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In an answer to one of my previous posts seen here: Verifying 802.3 auto negotiation, @Zac67 mentions that by using an oscilloscope I could observe normal and fast link pulses that are sent in order to determine ethernet data rates of newly connected ethernet devices. How could this be done?

I have a basic understanding of oscilloscope functions. Would a logic analyzer work as well? Thanks for your help.

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  • \$\begingroup\$ Ethernet will show up as quasi-random noise on a conventional oscilloscope. The packets are variable rate and length, so nothing recursive to lock onto. You are fooling yourself if you think a scope will substitute in any way for a packet analyser. You can lease one for about $1000 USD per month. \$\endgroup\$ – Sparky256 Jun 14 '18 at 21:56
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    \$\begingroup\$ @Sparky Not so. For any given Ethernet wire that has completed auto-negotiation (or is at a fixed speed selection) the symbol rate is NOT variable but fixed. (see this great answer: electronics.stackexchange.com/questions/151112/…). The vast majority of packet analyzers will not capture auto-neg pulses, and trying to sync to them requires both packet capture and external sync of the oscilloscope. \$\endgroup\$ – Jack Creasey Jun 14 '18 at 22:18
  • \$\begingroup\$ @JackCreasey. I stand corrected then, but for fast Ethernet a high-performance oscilloscope is needed, and the symbol rate should not be changing. \$\endgroup\$ – Sparky256 Jun 14 '18 at 22:29
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You need: A scope with (most likely) 500Mhz bandwidth and 1 Gigasamples/s
A differential probe with (most likely) 500Mhz bandwidth (You could go lower on the speed if you only want to look at 10Mpbs, size your probes appropriately to be faster than the speed you want to look at)

enter image description here
Source: http://www.learnabhi.com/ethernet-cable-categories-cat1-cat2-cat3-cat4-cat5-cat5e-cat6-cat7-cat8/

Then you wire up the orange pairs like this, and enter image description here Source: Tektronix vid with annotations

After that you can set the trigger level and decode it by hand. If you want to pay more some of the newer scopes have software features to automatically decode Ethernet packets (like an MDO3000).

A logic analyzer might work (I have not tried this) if it had a differential probe, all the logic analyzers I know of have a 0V to 30V span (or some upper limit) which would make it hard to detect the differential signalling of Ethernet.

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  • \$\begingroup\$ thank you for your detailed answer, I’ll get back with you once I have performed your steps. This may take a while. \$\endgroup\$ – Niko_Jako Jun 15 '18 at 17:49

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