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How can I know the latency of a digital oscilloscope in roll mode? By this I mean the time it takes from the signal reaching a certain value to the moment this value is shown on the screen, i.e. the time it takes for the scope to sample the signal, process the samples and draw on the screen.

What's the name of this parameter? Is it usually in the datasheet? What is a reasonable value for this?

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    \$\begingroup\$ A reasonable value (guaranteed by all useful scopes is) "much faster than you could possible ever notice" \$\endgroup\$ – PlasmaHH Apr 3 '18 at 13:28
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    \$\begingroup\$ @PlasmaHH Is it so fast that they don't even care to report it? How fast does the display refresh? Ordinary desktop screens refresh at 60Hz, which is 16 ms. I wouldn't expect scopes to have ordinary screens but still, 16ms could be a lot, they'd have to be much faster. \$\endgroup\$ – freejuices Apr 3 '18 at 13:35
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    \$\begingroup\$ They are pretty ordinary displays, I would have no reason to assume they are not running at 60Hz or similar. What benefit do you think a faster display would have? This is not a 3d shooter where you have to quickly react to something, which your brain takes awfully much longer than 16ms anyways. \$\endgroup\$ – PlasmaHH Apr 3 '18 at 13:39
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    \$\begingroup\$ That sounds like a pretty dumb idea, both are suffering from the same "lag" problem. You should rather setup one channel to capture the signal and another channel to capture a change in the display (e.g. a photo diode). Still this seems like an XY problem, that other devices latency doesn't seem to be important at all. \$\endgroup\$ – PlasmaHH Apr 3 '18 at 13:49
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    \$\begingroup\$ Re, "I wouldn't expect scopes to have ordinary screens." A modern, digital oscilloscope is a special-purpose computer. Its screen is a computer screen. \$\endgroup\$ – Solomon Slow Apr 3 '18 at 14:05
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Almost all analog 'scope display slightly behind real-time...perhaps by 50 to 100 ns.

Digital 'scopes likely add much more delay....
Waveform samples collected in real-time are often stored in digital memory. From there, a microcontroller transforms samples to drive the digital display. The microcontroller may be required to do other calculations (for example: calculate mean voltage, RMS voltage, peak voltage, frequency) that slows display.
Depending on processor speed and display interface, you cannot know the display latency. Samples stored in memory can sit undisplayed for a very long time.

For example, my digital 'scope states that up to 2000 waveforms per second can be displayed, if no additional waveform calculations are requested. No mention is made of display latency, which is an entirely different matter.

If a 'scope user must discover timing relationship between two events, use a multichannel 'scope, or one with an independent trigger input. The timing between a trigger event, and a displayed waveform is available to the user, with a decently defined specification in the manual. Similarly, timing between channels of a multichannel 'scope is well-defined.

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  • \$\begingroup\$ Moreover, the length of the probe can add a bit of latency on its own. With a velocity factor of 0.8, 1.5m probes add a delay of about 6ns. I remember a video with Bob Pease immediately noticing different cable lengths with a peek at the scope's screen. \$\endgroup\$ – Sredni Vashtar Apr 3 '18 at 15:45
  • \$\begingroup\$ @SredniVashtar, I'd guess we're not worried about nanoseconds if OP was considering using a rolling display to observe the signal. \$\endgroup\$ – The Photon Apr 3 '18 at 15:53
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Scope manufacturer here - I'm curious what you're trying to do? The human reaction time is 150-250ms, so in roll mode anything less than this shouldn't really be an issue. I'd fully expect a scope to respond in that amount of time, I'm certain our Keysight ones do.

To answer your question, though, I'd expect that the faster the waveform update rate of a scope the lower the delay for roll mode.

If you're looking to get a quick reaction, your best bet is to setup a good trigger and use a trigger-out signal.

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