Author Topic: Ethernet Card as Logic Analyzer?  (Read 1702 times)

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Offline Luu

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Ethernet Card as Logic Analyzer?
« on: December 18, 2016, 02:59:59 pm »
I was thinking, could a 1 Gbps Ethernet Card be used as a Logic Analyzer?

I get that one might need to write a custom firmware for it to not discard non-ethernet data and you'd have to translate signals to -1V - +1V before handing it to the card. But just speed wise wouldn't the hardware on an Ethernet Card be able to process the incoming pixies really fast?

I know some cards have a feature where they can detect cable problems. Like breakage in a cable, in what distance from the card the break occurred and in what wire. They would need to have some kind of raw access to do that I suppose?!

Do you know of any obvious problems that would prevent such use?

Do you know what kind of low level access Linux currently provides on ethernet cards? I.e. what's the lowest level exposed to userspace programs and what's the lowest a device driver typically sees? Is some decoding done on the card itself already or can you get true raw access to each individual wire state?
« Last Edit: December 18, 2016, 03:03:18 pm by Luu »
 

Offline lapm

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Re: Ethernet Card as Logic Analyzer?
« Reply #1 on: December 18, 2016, 03:15:16 pm »
What you want to logic analyzer is capture state of several signals in parallel. So etherned card would give you just one channel.

Or do you intent to do parallel to serial conversion witch would drop our real capture speed way belove that 1Gbps...

not to mention captur triggers implementations when using repurposed ethernet card...
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Offline stj

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Re: Ethernet Card as Logic Analyzer?
« Reply #2 on: December 18, 2016, 04:22:16 pm »
i'm pretty sure the packet framing and error correction are in the hardware and you cant get raw aquisition.
even if you could, they arent really gigabit because the pc side is not that fast.

it's assumed that there is more than one pair of cards sharing the net, so the gigabit speed is split between them.
 

Offline helius

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Re: Ethernet Card as Logic Analyzer?
« Reply #3 on: December 18, 2016, 05:23:32 pm »
1000baseT uses four balanced pairs in parallel with a symbol rate of 125 MHz, with deskewing and error correction in hardware. The coding is not binary; it uses five different voltage levels that are input to a descrambler before being stored. This is remarkably unlike a logic analyzer's acquisition section.

The TDR feature uses the clock pulse hardware that is already required: such a feature would not be implemented if it required additional hardware. All it does is send a clock pulse over one pair and listen for an echo.
« Last Edit: December 18, 2016, 05:30:53 pm by helius »
 

Offline radar_macgyver

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Re: Ethernet Card as Logic Analyzer?
« Reply #4 on: December 18, 2016, 06:04:41 pm »
Also, Ethernet is transformer coupled; the high pass effect makes LSA applications rather limited.
 

Offline evb149

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Re: Ethernet Card as Logic Analyzer?
« Reply #5 on: December 18, 2016, 06:32:37 pm »
The only good way would be to use the ethernet link as a cheap / "easy" transport of data into the PC.
To capture the data you would use at least an ethernet enabled MCU with somewhat synchronously capturing GPIO input pins or (preferably) a FPGA/SOC with the appropriate types of input pins.

Then you'd just packetize the streaming input data and send it over ethernet.
But with ethernet being limited to 1Gbit/s per port in common PCs the bandwitdh would be worse than using superspeed USB as a transport, and there are easier interface ICs for superspeed USB than Gb/s ethernet for the purpose of streaming logic capture.

 
 

Offline cdev

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Re: Ethernet Card as Logic Analyzer?
« Reply #6 on: December 20, 2016, 09:22:33 pm »
You can use an Altera FX2 series dev board as a sigrok compatible logic analyzer. Some of them are extremely inexpensive.  See sigrok.org
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