RIGOL DHO900: Logic analyser can only do fast

[D-Jan]

Hi,
just noticed that you cannot use the 16chan. logic analyser (LA) in the DHO924 at slow timebase speeds:
Whe the unit comes up it always has this annoying "Auto Roll mode" enabled.
Default timebase is 2µs. When I enable the LA and then reduce the timebase to 50ms the LA traces disappears. Pushing the LA button sais "Operation disabled"
When I disable the "Auto Roll mode" to have classic scope behavior the same happens at 200ms timebase setting.
     What is the problem of recoding digital data at slow speeds?
I don't get it.

Hey Rigol guys please read this and fix it.

[NE666]

It speaks to not having a variable sample rate for the LA section, I think.

I think this also speaks to selecting the right tool for the right job. IMHO the LA section of a MSO is principally there for tasks which require correlation of analogue and digital domains and the characteristics you see largely fit with that. For example, no-one has a major need for debugging signal integrity issues with rising edges in the order of 100's of ms. Other principally digital tasks such as long captures, long running protocol exchange traces and in your case slow recording speeds, are the domain of 'real' LAs. The good news is that 'real' LAs can be pretty inexpensive for low speed operation.

I coughed for the PLA2216 for my MSO5000 but I sill have and use a 'cheap' PC-based USB LA for what you describe. They are, again IMHO, different instruments with different use-cases. The LA section of the Rigol's isn't really a LA, it just provides a subset of that functionality.

[D-Jan]

Hi NE666,

I cannot believe that there is no variable sample clk available in the DHO. As you mention it also features 4 analog channels. When I go to 1sec/Div these ADCs  also need a slower sampling clock.
My old Agilent 54622D has no issues with that whatsoever. With slow timebase you can see analog and digital signals develop at the same time from left to right. For the DHO924 the LA updates the LA in one shot when the analog channels have reached the right border of the screen.
I still think an MSO is the correct tool for the job. My project features a MCU that generates PWM and controls various other circuits. With the MSO I can monitor the Buck output voltage in analog (up to 60V). For a single shot I have all my control signals at a view, plus I can zoom in to even measure PWM duty rate since modern MSO have huge memory depth.
I have a standalone LA (ikalogic), but I just like the benchtop better. Also a LA will never show you analog waveform matching with your digital data.
Having the MSO function in my 54622 I never wanted a standard oscilloscope.
At work I have a RIGOL MSO8204A and here the LA section is behaving as expected.

I have no idea if someone at Rigol has ever tested the LA in the DHO924: You can disable some channels for better readability, but every time you press the LA button to hide /show the LA traces all channels are automatically enabled again. It is a pain ...
It is nice bit of hardware, but the firmware of this thing is clearly still in beta stage.
I would expect Rigol to have some developers to fix the bugs in the firmware and provide SW-updates on a regular basis.

[NE666]

My old Agilent 54622D has no issues with that whatsoever. With slow timebase you can see analog and digital signals develop at the same time from left to right. For the DHO924 the LA updates the LA in one shot when the analog channels have reached the right border of the screen.

Hi.

That's what leads me to believe that the digital sampling system has a separate sample clock which is either fixed rate, or has a minimum rate for some architectural reason. It's not that the LA just doesn't work at low speed, it is being actively prevented from doing so, and there must be a design reason for that. If I had to bet, I would say it's because it's fixed sampling (or fixed within a narrow range) and doesn't have the memory depth to collect enough data to meaningfully decimate and recombine in post-processing with a slow analogue capture. Your other scopes use different hardware and architecture and therefore could well be different in how they handle this. However, I'm an enthusiastic amateur, not a qualified engineer, so I'll be the first to admit that's a hunch based on what you observe.

I have no idea if someone at Rigol has ever tested the LA in the DHO924:

If it's anything like the three Rigol products I have purchased, not much testing was done period. And probably no usability testing or customer feedback whatsoever. That might just sit ok with me, at this price point, if your other expectations of Rigol actually being interested in fixing defects and committing to a consistent, sensible FW release process and cadence. Sadly that's not been my experience either and thus why I'd be hard pressed to buy another Rigol product right now.

[Fungus]

The analog part goes into roll mode at slow speeds, too.

I'm guessing it because it simply isn't useful to take ten seconds (or two minutes) between screen updates.