The scope is on, the probe is on the signal, and the screen says it is waiting for a trigger. The trigger decides when the scope takes a picture, and it is waiting for a moment that is not coming. Here is what each setting means, the real case that sent us looking, and the settings for a serial line.
The scope samples all the time, but a screen of samples taken at a random moment shows a different slice of the signal every time. The trigger picks the moment: when the source channel crosses the level in the chosen direction, the slope (rising: upward through the level; falling: downward). Every capture lines up on that moment, so a repeating signal stands still on the screen.
| Sweep | What it does | Use it to |
|---|---|---|
| Auto | Waits briefly for a trigger, then captures anyway. There is always a trace, still when triggered, drifting when not. | find a signal and see roughly what it is |
| Normal | Captures only on a trigger and keeps the last capture on screen while it waits. | watch a repeating signal, or one that comes now and then |
| Single | Captures once, on the next trigger, then stops. | catch one burst, a power-up, a glitch |
Only Normal and Single can wait forever. On NovaOS (the trigger sheet's Sweep row has Auto and Normal; Single is its own key), when two seconds pass without a trigger in Normal or Single, a banner over the plot says so, for example "No trigger at 2.96 V on CH1", with a Level 50 % button, and the run status reads NO TRIGGER. In Auto the scope keeps capturing and marks the trace UNTRIGGERED. Before the first capture the screen says "Waiting for a trigger" and what is armed, such as "single shot armed on CH1". If the source channel is off, the banner says "Trigger source CH2 is off" and offers the first channel that is on.
The level must lie inside the signal's swing: a 0 to 3.3 V signal never crosses 4 V, and never crosses −1 V either. In the middle is safest: it is furthest from noise at either end and catches slow edges at a well-defined point. Level 50 % on NovaOS puts the level halfway between the lowest and highest points of the trace on screen. On a running signal that is the middle of its swing. On a flat line it puts the level on the line itself, where noise alone may trigger it: it cannot find a signal that is not there.
We armed Single with a rising edge at 2.96 V on an XBee's serial output, and nothing happened. The line sat at 3.41 V, its idle level.
The level was not the problem: 2.96 V lies inside the line's 0 to 3.41 V swing, and every time the line came back up after a low bit it would cross 2.96 V upward. The problem was that the line was idle: nothing was being sent while the scope was armed, so there was no edge at any level. Sending a command to the radio made the scope trigger at once.
Two things would still have been better. A falling edge catches the start bit, so the capture begins at the start of a character rather than somewhere inside one. And 1.6 V, the middle of the swing, leaves room for noise and for a line whose high level sags; 2.96 V is only 0.45 V below the idle level.
Once a decoder is set up, NovaOS can also trigger on the decoded bus itself (the decode sheet's "Trigger on B1"), for RS232, I2C, SPI, CAN and LIN.
| Check | What to do |
|---|---|
| Is the signal there at all? | Switch to Auto: a flat line means nothing is happening, or the probe is on the wrong pin. |
| Is the source the right channel, and on? | The trigger source must be the channel with the signal. |
| Is the level inside the swing? | Compare it with the trace in Auto; put it in the middle. |
| Is the slope right? | A line that only ever falls once (a reset going low) never makes a rising edge. |
| Is it a one-off? | Arm Single first, then make it happen. |
The scope is not triggering: in Auto it captures anyway, at random moments. Put the trigger source on the signal's channel and the level inside its swing, and it stands still.
Auto captures even without a trigger, so the screen never goes blank. Normal captures only on a trigger and holds the last capture while it waits.
A time after each trigger during which the scope ignores new ones. On a burst of pulses it makes the scope trigger on the first pulse of each burst rather than on any pulse.
Set the trigger for it, press Single, then make the event happen. The scope stops on it and keeps it.
An XBee radio's serial line, from the wiring to the decoded answer: the trigger on the start bit, the time base and the decoder settings.
60 Hz with nothing connectedMains hum on a floating input: why a probe on nothing shows a wave, why a 10× probe shows more of it, and how to tell hum from a real signal.
An XBee's power-up commandsProbe DIN and DOUT, capture the whole start-up in one deep record, and read the network, channels and addresses the device sets.