I'm attempting to add an RJ45 Ethernet jack to a commercial device without one by replacing an embedded WiFi module, which, according to its datasheet, is connected to the rest of the system as follows:


The section labeled "PHY" here is a PHY on the device (on a Micrel Ethernet switch chip elsewhere on the board), while the section labeled "HF-A11-SMT" is the WiFi module I'm attempting to replace with an ethernet jack.

I'd like to be able to connect the jack directly to a regular ethernet port on a computer, so I need some way to add a jack and magnetics, or a MagJack-equivalent, to the board without modifying any of the circuitry supposed to connect "transformerless" to the WiFi module.

There's plenty of examples out there for how to hook up a MagJack if I was able to remove the "transformerless" circuitry in the schematic above and connect directly to the Micrel chip, but since I haven't been able to fully trace this part of the board (it's quite a complicated design, and there's a lot of other unrelated stuff around this module that make trying to trace it difficult) and I'd like to interfere with the rest of the device as little as possible, I'm looking for a way to connect a regular RJ45 jack with magnetics to the pads meant for the WiFi module's RX/TX pins ("transformerless" circuitry intact).

I can't find any information on how adding a jack to a design like this might work. If there's either a circuit that could do this which I could design a simple board for, or some sort of off-the-shelf version of the same for testing devices like these, what are they? If not, what's the minimal amount of modification that would have to be made to the "transformerless" circuitry in order to make this feasible?


1 Answer 1


The normal and only way to make it by keeping the signals within required levels would be to remove caps and the 49.9 ohm resistors at the wireless module side so that magnetics are directly connected to only single set of termination and the Micrel PHY.

For test purposes, I have put transformers and 8P8C jack without termination after capacitive coupling and it works with some devices and not with others, so I can't recommend it.

So at minimum, just put a magjack in place of the wireless module.

The capacitive coupling is only meant for special links between boards, so the Micrel chip may have specially configured settings for this being a capacitive interface - usually capacitive interfaces are forced on with 100Mbps full duplex settings, so there is no auto-negotiation available, as autonegotiation and 10Mbps does not pass well over the caps. If this is true, and you can't change those settings, it may not work well with standard devices.

If you really need to make it work, an overkill solution would be to replace the wireless module with module that has a switch chip that has one true magjack port for the actual Ethernet and one port configured for the capacitive link.

  • \$\begingroup\$ Okay, sounds good. Could just putting a magjack in place of the module do any harm to the device, or would it just not work? I would think the capacitors would still isolate it? Sounds like the easiest thing to try first, but if it's risky, I don't want to kill my device. \$\endgroup\$ Commented Sep 22, 2019 at 20:36

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