Oscilloscope recommendations
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Re: Oscilloscope recommendations

by mojo on Wed Apr 21, 2010 1:15 pm

sirket wrote:16 bits for 9 is hardly a big deal on the USB bus.

As for the logic side- If they are, in fact, FPGA's with no buffers then I am definitely confused.
It's basically a parallel to USB connection- no need for processor cores or anything else. Still- I haven't bothered to design one so I probably shouldn't criticize.


We are drifting way off topic now but basically USB is a complex protocol which needs some processing in order to service. At it's simplest a parallel to USB converter needs to be able to interpret packets on the USB bus and respond with the correct set-up data (device and configuration descriptors), then process bulk transfers via an endpoint. There is also some protocol management stuff like checking CRCs and doing re-transmits if required. The Logic doesn't have much of a buffer itself either so there needs to be additional code to manage the buffer and signal buffer overruns if the USB interface can't keep up.

Maybe you could do all that as pure logic on an FPGA but I am unaware of any designs which do. They all have some kind of code execution because it's just so much easier to do that way.

Going back to the point about 9 bits padded to a 16 bit word, sure it's not hard to do but you are missing the point. Since it's either a choice of wasting the other 7 bits or going to all the extra effort of packing/unpacking 9 bit words you might as well just use all 16 bits. That is why most logic analysers have multiples of 8 inputs. Sorry if I'm repeating myself but I really don't think it's that difficult to understand.

I agree completely wrt to visualizing waveforms. As for the digital versus analog- digital is simple enough- almost every time I run into problems it's because of analog characteristics present in the digital waveform- ringing, bouncing, etc.


No offence but your inability to grasp why logic analysers tend to have multiples of 8 inputs suggests otherwise.

To give an example I have found my Logic very handy when working on timing issues or emulating existing but poorly documented protocols. For example I recently improved compatibility with some models of the Playstation 2. Of course there is no official documentation for the controller protocol in the public domain but based on what people have discovered and my own readings taken from a real PS2 controller I had a working system. For some reason a few models of PS2 and one type of PS2 to USB converter didn't work though. It turns out that it's because those devices were starting the next byte too soon and not leaving enough time for the controller to ACK the last one. Sony PS2 controllers work because they can ACK and read/write bytes at the same time, but my hardware and several third party controllers could not. It seems that Sony realised this too as after two revisions that had the problem (and one of them on only one of the two ports for some reason) they fixed it. Fortunately I was able to modify my code.
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Re: Oscilloscope recommendations

by sirket on Wed Apr 21, 2010 6:48 pm

No offence but your inability to grasp why logic analysers tend to have multiples of 8 inputs suggests otherwise.

Of course offense was meant- That's the only reason anyone says "no offense."

I also wasn't referring to logic analyzers in general- I was specifically referring to the super basic FPGA based ones:
"I really wish the Salae and other basic logic analyzers would come with more than 8 channels."

The Salae is a beautiful device. I own one and I like the ergonomics of the design as much as it's capabilities. It's small, simple, quick to set up and configure. Adding 1 more channel to trigger on would not have impacted the case design, or really added to the probe cost. Adding 8 more wires and ez-hooks would have increased just the material cost by about $30 (based on the pricing for those accessories on the Saleae web site) without taking into account the price of the newer, larger case design (or possibly having to move to a larger FPGA or any one of a hundred potential differences). That said- I know nothing of the economics involved so I could be even more wrong.

Not to mention- having to break out another 8 channel pod- when all I want to monitor is one additional channel- is frustrating. It's 7 additional wires in the way.

I spend a lot of my time tripping over poor design decisions. "Why not add another 8 channels" is the kind of mantra that engineers love and bad designers accept. (Like when Agilent/HP first went to digital scopes and replaced all the standard controls with menus and buttons and so on). A good tool doesn't have superfluous trappings. 12 channels is useful for an 8 bit system but 9 would be enough. 17 would be enough for a 16 bit system.

I understand why larger logic analyzers have data paths that are multiples of 8- the storage RAM is all in multiples of 8- the buffers, amplifiers and so on also all come that way (Although 12 channel buffers and such are often available). I was specifically referring to inexpensive USB based LA's that don't actually have any of these constraints. (Mixed signal devices all provide additional channels and are thus incredibly useful as a result).

If you're going to design a new product- you should do a lot of real world research into usage patterns, competitors shortcomings, etc. Just because everyone else does something one way- doesn't mean you have to too. In fact- it's probably a good way to differentiate your product.

If you want a much better idea- Rather than waste those additional 8 bits on 8 more data channels- why not provide a single extra channel with a rudimentary scope like functionality with 8 bits of resolution. Now you've given people a trigger channel that they can also use to help find oddball analog problems, along with 8 digital channels. It doesn't need to be super accurate or fast- just enough to keep up with the digital channels and hopefully provide a tiny bit more insight into a problem.

... For some reason a few models of PS2 and one type of PS2 to USB converter didn't work though. It turns out that it's because those devices were starting the next byte too soon and not leaving enough time for the controller to ACK the last one. Sony PS2 controllers work because they can ACK and read/write bytes at the same time, but my hardware and several third party controllers could not.

You should document your troubleshooting experience (if you haven't already) and post it here (on the forums obviously- not this thread :) ). People might find the process relevant to their own problems- the inferences you made- the missteps you took, etc. It would be doubly useful since you used the Saleae and that's going to be what a lot of people here end up using.
Last edited by sirket on Wed Apr 21, 2010 10:24 pm, edited 3 times in total.
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Re: Oscilloscope recommendations

by sirket on Wed Apr 21, 2010 8:11 pm

It just occurred to me that I was so busy arguing with you that I had almost forgotten my original point- as well as an important consequence therein.

If you read through my posts you can see that I was primarily interested in having an extra LA channel so that I could trigger on it.:
"it would cost trivially more to add an extra line and then you could trigger on a clock while watching the data bus."

Well it occurred to me that I don't really need to send that input anywhere- I just need to be able to trigger on it. As such- all I need to be able to do is tell the LA itself to use that input and to do something with it such as: only send me data when that signal is high or low, or to start sending data when the signal transitions, or whatever (possibly to send me an extra packet whenever that trigger line trips). All things that could easily be done without dealing with USB bus padding or any of the other concerns you raised.
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Re: Oscilloscope recommendations

by mojo on Wed Apr 21, 2010 8:54 pm

sirket wrote:You end up padding a few bits in your USB packets- who cares?


The person designing a low cost logic analyser that has almost no buffer memory and so has to keep sending a constant stream of data over the USB bus as fast as possible.

The art of cost reduction is a difficult skill to master. It would be nice if everyone could spend thousands of Pounds on a LA but unfortunately that isn't the case so compromises have to be made. I was simply trying to explain the logic, if you will forgive the pun, behind that decision. Clearly the Logic is not designed for parallel buses, and fortunately most hobbyists don't encounter them very often.

You should document your troubleshooting experience (if you haven't already) and post it here (on the forums obviously- not this thread :) ). People might find the process relevant to their own problems- the inferences you made- the missteps you took, etc. It would be doubly useful since you used the Saleae and that's going to be what a lot of people here end up using.


http://blog.world3.net/2010/04/playstat ... -io-error/

I was looking for the Famicom controller pinout and was amazed to discover that the only two sources on the web are both wrong:

http://blog.world3.net/2010/04/famicom- ... rt-pinout/

The "least wrong" one has the DB15 lines back-to-front, a surprising error considering that most connectors have the pin numbers embossed on them. Now I just have to figure out the trackball protocol.

Anyway,I'm not wasting any more time on this. Yeah, I went to university too and I have products out there. I even have a Gold Swimming Certificate. The great thing about the intertubes is that none of that matters because all you see is my username and my argument. Feel free to post pics of your certificates, but it won't make you any less wrong.

I think my work here is done.
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Re: Oscilloscope recommendations

by sirket on Wed Apr 21, 2010 10:07 pm

*Edit* Removed off-topic comments.

I hadn't really thought a lot about low end logic analyzers- but I went into the discussion assuming that there was a solution. *edit* The solution I described (that of letting the hardware do the trigger) is exactly what the Openbench Logic Sniffer does (although they buffered the inputs and thus have the same 8 bit constraints- something the Saleae could have avoided).
Last edited by sirket on Thu Apr 22, 2010 12:31 pm, edited 2 times in total.
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Re: Oscilloscope recommendations

by adafruit on Thu Apr 22, 2010 11:38 am

sirket wrote:No offense- but ...


ok gang, (phil here) this has been flagged as having personal attack - it looks like it spun out of control. how about sirket you apologize and just remove the name-calling parts of your post, mojo maybe you can say something nice about this lively debate and we'll move forward?

the alternative is that i remove posts and lock the thread, let's not do that - let's just work together and keep up a good conversation, ok?
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Re: Oscilloscope recommendations

by mojo on Thu Apr 22, 2010 12:18 pm

Well I think there has been some useful information posted here so it would be a so it would be a shame to close it. I was planning to start a thread for GW Instek 'scopes once I have had more opportunity to use mine (I am very busy at the moment with digital stuff).

As I said before I think having a 'scope is very helpful when you are trying to learn about electronics. Most of the talk here has been about debugging but I find them useful as a learning tool as much as anything. For example I have been working with radio time receivers lately. Before I had little experience with radios and was hoping that the modules would just work, but due to signal strength issues I had to replace the antennas and eventually decided to build the whole circuit from scratch. Although I had managed to make it work based just on what I read in books and the schematics I found being able to probe at different points and get a visual representation of what was going on very helpful to my understanding. Well, understanding might not be exactly the right word. I can understand the circuit and the mathematics involved, but they don't give you a "feel" for it.

It's good that digital oscilloscopes are now becoming quite affordable. Unfortunately there isn't a lot of good tutorial information online, but there are plenty of books on the subject.
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Re: Oscilloscope recommendations

by rct on Thu Apr 22, 2010 12:20 pm

adafruit wrote:
sirket wrote:No offense- but ...

how about sirket you apologize and just remove the name-calling parts of your post, mojo maybe you can say something nice about this lively debate and we'll move forward?


I don't think this thread has gotten that out of hand, yet. I have learned a few things. I think things stopped being congenial a bit earlier (and without provocation from what I remember.)

mojo wrote:...
Sorry if I'm repeating myself but I really don't think it's that difficult to understand.
...
No offence but your inability to grasp why logic analysers tend to have multiples of 8 inputs suggests otherwise.


In any case, I agree it doesn't need to be personal.
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Re: Oscilloscope recommendations

by sirket on Thu Apr 22, 2010 12:31 pm

I think the first personal attack started with:
mojo wrote:"No offence but your inability to grasp why logic analysers tend to have multiples of 8 inputs suggests otherwise.

This was after I tried to be conciliatory wrt to mojo's comment on visualizing waveforms.

That said- I apologize if I offended anyone - and I apologize for letting the thread veer so far off topic.

I've edited my post and removed my personal comments.
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Re: Oscilloscope recommendations

by sirket on Thu Apr 22, 2010 12:37 pm

A good oscilloscope becomes invaluable when dealing with high frequency signals because some of the phenomena you see just aren't a concern at lower speeds. Andrew Huang has a great example over at his blog:

http://www.bunniestudios.com/blog/?p=197
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Re: Oscilloscope recommendations

by adafruit on Thu Apr 22, 2010 1:16 pm

sirket wrote: That said- I apologize if I offended anyone - and I apologize for letting the thread veer so far off topic. I've edited my post and removed my personal comments.


thanks gang, this post and thread will stay as is - keep up the great conversation.
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Re: Oscilloscope recommendations

by mojo on Wed May 19, 2010 6:15 pm

I have been speaking to a few other DSO owners recently and an interesting issue came up. 'Scopes with large memories display the contents of those memories in different ways.

Say you have a 1M sample buffer but the screen is only 640 pixels across. Obviously scrolling through 1,000,000 pixels is impractical so you zoom out. Now each pixel covers ~1500 samples. Some 'scopes just display every 1500th sample which is pretty useless because only 0.064% of the waveform is shown and if the bit you are looking for lies between two pixels it isn't visible.

The GW Instek I own has a 2M sample buffer. When you zoom out to it displays a kind of weighted average of the signal with the minimum and maximum peeks. In technical terms it finds the minima and maxima and displays the excursion. That way you can usually see the signal you are looking for and zoom in on it.

It's quite surprising which 'scopes have the proper display algorithm and which don't. The Instek ones do, as do Tekway. Rigol and cheap Agilent stuff (which is made by Rigol) don't, but the big surprise was Tektronix which also don't display properly! Tektronix have a well deserved reputation for quality instruments, but it seems like they have really dropped the ball on this one!

A friend of mine emailed them to ask about this and one of their techs confirmed that their entire range is like that.

My guess is that they are playing catch-up with the Chinese manufacturers who have the advantage of starting out with FPGA designs that are suited to this kind of processing where as Tektronix use less generalised hardware that makes processing of large memories difficult. The arrival of affordable large memory 'scopes has forced them to offer similar features but they had to bolt them on to the existing hardware.

It's a shame because long sample memories are not that useful with the simple displays they offer. Remember that the whole point of having a long memory is to be able to record a longer period of time and then move around it quickly rather than having to get the trigger in just the right place. Obviously your ability to do that is severely limited when you can only see 0.064% of the waveform and entire artefacts are hidden. It also saves wear on your scroll knob :-)

Anyway, I think this is an issue that is well worth considering when looking at digital oscilloscopes. It's something I didn't really think about (it doesn't seem to be marketed much) but I'm really glad I got the Instek now. I was considering Owon too, who have yet to reply to my email, incidentally.
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Re: Oscilloscope recommendations

by charliex on Wed May 19, 2010 8:04 pm

I doubt any of the digital scopes would have a problem running an average on such a small amount of data. Its hard to imagine you can make out much of a trace over that side buffer on such a small screen even with any averaging or min/max.

I'd just view it on my PC via the USB.
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Re: Oscilloscope recommendations

by mojo on Thu May 20, 2010 6:08 am

Perhaps I didn't explain it very well. I am waiting to receive some pictures from a Tektronix to illustrate the problem.

Obviously when zoomed out you can't see fine detail, but the point is that on scopes without a proper display you can't see anything. The bit you are looking for simply will not appear on the display, where as with a proper min/max display you can see where it is an zoom in for a better look.
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Re: Oscilloscope recommendations

by charliex on Thu May 20, 2010 12:24 pm

I think i see what you're saying, i haven't ever really found it to be a problem with my TDS but then i may never have needed to use it in a situation it was warranted. If i'm using it on the bench its much better to plug it into labview or the standalone viewer anyway, since the LCD's on scopes are pretty basic mostly 1/4 vga. thats a lot of data to fit in. Even if i'm on the road i've still got a laptop to review with.

I guess i'd have to see the scenario to see what i'd do about it, apart from load it into labivew/signalview.
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