Recently, I got a new Asus RT-N12. As an electrical engineer, I like to modify electronic devices for fun.
For a little background info, I started playing with electronic kits when I was in first grade, started working with OpenWRT in 11th grade, and I'm currently a senior at Texas A&M University. Therefore, I'm very familiar with how to design and modify circuits.
On the routers I have worked on before, I installed a serial console port since it is extremely useful for debugging. On the previous ones I have worked on (two F5D7230-4s and a WRT54G), there was more than enough room on the back for a proper serial console port. But not on the RT-N12. And the case was just too beautiful for me to just cut a hole for the port elsewhere.
After a little thinking, I realized that only 4 wires are used in a standard 10/100 Ethernet port. That leaves 4 wires that I can use for a serial interface. Therefore, I can add a serial port to one of the Ethernet ports without losing any functions. I found an adapter to convert the 9 pin serial connector to RJ-45, originally intended for the console port on Cisco equipment. The pinout would interfere with the Ethernet pinout, so I made an adapter to solve that problem.
Here it is:
That's how to have your cake and eat it too. It would work on any router without Gigabit. (And yes, there are a few other "toys" I work with in that picture. I'm on winter break and the temporary "lab" I set up at home is a little crowded.)
I just finished drawing the schematic:
Note that TX, RX, and DTR on the RJ-45 is from the device perspective. They would be wired to RX, TX, and DSR/CD on the serial port. Serial settings are the common 115,200bps 8N1.
I just wired it up freehand. After testing to make sure it works, I covered it in sealant to protect it from vibrations.