trigger level voltage is changed.
If the trigger source channel is not in the waveform area of the screen, the
trigger level marker will not be displayed.
If there are multiple occurrences of the trigger source waveform in the
waveform area of the screen, only the uppermost occurrence of the trigger
source waveform will display the trigger source marker.
The trigger level marker will only appear when the trigger menu is selected.
The default value for the Level field is 1.620 V (TTL preset value).
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