Marc.S
Posts: 2
Joined: Mon Aug 02, 2021 8:54 am

capacitive coupling ethernet

Fri Aug 05, 2022 9:04 am

HI,

I have made a carrier board for the CM4. on this board I also integrated a 5 port switch KSZ8995MLI. with 4 external ethernet ports and 1 internal port for the CM4.
The internal port is connected to the switch without magnetics, these are replaced with some capacitors of 100nF and 50ohm pull-up resitors on each wire.
The incoming signal to the CM4 module is working fine. the outgoing pair works but is extreme slow.
I get some VNC reaction but in it does not show a complete scree, it times out before it can complete.
If I look at my local screen I can use the mouse and keyboard via VNC. and start programs on the PI normally. use the browser and look at some youtube movies.
It looks as if my termination on the driver side is not working as expected. Does anybody has any information about the BCM2711 ethernet hardware inside the chip?
Thanks for any support.

pidd
Posts: 3703
Joined: Fri May 29, 2020 8:29 pm
Location: Wirral, UK

Re: capacitive coupling ethernet

Fri Aug 05, 2022 11:12 am

The use of capacitors is going to limit the lower frequencies of the spectrum (ethernet has a very wide bandwidth/spectrum).

You could try increasing the value of the capacitors and see if that helps.

For want of a better diagram ....

Image

I am assuming you are using the capacitors as dc isolation?

dp11
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 899
Joined: Thu Dec 29, 2011 5:46 pm

Re: capacitive coupling ethernet

Fri Aug 05, 2022 11:21 am

Show us your schematics.

Marc.S
Posts: 2
Joined: Mon Aug 02, 2021 8:54 am

Re: capacitive coupling ethernet

Fri Aug 05, 2022 1:22 pm

This is the LAN part of my carrier bo
cm4_lan.png
CM4 to KSZ8995MLI
cm4_lan.png (101.22 KiB) Viewed 235 times
ard

dp11
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 899
Joined: Thu Dec 29, 2011 5:46 pm

Re: capacitive coupling ethernet

Fri Aug 05, 2022 1:40 pm

On the CM side try removing all the stuff bar the coupling capacitors. For the KSZ side try reading http://ww1.microchip.com/downloads/en/A ... ormers.pdf

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