Pausing and inspecting signals on USB oscilloscopes - Page 1
Forum section: electronics / beginners

author: midix
author_rank: Contributor
timestamp: September 26, 2022, 02:51:45 pm
content: Very likely, I'm doing something wrong or the software is designed badly.I have OSC482 from Loto Instruments. Seemed like a decent scope for occasional experiments.Now I'm trying to debug a practical problem (and learn as I go). There is a device sending MIDI protocol bytes. Essentially it's UART start-8-stop at 31250 baud rate.The device is sending a byte 0xFE about every second.I connected my scope to the optocoupler that receives the data. I could see something coming in, yay. But the data flashes by too fast. That's what the triggers are for, right? I can set a trigger, it will pause the input and then I preview it zooming and scrolling left & right. I had to fiddle a bit with the time division settings because the osc disables the trigger for some time values. Finally, I managed to enable the trigger, move its handle to the expected voltage (around 2V), set it to trigger on fall-off (because UART is high by default) and hit Single button to capture a packet.I see it has been captured in the cache - there is a peak in the preview. But I cannot select it in any way to investigate in the main view! There seems to be no way to scroll the frozen view back in time nor any way to activate those cached frames and inspect them.I know that OSC482 has software and not hardware trigger, so it can be inaccurate. But this time it seems mostly a software problem. I had to hit Single like a thousand times to finally get lucky and get a byte in the main view. At the same time, I see it did capture a byte in every small preview rectangle, I just cannot access them somehow.Then I thought it would be easier to record a fragment and then inspect it... no luck, the recording seems to have the same problem - I cannot pause it and scroll to wherever I want (or scrolling could be hidden somewhere).I also tried out UART decoding feature, setting all the parameters as needed (31250, start, 8 bytes, stop, no parity), but it did not yield any bytes. Maybe this function is not supposed to work with the analog probe.This makes me consider upgrading to a better oscilloscope, but I'm not sure if it's worth it. OSC482 seems to be totally enough for me with its hardware specs. I wish there was alternative software for OSC482.I don't have a place for a full-size scope and also I'm visually handicapped, so computer scopes are much more convenient for me. Analog Discovery 2 or PicoScope seem to be the safest choices, but they are a bit too pricey though.Still, I'm wondering, how would the same scenario work with AD2 or PicoScope? Do other scopes have the ability to easily zoom / scroll the view back&forward in time after capturing a triggered signal (assuming the signal was captured with proper resolution)?

author: jwet
author_rank: Frequent Contributor
timestamp: September 26, 2022, 04:43:07 pm
content: There is an open source scope software project called SigRok that might support your hardware. Your particular scope is listed directly but there a lot of the USB scopes are pretty similar. They also have an active forum where you could ask some questions about your hardware. Might be a good place to start. I used it for a multimeter interface a long time ago and it was pretty solid. Good luck.

author: ledtester
author_rank: Super Contributor
timestamp: September 26, 2022, 08:09:41 pm
content: It appears there is someone from Loto Instruments on the forum -- see this thread:https://www.eevblog.com/forum/testgear/how-to-promote-an-usb-oscilloscope-to-individual-engineers-i-am-the-loto-instru/Also, on their web site:http://www.loto-ins.com/en/there is an email address at the top which might be a source of help.Update: Oh -- I see you're already aware of that thread!

author: Gyro
author_rank: Super Contributor
timestamp: September 26, 2022, 08:54:04 pm
content: Wow, that UI sucks. Any chance of getting your money back?It really would be worth trying to get it running under Sigrok Pulseview (if not).Edit: It looks as if it is planned...https://sigrok.org/wiki/Loto_OSC802(at least the 802, it may be worth submitting a request).

author: midix
author_rank: Contributor
timestamp: September 26, 2022, 09:22:13 pm
content: Unfortunately, my dispute time on Aliexpress is over. And, technically, there's not much to complain about - it's working as advertised, it's just not working the way I imagined it should.That sigrok wiki page hasn't been updated for almost 3 years, so I don't have much hope. However, I see Loto updating their own software sometimes, so maybe they will improve it one day.

author: james_s
author_rank: Super Contributor
timestamp: September 26, 2022, 09:56:54 pm
content: I've never liked UIs that try that hard to emulate a physical device. You end up with the worst of both worlds, trying to operate an emulated knob with a mouse pointer is like trying to operate a real one with a stick.

author: thm_w
author_rank: Super Contributor
timestamp: September 26, 2022, 09:58:19 pm
content: For MIDI decoding, likely better off getting a $10 USB logic analyzer that is compatible with sigrok. If your budget is not that high.Of course, good to have both LA and scope in some cases.Quote from: james_s on September 26, 2022, 09:56:54 pmYeah I see no need for the vitual knob, unless its for VERY senile EE's that can only use an analog scope or something.Probably best to just copy what Picoscope does, left/right arrows and a dropdown box.

author: midix
author_rank: Contributor
timestamp: September 27, 2022, 06:56:13 am
content: Loto replied on Youtube. It turns out, you have to hit Stop device button and then, finally, those PC cache thumbnails become available and you can scroll the signal into view. A bit convoluted, but it works. As I understand, that's mostly necessary only for their lower-end models with software triggers because those fail to "focus" the signal into the main view. But even with a properly focusing trigger you might want to be able to scroll a long signal, and then you have to stop the device.I agree with the concept that it's not good to emulate physical UIs because what works well in the physical world is not that convenient on a computer. For example, turning a knob feels natural in the physical world, but on a PC it's a bit awkward mouse gesture. Sliders feel much better for the purpose.I just tried Picoscope and Analog Discovery software in Demo mode. I liked Picoscope 7 (Early Access) better because it's more intuitive. In the very first minute, I understood what I'm doing and what's the meaning of most of the controls, and how to zoom and pan the signal.With AD Waveforms it felt somewhat more convoluted. You have to figure out what those unnamed dropdowns do, and some of them change the functionality of other dropdowns and buttons. Of course, Waveforms is powerful and it can be a great tool for a person who is using it daily and has "muscle memory" for it, but Picoscope might be better for occasional users who want to immediately recognize all the controls.Quote from: thm_w on September 26, 2022, 09:58:19 pmYup, I have a cheap logic analyzer. But this time I suspected one of the devices in my chain is generating dirty square waves or baud rate might be a bit off, so I wanted to inspect the health of the signals with an analog scope first.

author: midix
author_rank: Contributor
timestamp: September 27, 2022, 11:26:28 am
content: I found a detailed user manual for Sainsmart DS series which is rebranded LOTO.Here's how it's supposed to work officially:QuoteSo, not the most convenient thing but at least possible when you manually stop the device every time after it has hit a Single trigger.

author: midix
author_rank: Contributor
timestamp: September 27, 2022, 12:12:08 pm
content: So finally it works, and I even can use UART decoder with custom baud rate on the analog probe. Nice.And now I see what's wrong with my setup - for some reason, it matches twice the expected baud rate - I can decode the actual byte AA I'm sending only when I set baud rate 62500 and not 31250, as needed by MIDI. Ouch, one of my Arduinos it ticking too fast, very likely it got confused with the fuses and the crystal frequency.

author: thm_w
author_rank: Super Contributor
timestamp: September 27, 2022, 09:58:53 pm
content: Quote from: midix on September 27, 2022, 06:56:13 amLA will tell you if baud rate is a bit off in this case, since LA sample rate is up to ~20MHz.Its also easy to drop in horizontal measurement and it will show you the frequency (baud) of the signal at the top. Although I'm sure this Loto tool can do the similar, since you showed it with measurements at the bottom already.But yeah, noisy signals you'd want scope.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 28, 2022, 12:14:40 am
content: Quote from: james_s on September 26, 2022, 09:56:54 pmAn excellent description of the problem!I, from time to time, use a remote ham transciever (just for reception, so far, as I don't trust the microphone on the laptop).Attempting to turn the emulated VFO knob is very frustrating for that same reason.They do, at least offer the option of clicking to change the frequency by various increments.