ZOYI ZT-703S Unexpected Oscilloscope Behaviour - Page 1
Forum section: electronics / testgear

author: CJSouthern
author_rank: Newbie
timestamp: October 02, 2025, 05:44:02 am
content: Hi folks,I've built a "breakout box" that allows me to monitor various power supply voltages and signals between an ATX power supply and it's motherboard. This afternoon I've had PS_ON# on channel 1 of my ZOYI ZT-703S (set to 2V per div, DC coupling) and PG (Power Good) on channel 2 (same parameters). Triggering is set to CH1, single, falling edge @ 2.5v.When I press the power button PS_ON# should drop to low (and stay low) and PG should go high (and stay high) when it's had sufficient time to get all the rails up to spec. In reality, this is what is actually happening as evidenced by (a) the computer actually powering up and staying on and (b) my bench scope - set to the same conditions - is showing exactly what I'm expecting (the PSU is taking about 145mS to come online). All looks perfect. However.At the same time (literally - both scopes are connected at the same time - although I also get the same behaviour with just the ZOYI connected) I'm seeing PS_ON# drop - the scope triggers - PG rises at the expected time - but then the ZOYI scope shows PS_ON# rising and PG dropping about 300mS from where it first triggered. It's wrong, but I don't know why.I get the same behaviour at 20mS and 100mS. I'm not seeing the same behaviour at 200mS and slower timebases - but - at 200mS or slower the scope goes into roll mode with auto triggering and thus just updates the screen in real time.Just wondering if anyone can explain why I'm seeing this false information on the Zoyi?Many thanks.

author: Aldo22
author_rank: Super Contributor
timestamp: October 02, 2025, 08:54:57 am
content: Looks like it doesn't buffer the right part. There's no noise there either, just a straight line.I don't have a ZT-703S and can't check it. In any case, my Zeeweii doesn't do that.Is there not enough memory?

author: CJSouthern
author_rank: Newbie
timestamp: October 02, 2025, 09:19:12 am
content: Quote from: Aldo22 on October 02, 2025, 08:54:57 amThanks for your reply.Are you saying that what's displayed on the right-hand portion of the screen may just be what happened to the left of the left-hand edge of the screen because it's displaying the contents of the pre-trigger buffer in the wrong place?Rather than using an actual PSU I'm considering switching to an Arduino Uno to simulate the signals. I've written a quick sketch tonight but can't test it until tomorrow as the ZT-703s is at work. I might be able to test the pre-trigger buffer hypothesis using an analog port on the Arduino to output a signal that I can recognise on the ZT-703s, but one that doesn't drop low enough to trigger a sweep. If I see that pattern to the right then I'll at least know where it's coming from.I've no idea about it's memory; I assume that it adjusts the sample rate depending on the timebase but I have no visibility as to what it's doing in that regards.

author: csuhi17
author_rank: Frequent Contributor
timestamp: October 02, 2025, 09:38:20 am
content: Didn't you swap the channels on the zoyi? CH1 is on the right.

author: CJSouthern
author_rank: Newbie
timestamp: October 02, 2025, 09:45:56 am
content: Quote from: csuhi17 on October 02, 2025, 09:38:20 amNope. It wouldn't trigger until PG dropped if that were the case - and that only happens when the PSU is shut down.

author: Aldo22
author_rank: Super Contributor
timestamp: October 02, 2025, 11:01:54 am
content: Quote from: CJSouthern on October 02, 2025, 09:19:12 amI think it just doesn't have any data stored for this part and is somehow connecting two points with a straight line.Take a closer look, the left part is a bit noisy, the line isn't completely uniform.The right line (yellow) is completely without variations.That doesn't look like real measurement data. In "dots mode" (if it had one), there would probably be nothing there.But it's best to have someone who owns the device confirm this.To me, it looks like a bug.

author: indman
author_rank: Super Contributor
timestamp: October 02, 2025, 01:51:46 pm
content: Quote from: CJSouthern on October 02, 2025, 05:44:02 amWatch the video on youtube.which is attached in this topic and you will understand everythinghttps://www.radiokot.ru/forum/viewtopic.php?f=10&t=192945As I have repeatedly written in the main topic - the oscilloscope in this device is just a toy, the readings of which cannot be trusted yet.The software contains a lot of known as well as hidden bugs. The multimeter, fortunately, can be used safely, as the ZOYI team was unable to damage it, or its shell was created by another programming team. This is my opinion.

author: CJSouthern
author_rank: Newbie
timestamp: October 02, 2025, 07:57:44 pm
content: Quote from: Aldo22 on October 02, 2025, 11:01:54 amInitially I thought you might be on to something, but I now think it's just a case of when there's +5v it's clean and when there's (close to) 0v it's noisy anyway. There's still noise in channel 2 visible after "the event", and no noise in channel 1 pre-trigger. So I suspect that it has a memory pointer bug and from centre screen onwards it's actually displaying what was captured in pre-trigger memory. I'll test today with an Arduino via an analog channel to confirm.

author: CJSouthern
author_rank: Newbie
timestamp: October 02, 2025, 08:13:28 pm
content: Quote from: indman on October 02, 2025, 01:51:46 pmThanks for that - it was very helpful.Personally, I like the device, but feel that it's a mixture of "wheat" and "chaff". I've read quite a lot from others on it and have come to the conclusion that it has all the "raw ingredients" but the software definitely has some issues; If I can understand those issues well enough to work around them then I can feel more confident in the results. It looks like the "rule" here is "avoid triggering left of centre at long timebases - and be weary of unexpected behaviour that starts exactly midscreen".Out of interest, I also own a Fnirsi 2C53T; it doesn't have this issue, but it does have issues if I set the trigger point too far to the left (it either triggers somewhere on the right or not at all) - so that's another "gotcha". On the plus side, it doesn't go into roll mode when set to single trigger; I can even have it as 20 seconds per division and it'll still trigger on cue (providing that the trigger event is long enough as, understandably, sampling per second is extremely low at those timebases.Overall, I prefer the user interface of the Zoyi - but the bugs are concerning (I've noted a few others too). It would be fantastic if there was an address that I could report them to so they would get fixed; I really like the potential of both units and would be more than happy to be a beta tester for them.

author: adam4521
author_rank: Regular Contributor
timestamp: October 02, 2025, 08:56:22 pm
content: That little waveform mimic at the top of the screen -- is that supposed to be showing the overall buffer length? Are the two shaded ends displayed like that in normal operation (ie when the buffer isn't wrapping)?

author: CJSouthern
author_rank: Newbie
timestamp: October 02, 2025, 09:19:54 pm
content: Quote from: adam4521 on October 02, 2025, 08:56:22 pmOff memory, the only thing that changes on that graphic is the pointer.I'm assuming (which may or may not be correct) is that it's supposed to work like this:1. The unit samples all the time and puts what it finds into memory - using internal "pointers" to mark where the oldest data finishes and the newest data starts.2. When a trigger event occurs, the unit displays pre and post trigger data either side of the user-selected horizontal trigger point (in essence, just a way of telling the device where you want the portion of the waveform of interest to start along the horizontal axis).3. More data is in memory than is displayed on the screen - so it's possible to scroll left/right to move the "window" on the data; in essence, that's what that graphic is showing - which point of the data one is looking at.To be honest, I usually don't pay much attention to it because I set things up to display what I want within the bounds of what's displayed; scrolling into the other areas seldom produces anything of interest for me which, in retrospect, may be because "that's just the way it is" or may be "the way it's looking due to this bug".At present it's almost like it's mismanaging memory in that it starts the display at the correct place but hasn't told the portion of code responsible for determining where memory data should be displayed from, that the trigger point has been moved from the default (ie in the centre) - so it's happily starting where it should start - doing what it should do until it gets to the middle - and then displaying pre-trigger data from the middle onwards instead of continuing with post-trigger data. Either that or it's stopped sampling the data from when it got to the centre - or something like that.I'll do some more testing shortly.

author: CJSouthern
author_rank: Newbie
timestamp: October 03, 2025, 02:14:55 am
content: Hi folks,Update: I've been doing some experiments and the bottom line is that the Zoyi ZT-303s on firmware 1.5.7 appears to "have issues" when either the 50mS or 100mS timebases are selected, and triggering is set to anywhere on the left hand 1/2 of the screen. When in that configuration it appears as if what one sees on the display is only valid until approaching the centre of the screen; after that it appears to be presenting some portion of pre-trigger buffer as best I can tell.I've been feeding it a low-frequency step-down waveform (in the region of 5 to 15Hz) with triggering set to falling edge at around 300mV on a 3v peak-to-peak signal with the horizontal trigger point set anywhere from "all the way to the left" to "centre". With normal triggering one can often see a malformed signal around the centre at either of those two sweep speeds. A simpler test is to just feed it the output from a power supply (as shown by @indman in his linked YouTube video).I'll try to contact them to get a bug fix. Fingers crossed.

author: CJSouthern
author_rank: Newbie
timestamp: October 03, 2025, 04:10:22 am
content: Update 2: I've made another interesting discovery; if I set the run mode to high, the unit appears to behave as it should on the 50mS timebase with the trigger point set well to the left (although not in an extreme position). This is the only difference between the two attached photos where I'm simply catching the rising voltage from a PSU when turned on. Note that the unit still misbehaves when set to a timebase of 100mS even when in high run mode.Note that the unit only stays in high run mode until powered off. Also note that changing the run mode also changes the unit to Auto trigger - although it can be reset to normal or single after changing to high run mode.Hope this helps someone.