Difficulty when using oscilloscope to observe Ethernet signal - Page 1
Forum section: electronics / testgear

author: houkensjtu
author_rank: Contributor
timestamp: April 13, 2016, 03:24:09 pm
content: Update: For those of you who may interested in the result, I have actually successfully captured several sections of a Ethernet packet, each was shifted by utilizing the "Delayed time base". And by gluing them altogether, I actually saw the waveform of 56 bits of preamble and 8 bits of SFD.Here is the link :https://goo.gl/photos/3EQojfnYpNPd4fWz7Hope you enjoy it too!Hi all! I've been trying to use my old digital oscilloscope (Tektronix TDS 340AP) to observe an Ethernet signal.Basically what I have done is only to force the NIC (Network Interface Card) to work at 10Base-T (10Mhz). Thus the signal is basically 10Mhz and encoded by the so called "Manchester Code", which is pretty easy to interpretate. You can check the wave form I got from the below link:https://goo.gl/photos/3EQojfnYpNPd4fWz7The problem is, as described in Wikipedia (https://en.wikipedia.org/wiki/Ethernet_frame), every ethernet packet has a49bit56 bit "preamble" header. No matter however I change the setting of trigger function, the oscilloscope will always trigger at the first slope of the preamble header signal. Since the preamble is supposed to be simple repeat of 0 and 1, I really want to "skip" the preamble part and trigger at a later timing. More importantly, since the memory depth of my scope is limited, it's impossible for me to capture the whole signal of a ethernet packet.My question is, is there any setting or trick can make it possible to delay the trigger timing?

author: jboard146
author_rank: Contributor
timestamp: April 13, 2016, 03:57:41 pm
content: I've not used that particular scope, but what you are looking for is "Trigger Holdoff". According to quick google search for the users manual on Pg 3-13 at the very bottom of the page. I've attached a screen shot of the crappy copy of a copy I found.

author: nctnico
author_rank: Super Contributor
timestamp: April 13, 2016, 05:03:45 pm
content: Hold-off is not what you want because that is a forced dead time between 2 triggers. What you want is delayed trigger which should be possible by just scrolling the section you don't want off screen using the horizontal position knob.

author: kripton2035
author_rank: Super Contributor
timestamp: April 13, 2016, 05:16:49 pm
content: I just noticed that the scopes that trigger on ethernet signals are very high end keysight scopes ... like the one actually in the contestso may be it's quite difficult to achieve ?

author: stmdude
author_rank: Frequent Contributor
timestamp: April 13, 2016, 06:45:05 pm
content: Quote from: kripton2035 on April 13, 2016, 05:16:49 pmThat's a whole different kind of triggering. That's triggering on specific contents in the ethernet packets, so it does demodulation and protocol decode for the trigger.I.e, you can (for example) set up a trigger for all packets with an ethernet source-address of BA:DC:AF:EB:AB:E0.

author: nctnico
author_rank: Super Contributor
timestamp: April 13, 2016, 06:48:00 pm
content: If you want to look at the packet contents then you are much better off with software tools like Wireshark. I'd never use an oscilloscope to do ethernet decoding.

author: stmdude
author_rank: Frequent Contributor
timestamp: April 13, 2016, 08:42:57 pm
content: Quote from: nctnico on April 13, 2016, 06:48:00 pmWireshark cant look at layer1 stuff. A scope can.Usefull if you're, for instance, designing network gear

author: Keysight DanielBogdanoff
author_rank: Unknown
timestamp: April 13, 2016, 09:56:10 pm
content: Maybe you could rig up something on the breadboard that sends a trigger-in to the scope after the preamble?

author: nctnico
author_rank: Super Contributor
timestamp: April 13, 2016, 10:10:59 pm
content: Quote from: stmdude on April 13, 2016, 08:42:57 pmThat is a valid point but it would mean you'd be involved in designing semiconductors so it is very unlikely there isn't a budget for specific tools.

author: houkensjtu
author_rank: Contributor
timestamp: April 13, 2016, 11:25:06 pm
content: Quote from: nctnico on April 13, 2016, 05:03:45 pmYou are totally right. But as I said, the memory depth is very limited on this scope, I can only get the very begining part of a packet, and sadly most of them are occupied the preamble. That's why I want to delay the trigger.

author: houkensjtu
author_rank: Contributor
timestamp: April 13, 2016, 11:26:54 pm
content: Quote from: stmdude on April 13, 2016, 08:42:57 pmYeah that's the point! Except that I am not designing network gear, I was just curious

author: nctnico
author_rank: Super Contributor
timestamp: April 13, 2016, 11:37:37 pm
content: Quote from: houkensjtu on April 13, 2016, 11:25:06 pmWhat happens if you scroll the horizontal position off-screen (in run mode!) ? I'm not sure about the TDS300 series but I think you should see the rest of the packet due to delayed triggering.

author: borjam
author_rank: Unknown
timestamp: April 14, 2016, 07:03:22 am
content: Quote from: stmdude on April 13, 2016, 08:42:57 pmStill, if debugging physical problems, an eye pattern is likely much more useful than capturing whole packets. Capturing whole packets makes of course would help detect stuff that an eye pattern won't, like packets cut short, etc.

author: Jeroen3
author_rank: Super Contributor
timestamp: April 14, 2016, 07:36:51 am
content: If I recall correctly Manchester code guarantees an edge. So you just set your trigger to trigger after 22 triggers.

author: tautech
author_rank: Super Contributor
timestamp: April 14, 2016, 08:30:42 am
content: Quote from: houkensjtu on April 13, 2016, 03:24:09 pmNot sure how complex your trigger suite is but if you had them:Pattern triggerInterval triggerPulse triggerAny or all could be of use to help overlook the preamble and then trigger on subsequent packets of interest.

author: rf-loop
author_rank: Super Contributor
timestamp: April 14, 2016, 09:02:10 am
content: Quote from: tautech on April 14, 2016, 08:30:42 amWith ancient TDS300 series scope. No Way. Afaik, it do not have any advanced trigger ffunctions. Also it have old analog trigger system. Also need note that it have up to 1000 points (yes 1k) sampling memory.

author: houkensjtu
author_rank: Contributor
timestamp: April 14, 2016, 12:05:19 pm
content: Quote from: rf-loop on April 14, 2016, 09:02:10 amSeems that's the truth. Too sad if I have to give up here...

author: houkensjtu
author_rank: Contributor
timestamp: April 14, 2016, 12:07:49 pm
content: Quote from: Jeroen3 on April 14, 2016, 07:36:51 amI'm afraid that there is no such functionality on my TDS340AP. Maybe plugging in some external trigger source can do the trick, but I still have no idea.

author: macboy
author_rank: Super Contributor
timestamp: April 14, 2016, 12:44:19 pm
content: Quote from: Jeroen3 on April 14, 2016, 07:36:51 amThis would be the solution, if this old Tek didn't try so very hard to be just like an analog scope. It has no capability to trigger after N trigger events. It also has no simple trigger delay to start the capture N divisions or microseconds before or after the trigger event. It just has basic slope and level controls, plus holdoff. I have an old Philips digital scope from around the same era (probably slightly earlier) and it does have the event/N trigger and delayed (or advanced) triggering. It does not have holdoff, which I think is absurd. Back then, adding features like that usually required additional hardware, not just software, so I guess they had to pick and choose.One way to achieve delayed trigger is to split (or tap) the signal, and send part through a delay line before entering the external trigger input of the scope. The "delay line" is nothing more than a coax cable, cut to appropriate length to give the required delay. This is more commonly used to pre-trigger (sending the signal directly to trigger input, and through the delay before the channel input), but it works fine the other way too. The speed of propagation of the signal through the coax will be approximately 2/3 that in air, or 200 000 000 m/s. You want about 6 us delay (about 60 bits at 10 Mb/s) so... about 1200 meters of cable. Oh well, it was a thought anyway. For this large amount of delay, you might be able to design and build a delayed trigger pulse circuit of some kind.

author: houkensjtu
author_rank: Contributor
timestamp: April 14, 2016, 02:43:07 pm
content: Quote from: macboy on April 14, 2016, 12:44:19 pmThis seems a little bit difficult for dumb like me, but I got the idea!So I need to research for a while how I could delay a digital signal for exactly several micro seconds.

author: Fred27
author_rank: Unknown
timestamp: April 14, 2016, 05:09:31 pm
content: Would a logic analyser be a simpler and more flexible choice for the job?

author: PA0PBZ
author_rank: Super Contributor
timestamp: April 14, 2016, 07:07:47 pm
content: Just AC couple the signal, rectify it and charge a cap. Then trigger on the voltage level of the cap using external trigger or CH2?

author: nctnico
author_rank: Super Contributor
timestamp: April 14, 2016, 07:13:19 pm
content: Quote from: macboy on April 14, 2016, 12:44:19 pmWith all this effort it would make more sense to get a different oscilloscope with at least pre/post trigger and perhaps much more memory. Ethernet signals up to 100Mbit have a frequency content not higher than 125MHz so a reasonably priced scope will do fine.

author: femtohertz15
author_rank: Contributor
timestamp: April 15, 2016, 12:44:44 am
content: Just browsing a service manual, I think TDS340 has Delayed Sweep capability. Should do the trick. Looks like a zoom tool in the Tek menus. but special: Trigger, wait <x> amount of time, capture.

author: siggi
author_rank: Regular Contributor
timestamp: April 15, 2016, 02:20:40 am
content: Quote from: femtohertz15 on April 15, 2016, 12:44:44 amYups, looks like the 340 has delayed time base. Tap the horizontal menu button, select "Time Base" from the soft key below the screen, then pick "Delayed Only" from the side menu. You'll then be able to dial in the delay from trigger event to capture.

author: houkensjtu
author_rank: Contributor
timestamp: April 15, 2016, 02:35:26 pm
content: Quote from: siggi on April 15, 2016, 02:20:40 amI have given that a shot and everything works perfectly now!It's proved again that I actually know NOTHING about my scope! THX!

author: siggi
author_rank: Regular Contributor
timestamp: April 15, 2016, 02:41:11 pm
content: Quote from: houkensjtu on April 15, 2016, 02:35:26 pmWell, in fairness the UI on these TDS scopes can only be called "intuitive" for large negative values of "intuitive". I always have to hunt around for this on my TDS784D :/.

author: tautech
author_rank: Super Contributor
timestamp: April 15, 2016, 08:19:45 pm
content: Quote from: siggi on April 15, 2016, 02:41:11 pmWell done guys.Sounds similar to the Interval trigger setting I mentioned in reply #14.

author: femtohertz15
author_rank: Contributor
timestamp: April 17, 2016, 01:50:55 pm
content: NopeI've seen things called names like interval trigger, usualy gap more or less than a threshold of time between pulses. Not the same! "delayed sweep" has been called that for more than 45 years at least. It was the defining feature of a high performance lab CRO, much better if you had a 10-turn pot and the scope didn't have a fancy readout. TDS's had it, HP54xxx have it. It has gone out of fashion with deep memory scopes, if you don't have enough memory to span the the time delay you want, you're out of luck. Which is sad, a coarse counter would be better than nothing.

author: nctnico
author_rank: Super Contributor
timestamp: April 17, 2016, 02:31:59 pm
content: Quote from: femtohertz15 on April 17, 2016, 01:50:55 pmNot true. On DSOs you typically set the horizontal position but it may vary between brands.

author: femtohertz15
author_rank: Contributor
timestamp: April 17, 2016, 04:15:21 pm
content: Sure, i haze a nice new rigol too. You can shift horizontal and crank up time per division until you are out of sample ram for the desired time shift. That is good, but it is zoom. A delayed sweep (or acquisition maybe in modern terms?) lets you capture after a delay many times the length of sample memory at max sample rate. The hardware counts off time during the programmed delay, bit doesnt waste sample memory until it is time.

author: Performa01
author_rank: Super Contributor
timestamp: April 17, 2016, 04:33:23 pm
content: Quote from: femtohertz15 on April 17, 2016, 04:15:21 pmI've yet to come across a DSO that wouldnotdo exactly that. Also 'Zoom' on a DSO with deep memory works exactly as it did on the analog scopes. Btw. my analog scope also has a 10-turn pot and a nice 3-digit digital readout for the delayed 2nd timebase

author: Kilrah
author_rank: Unknown
timestamp: April 17, 2016, 05:04:05 pm
content: My old Philips PM3214 doesn't have the digits or anything digital at all, but has the 10-turn potShould take it out again one day.

author: femtohertz15
author_rank: Contributor
timestamp: April 17, 2016, 08:09:01 pm
content: Love it!Turn, turn, turn, click!I have learned: zoom wtf out to set up a large delay on a new digital, rough the delay, zoom in.Did work on rigol 100ms delay to 5ns/div.

author: houkensjtu
author_rank: Contributor
timestamp: April 18, 2016, 01:13:25 pm
content: I have updated the photo link with the wave form I captured by using "delayed time base". See the update information in my original post.