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I have a device that works on 100 Mbps using a CAT5E cable.

Any idea of how can I show the data communication (capturing the signal integrity eye pattern) of this device using an oscilloscope (4 GHz/ 40 GS/s)?

  • I can connect the probes to the inner cables of the CAT5e cable.
  • I have differential probes.
Transistor
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anxiousPI
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    What do you actually want to do? Capture the actual data, and decode the protocol, or analyze analog signal integrity? – AndrejaKo Feb 06 '20 at 13:44
  • What is the bandwidth of the oscilloscope? – Scott Seidman Feb 06 '20 at 13:53
  • i want to capture the actual data, bandwidth is 4Ghz, 40GS/s – anxiousPI Feb 06 '20 at 13:54
  • For 100Mbps, you can capture the actual data just by splitting the cable (like with a [throwing star tap](https://shop.hak5.org/products/throwing-star-lan-tap)) however you should be aware that it will degrade the signal quality so keep the wires short. – user253751 Feb 06 '20 at 14:21
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    If one of the devices is a PC, you could also run Wireshark on it. Or you could route the communications through a PC, by setting the PC to forward packets back and forth between two connections (this is called a *network bridge* in most OSes) and then running Wireshark on it. – user253751 Feb 06 '20 at 14:22

2 Answers2

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An oscilloscope is the wrong tool to view ethernet data. Unless you expect signal integrity problems. Then you need the scope.

The best tool for the job is a switch with port mirror and a spare network card on a PC to run pcap with Wireshark.

You can also use a ethernet hub, but you can't buy those anymore. So unless you have from the 90's, you're better of buying a managed switch/router with port mirror capability.

eg: Mikrotik RB260GS or RB750r2.

Jeroen3
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  • i want to check for signal integrity issues. ethernet data is not observable like RS232 data for example with an oscilloscope? – anxiousPI Feb 06 '20 at 14:20
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    @anxiousPI First you said you want to see the actual data, now you say you want to see the signal integrity? – user253751 Feb 06 '20 at 14:22
  • @anxiousPI You'll need a 2 GHz scope with the appropriate probes (unless scope has independent adcs). You won't be able to see the actual frames, only the signal. I'm not aware of any products that can do both. – Jeroen3 Feb 06 '20 at 14:23
  • @user253751 sorry for the misunderstanding, I meant I want to visualize the data transfer electrically. I don't care about what packet protocol is sent and if packets are received, etc.. – anxiousPI Feb 06 '20 at 14:28
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    @user253751: You need to edit your question so that readers don't have to trawl through the comments. The title says "data" where it should be "signal". – Transistor Feb 06 '20 at 14:56
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You can just chop a cable in half, carefully wire it to a breakout (keep the exposed wire straight and short as possible), and put the scope probes across either the TX or RX pair depending on which direction you want to look at.

I have done this myself to debug an autonegotiation problem, which fortunately is done with lower speed data.

pjc50
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  • How was it connected? I did this already but the data I got did not make sense. I connected 1 probe to the Rx and another to the Tx, AC coupling with subtracting the 2 channels but nothing. – anxiousPI Feb 06 '20 at 16:12
  • Is it because when the link is up, there is constant IDLE character being sent, with scrambling? – Justme Feb 06 '20 at 19:18