I have been searching on how to connect two Ethernet switch ICs together. I have designed a custom PCB. Beside some other features, it accommodates an Ethernet Switch IC. This works OK. The bandwidth I use is not more than 100 Mbps.

It is time now to expand Ethernet ports by connecting two Ethernet switch ICs together. They will be very close to each other, a few inches perhaps.

*IC's datasheet: RTL8306E*

I have walked through the datasheet but I am stuck at the 6th port. RTL8306E has 6 ports. I use 5 ports of it as they have Rx+- and TX+- pins.

In order to connect to the other RTL8306E Ethernet switch, I have only port 6 at hand. But this has different interface so called MII.

enter image description here

And some configuration table from the datasheet:

enter image description here

Q1: Is it possible to use MII interface between RTL8306E Ethernet switch? If so, how?

Q2: If not, can I turn port 6 MII interface into a regular port like the others on the IC? So it would be easier to connect it to any Ethernet switch either by magnetics or without magnetics or just capacitor coupling.

Since I am new to those, I like to hear your opinions and experiences.

Any input greatly appreciated.


1 Answer 1


Q1: It's difficult to say with only the datasheet. There might be application notes about this. If you want the largest amount of RJ45 copper PHY ports, P4 should be left with UTP, so port 5 is available for MII. As you can see, the MAC5 can be set to MAC mode MII or PHY mode MII. So either they both should be in MAC mode with crossover connection, or maybe one in MAC mode and other in PHY mode. But it all depends on the details how it works in MAC and PHY mode.

Q2: If direct crossover with MII connection is not possible, you can add a PHY chip with MII connection to both chips for an extra copper PHY port between them, and it should be possible to connect them together without the magnetics. Most PHY chips have datasheets and application notes to describe if they support transformerless operation and if they suppport a DC coupled link or AC coupled link.

  • \$\begingroup\$ Thanks a lot for your input. For the Q1, I contacted to Realtek for some application notes but you don't want to deal with their bureaucracy. I still wait an answer from them. They don't just give away any information other than datasheet. For Q2, as you mentioned and I have also discovered recently, I need to use an additional PHY chip. Although this looks a bit extra cost, I don't think 20 pins MII connector and ribbon cable will be any lesser than PHY chip and a smaller connector. \$\endgroup\$
    – Sener
    Jul 15, 2020 at 11:01
  • \$\begingroup\$ Do you have any recommendation for a single port, 10/100 Mbps, MII <-> PHY with a reasonable cost? \$\endgroup\$
    – Sener
    Jul 15, 2020 at 11:04
  • \$\begingroup\$ And, one another question comes my mind that can we use any PHY supports MII from any manufacturer? Or how about this RTL8201F-VB-CG? \$\endgroup\$
    – Sener
    Jul 15, 2020 at 15:44

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