I am designing a PCB that will use a Microchip KSZ9896C 6-port Gigabit Ethernet Switch. In the datasheet (section 7.2) they state that :

"the four transformer center tap pins on the KSZ9896C chip side should not be connected to any power supply source on the board; rather, the center tap pins should be separated from one another and connected through separate 0.1μF common-mode capacitors to ground. Separation is required because the common-mode voltage could be different between the differential pairs, depending on the connected speed mode."

I want to connect the switch to a 2x2 Gigabit modular jack. The confusion I am having is that most, if not all, ganged modular jacks have the transformer center tap pins from the common mode filter connected together and sent out one pin (for each individual port). Even if the pin is left unconnected, all of the center taps are tied together in the jack and, therefore, not isolated. Correct?

I am totally new to Ethernet magnetics so I am trying to figure out if a 2x2 ganged jack will work. Any help?

KSZ9896C : http://ww1.microchip.com/downloads/en/DeviceDoc/00002390A.pdf

BelFuse 2x2 Gigabit MagJack : https://belfuse.com/resources/drawings/magneticsolutions/dr-mag-0879-2c2r-54.pdf

  • \$\begingroup\$ Welcome to EE.SE. No, that transformer will not work, because the IC side has the center-taps tied to a common point, often a bias point for other physical layer IC's. You need one with no taps on the IC side or isolated taps you can ignore. \$\endgroup\$ – Sparky256 Feb 2 '18 at 1:06

You need to change the magnetics your using or the Ethernet physical layer IC to one that uses the center tap for bias current. The KSZ9896C uses a differential output and a common center tap could make it not work.

I realize these parts are not cheap, but you cannot mix apples and oranges. Check the datasheets first before buying. They must be an exact match.

  • \$\begingroup\$ Thanks again Sparky256. I have the initial prototype board with the PHY and 2x2 jack. I am currently testing, verifying and making a list of required changes for the upcoming spin. I went ahead and tested the functionality of the ports at all of the different speeds and , so far, 2 ports are bridged through the PHY and running at GbE, one port is running at 100Mbps and one port is running at 10Mbps. All ports have been running for about 40hrs without any packet loss/errors. \$\endgroup\$ – ErikC Feb 7 '18 at 15:38
  • \$\begingroup\$ I would love to follow the datasheets requirement but : \$\endgroup\$ – ErikC Feb 7 '18 at 15:40
  • \$\begingroup\$ Arg...I was not done.... I would love to follow the datasheets requirement but : 1. Having a 2x2 RJ45 modular jack is a hard requirement 2. I cannot find any 2x2 jacks with the required magnetics. 3. Discrete magnetics are way too big for the board. Changing the board size is not an option. 4. Changing the PHY chip is not an option. So...I am stuck with what I got. Are you able to help me understand, electrically, why it seems to work with the grounded but non-isolated CT and under what conditions it may fail? \$\endgroup\$ – ErikC Feb 7 '18 at 15:48
  • \$\begingroup\$ Sorry, but I would need a setup like you have to judge non-standard functions. \$\endgroup\$ – Sparky256 Feb 7 '18 at 15:53

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