Ethernet switch design questions - Page 1
Forum section: electronics / projects

author: enz
author_rank: Regular Contributor
timestamp: June 28, 2018, 07:30:21 am
content: Hello,I could use some help regarding the design of an ethernet connected device.We are developing a consumer device which will be connected to an ethernet cable.To give the customer the oppurtinity to still connect another device and not loose the port on his switch, we want to provide two ethernet ports at the outside of our device, to "pass through" the LAN signal.So we need a 3-port switch, two ports to the outside and one to our internal ethernet module.And this is where my questions start:We considered something like a Microchip KSZ8863RLL 3-port switch and a Microchip KSZ8081RNA PHY.Is this a sensible choice, or do you have any other suggestions?The KSZ8863 does have two 10/100BASE-T/TX transceivers and one RMII interface, hence the need for the KSZ8081RNA PHY.We would use the two transceiver interfaces for the connections to the outside and the KSZ8081RNA PHY for the connection to our ethernet module.But, what I don't like is the fact that we would need two ethernet transformer to connect the PHY to our module.Is this really necessary for maybe 10cm of pcb track length?The ethernet module is already specified and not replaceable.It provides only the four signals ETH_TRXP0, ETH_TRXN0, ETH_TRXP1, ETH_TRXN1 which would normally be connected directly to a Mag-Jack.Would it be possible to connect the ethernet module ac-coupled to the PHY, without using transformers?The parts are all powered from the same power supply on the same circuit board.Thanks in advance for your answers.Martin

author: capt bullshot
author_rank: Super Contributor
timestamp: June 28, 2018, 07:36:14 am
content: For this kind of application, the ethernet transformes can be left out and replaced by small capacitors (for AC coupling).There are some application notes around related to a particular PHY, but this works in general with almost any PHY.The circuit is simple:Route the TX pair of one side to the RX pair of the other side and put a 100n cap in series with each signal. Keep the termination resistors as specified by the PHY datasheet at each side. Depending on the PHY, ensure that a supply voltage is fed to the RX / TX pins through the termination resistors.

author: shebu18
author_rank: Frequent Contributor
timestamp: June 28, 2018, 07:37:26 am
content: Hi enz,you can take a look at this two documents:http://www.ti.com/lit/an/snla088a/snla088a.pdfhttp://ww1.microchip.com/downloads/en/AppNotes/Capacitive%20Coupling%20Ethernet%20Transceivers%20without%20Using%20Transformers.pdfThey describe how to connect two ethernet PHYs without the magnetics.Cristian

author: Siwastaja
author_rank: Super Contributor
timestamp: June 28, 2018, 07:38:23 am
content: Don't build a 10/100 switch in 2018.Ifanyonehas a wired ethernet today, it's a gigabit ethernet, and in many cases, they really want or need it to be that way.This means the customer doesn't benefit from the "pass through" feature of your 100M switch, since its' going to be an unused feature. In addition, it's going to generate confusion and disbelief.So IMHO, either get rid of the passthrough idea, making your device cheaper, smaller and simpler, so that people can use an external switch (like they have to do with your 10/100 device anyway), or implement it properly (i.e., gigabit).Your own part of the circuit can, of course, use a 10/100 PHY.

author: enz
author_rank: Regular Contributor
timestamp: July 11, 2018, 09:05:45 am
content: Hi guys,thanks a lot for the quick responses, very appreiciated.Sorry for the delay on my site, but I was on a business trip.Quote from: capt bullshot on June 28, 2018, 07:36:14 amThanks for the detailed information. Thats how I assumed it should work, but I wasn't shure.Quote from: shebu18 on June 28, 2018, 07:37:26 amThanks for the links. Exactly what I was looking for. Somehow I didn't found them myself.Quote from: Siwastaja on June 28, 2018, 07:38:23 amValid point. But in our case there is no need for the connected device to have gigabit ethernet and ports are often rare because in many cases the available ethernet connection is only done by a powerline adapter. These often only provide two ports. So in this environment the "pass through" makes sense.Thanks for your help + best regards,Martin

author: Whales
author_rank: Super Contributor
timestamp: July 11, 2018, 09:48:21 am
content: What sort of data rates are you going to be transmitting? If it's 10/100 and you're only doing tiny bits of data then you might be able to get away with making a passive hub.EDIT: bugger even that, if it's 10/100 then you cando the good ol split of wires.

author: shebu18
author_rank: Frequent Contributor
timestamp: July 11, 2018, 10:24:14 am
content: If I'd use cables i could do that, but it will be on the same PCB, currently 170x170mm.

author: dmills
author_rank: Super Contributor
timestamp: July 11, 2018, 07:30:01 pm
content: Why are you even messing around with a PHY, just go RMII or whatever out of your processor into that set of pins on the switch chip, job done (Might need to add a little EEPROm with a mac address, but that should be about all).Putting two PHYs on a ethernet board a few centimeters apart is seriously doing things the hard way.Regards, Dan.

author: Whales
author_rank: Super Contributor
timestamp: July 12, 2018, 06:48:42 am
content: Quote from: shebu18 on July 11, 2018, 10:24:14 amA PCB is just wires. Wires that have been put onto a flat piece of board.