We are trying to connect two ethernet PHYs - KSZ8999 and AR8035. Here is the schematics.

enter image description here The schematics comply with the following document (figure 4) - SimpliPHY Transformerless Ethernet Designs - Microsemi The problem is that in one direction (from AR8035 to KSZ8999) link does not work (we use 100Base-TX). Though the signal is present on the link and looks pretty clear (status LED is on), and activity LED of KSZ8999 is blinking (as if some packets was captured), but they are not going up to the MAC layer (they are probably dropped because of errors).

We are pretty sure the problem is on the link layer. (We did a lot of experiments.) We can't use transformer. What may be wrong? How can we improve the performance of that connection?

  • \$\begingroup\$ Autonegotiation pulses do not pass capacitive coupling. Have you disabled autonegotiation, and forced both interfaces to use 100Mbps full-duplex mode, and forced the turned the interfaces on? \$\endgroup\$
    – Justme
    Jan 18, 2020 at 19:36
  • \$\begingroup\$ Yes, I put both interfaces in 100Base-TX mode and I turned off Autonegotiations. I can see MLT-3 signal on both diffpairs... \$\endgroup\$
    – Andy
    Jan 19, 2020 at 7:59
  • \$\begingroup\$ Will it be of any use if I put my capacitive coupling closer to AR8035 IC? \$\endgroup\$
    – Andy
    Jan 19, 2020 at 8:13


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