Oscilloscope Triggering: When is Normal not so Normal?

Oscilloscopes have two primary modes of triggering: AUTO and NORMAL. However, NORMAL is not the normally used mode of triggering. AUTO is. The default trigger mode in all of today’s oscilloscopes is AUTO. There is a lot of confusion these days among oscilloscope users as to exactly when to use which mode of triggering. Let’s first define what these terms mean and then discuss how these modes of triggering came to be called what they are.

AUTO simply means “automatic”. In the AUTO trigger mode, the scope will trigger on the signal under test if a trigger condition is met, such as a rising edge. But if a trigger condition doesn’t occur within a predetermined amount of time, the scope will begin to generate its own automatic triggers, which are not synchronous to the signal under test. This means that the scope will show of blur of waveforms when this happens. So if AUTO is always the default trigger mode, why would you ever want to see a blur of waveforms? One reason is that a blur of waveforms will show you where the signal is relative to your trigger level. Perhaps you have the trigger level set above (too high) or below (too low) the signal under test. With AUTO trigger you can see what’s wrong and make adjustments. Setting up an oscilloscope is an iterative process of seeing what’s there and then making adjustments (V/div, sec/div, trigger level, etc.) until it is right. Another reason the AUTO trigger mode is the default mode of triggering is that you may want to simply view the DC level of a power supply. Scopes can’t trigger on DC, unless the DC includes lots of switching noise, in which case it is not purely DC.

The NORMAL trigger mode means that the scope triggers if and only if a trigger condition is met. If you’ve got your trigger level set above or below the signal under test, then you’ll be looking at a blank screen on your scope. So when should you use NORMAL triggering? If the signal you want to trigger on occurs very infrequently, perhaps once every three seconds, then you should use the NORMAL trigger mode so that the scope will display synchronized representations (waveforms) of your signal only when trigger event occurs, and not generate automatic and asynchronous triggers between qualified trigger events and thereby show you blurs of waveforms.

So why is this trigger mode call NORMAL? I can only guess. Back in the old analog scope days, this trigger mode was not called NORMAL triggering. It was call the TRIGGERED sweep mode, which makes sense. When a trigger qualification was met, such as a rising edge, the analog scope would trigger a linear sweep of an electron beam across the scope’s cathode ray tube (CRT). But when digital storage oscilloscopes (DSOs) came along about 30 years ago, the representation of waveforms on the scope’s display changed from the sweep of an electron beam that excites phosphor on the CRT to the digitization and storage of discreet waveform points using an analog-to-digital converter (ADC) and then represented as pixels on a scope’s display. Since newer technology scopes stopped sweeping, most oscilloscope vendors began calling it a “trigger” mode instead of a “sweep” mode. And if they had kept using the same old analog scope terminology it would have become the TRIGGERED trigger mode, which sounds redundant. So some genius marketing guy must have said, “Let’s call it the NORMAL trigger mode — maybe because it was the trigger mode that he or she normally used.

Note that some DSOs still call it an AUTO and TRIGGERED sweep mode. I feel sorry for the younger engineers that have no idea what a sweep is.

In my opinion the AUTO and NORMAL trigger modes should be called AUTO On/Off. To me, this makes more sense. But I know that’s not going to happen, just like Australians will never stop calling their oscilloscopes their “crows”, which I think will be the topic for my next blog.

Anyone out there know for sure how this mode of triggering came to be called NORMAL?