Help: PC based mixed signal oscilloscope, Hantek 3254(A) VS Picoscope 2206(B)

[rsjsouza]

And roll should restart if I change timebase or input att. What is use for a graph with random axes ?

That's how I remember the Tek behaviour (a TDS3054). I re-tested it this morning on my Rigol DS4014 and it also clears the screen at a change on the timebase.

[Performa01]

Exactly. Roll mode has no trigger...

Interestingly, triggered roll mode (as a additional option) is coming into fashion and also Siglent have introduced it at one point. The important difference to the normal triggered acquisition is that it only works at slow timebases (which applies to roll mode in general), where normal mode leaves you with just an activity bar and the message "slow acquisition" until you finally see a graph (which might take several minutes, depending on timebase settings). By contrast, triggered roll mode gives you the joy of watching the trace developing as soon as the trigger fires, but then it also stops once a full screen width has been recorded, just like a standard acquisition would. This works in normal as well as single trigger mode.

It might sound like a good idea, because watching the trace develop is less boring than staring at an activity bar, but on the other hand this variant still has all the usual drawbacks of roll mode, i.e. lower samplerate and maybe also limited memory - which is the compromise required for continuous recording without blind time and continuous updating of the screen in parallel.

And roll should restart if I change timebase or input att. What is use for a graph with random axes ?

This. Roll mode originally was meant as a substitute for the classic stripchart recorder - probably you were able to change the transport speed during operation back then - even though I very much doubt anyone serious about their work would have actuall done that.

Anyway, Roll mode on a scope as a substitute for the chart recorder is just a crouch at best, because max. recording time is limited and you get no long physical chart for documentation purposes. Unless your scope happens to have a thermal printer built in, you need to dump the memory and post-process the data after the recording has finished - and for that you could just as well (or even better) use standard acquisition with long memory. Apart from that, I don't think a standard DSO with its limited accuracy and resolution (even if its 12 bit) could be a substitute for the real strip chart recorder applications.

I think there is a reason why logging multimeters as well as dedicated data loggers exist.

So the real use for roll mode on a scope is just the permanent visual observation of a slow signal and for this it is certainly not important what direction the trace initially runs until it starts rolling or that it clears the screen when the recording speed (timebase) is altered.

I can think of many reasons why it would not be a good idea to try to continue roll mode on a different speed. If it did not restart, we get a wrong timescale for the existing data on the screen. If we try to rescale and redraw them, this would not only create a time jump, because the additional data manipulation and transfer would interrupt the acquisition.
The rescaling would also be lossy if the speed is increased, because this usually means a higher sample rate (as the memory remains constant at it maximum supported length) and samples that have not been taken initially cannot magically emerge all of a sudden.

[2N3055]

Well, thinking about it, Picosope has a sort of a combined mode. It has slow scan mode (configurable when it kicks in), that basically does Roll mode until it gets trigger and then it switches to slow scan mode to the end of the  screen where it stops.  If you put trigger point at 50% of the screen, combined with large computer screen and 500MSa of memory, what you get is best of both worlds.

[MrW0lf]

Interestingly, triggered roll mode (as a additional option) is coming into fashion and also Siglent have introduced it at one point. The important difference to the normal triggered acquisition is that it only works at slow timebases (which applies to roll mode in general), where normal mode leaves you with just an activity bar and the message "slow acquisition" until you finally see a graph (which might take several minutes, depending on timebase settings). By contrast, triggered roll mode gives you the joy of watching the trace developing as soon as the trigger fires, but then it also stops once a full screen width has been recorded, just like a standard acquisition would. This works in normal as well as single trigger mode.

The ones whose heads rolled pointed out that with Pico system and historical lengths of timebases there is issue of waiting full screen width (or pre-trigger gap) time for initial trigger possibility. How and if has Siglent solved this?

As alternative for changing timebases one could use some large timebase and surf around with zoom window(s). However trace starting from left introduces little problem - you cannot leave zoom window in fixed position because it will lose leading edge of trace sooner or later. If roll trace would indeed start from right (or trigger point!) one could leave fixed zoom window there.

[nctnico]

Exactly. Roll mode has no trigger...

Interestingly, triggered roll mode (as a additional option) is coming into fashion and also Siglent have introduced it at one point. The important difference to the normal triggered acquisition is that it only works at slow timebases (which applies to roll mode in general), where normal mode leaves you with just an activity bar and the message "slow acquisition" until you finally see a graph (which might take several minutes, depending on timebase settings). By contrast, triggered roll mode gives you the joy of watching the trace developing as soon as the trigger fires, but then it also stops once a full screen width has been recorded, just like a standard acquisition would. This works in normal as well as single trigger mode.

Sorry but this isn't roll mode. It is just showing the signal as it is recorded which isn't new. The older Agilent DSO7000A series are already doing this for example.

[Performa01]

By contrast, triggered roll mode gives you the joy of watching the trace developing as soon as the trigger fires, but then it also stops once a full screen width has been recorded, just like a standard acquisition would. This works in normal as well as single trigger mode.

Probably I've got this wrong and the trigger actually stops the acquisition rather than starting it.

I rarely ever use roll mode, hence haven't investigated it thoroughly in conjunction with a trigger. I will have to check this next weekend.

[Performa01]

I can now confirm that triggered roll mode on a Siglent X-series scope works similar to the Pico:

The trace rolls from right to left until it has filled the screen width. Then the trigger gets armed. The trace keeps rolling until the trigger fires. Then it stops briefly and restarts (normal trigger mode) or stops completely until you press the [Single] key again (single trigger mode).

The only difference is that you can set the trigger position wherever you like, the trace will always start at the far right, but will only stop after the trigger has been armed and the trigger condition has been met at the arbitrary trigger position.

In auto trigger mode, we get the familiar untriggered roll mode.
 

[KlausF]

Dave, I watched your video showing a teardown of a Picoscope at a fair.
Why have you never done a review ?