I am wondering why the signal that was traced on my oscilloscope started on a negative slope when the trigger slope = rising.

I externally triggered the oscilloscope by the TTL/CMOS output of a function generator and connected it to both the EXT INPUT connector and the CH2 signal input connector of the oscilloscope. My settings on the Trigger Menu are Source: Ext, Coupling: DC, Mode: Normal, Slope: Rising. Then I connected the output of the function generator to CH1 on the oscilloscope.

I displayed a 10kHz sine wave and the triggering pulse at the same time, making note that the function generator DUTY control is in the CAL position.

Can someone explain why the displayed sinusoid on channel 1 began at the negative-slope portion of its slope even though the trigger slope was set to rising? It remained the same whenever the setting changed to falling.

Anything will help!


Problem Statement

Our output

  • \$\begingroup\$ I'm having a little hard time understanding your question and visualizing how you're setting up your probes. Do you have any schematics of how you're setting up your probes and some screenshots of what you're seeing? (and perhaps your trigger settings as well on the oscope) Perhaps you may need to set the trigger level to be higher. \$\endgroup\$
    – user103380
    Commented Jan 29, 2019 at 20:03
  • \$\begingroup\$ "start" where? On the left edge of the screen, or where the trigger is set on the horizontal axis? Post a picture to get better help. \$\endgroup\$
    – Aaron
    Commented Jan 29, 2019 at 20:07
  • \$\begingroup\$ Start on horizontal axis. And the answer to the last question is there is no change in the signal when the setting is changed to falling. \$\endgroup\$ Commented Jan 29, 2019 at 20:13
  • 1
    \$\begingroup\$ Looks like it's triggering on the sinewave. \$\endgroup\$
    – Aaron
    Commented Jan 29, 2019 at 21:52
  • 1
    \$\begingroup\$ I'm with @Aaron...see that little downward-pointing arrow near the top of the screen? That arrow marks the time where the 'scope is triggered. Not the left edge. \$\endgroup\$
    – glen_geek
    Commented Jan 29, 2019 at 22:34

1 Answer 1


The screen photo helps explain what's going on:
TDS210 scope scree
Inside the red circle is a downward-pointing arrow. It marks the time (on the horizontal scale) where the 'scope triggers. The horizontal position of this arrow can be slid left or right with the front-panel horizontal position knob.

At the top of the screen appears text M Pos: 0.000s This tells you that the trigger point is exactly at the centre of the screen (equidistant from the display left edge and right edge). This display is very useful for measuring time spans: its number will change as the trigger control moves the downward-pointing arrow left or right. It displays a signed number of seconds, milliseconds, microseconds, nanoseconds between cursor and centre-of-screen. When the downward pointing arrow is at screen centre, its display is 0.000s.

The black T along with Trig'd tell you that the 'scope circuitry has successfully found a point on the waveform (of the proper slope) on which to trigger.

At the right edge of the screen is a left-pointing arrow. This arrow slides up and down, adjusted by the trigger level control. It controls the voltage level of the trigger point (as marked on the photo by the orange horizontal line). As this trigger point is moved up, the sine wave would shift left a little on the display. If the trigger voltage moves above its topmost peak, the 'scope becomes untriggered, and the nice stable display goes all squirrelly. The black T would also change, indicating that the display is untriggered.

Most digital 'scopes default to trigger at the centre of the screen. Unlike old analog 'scopes that must default to trigger at the screen's left edge.

Coordinating all these knobs, displays takes some time to master. Much thought has gone into making scope measurements versatile yet simple - two conflicting issues.


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