The Nokia n810 has a Serial Console connector under the battery. Unfortunately it's hard to access mechanically. Especially, if you don't want to alter your device by soldering cables on the pads.
The n810 has pads for power, USB and serial on the backside. There are more pins, but I don't know the function of these pins, yet.
The USB pads are the same port as the external USB connector. Most likely they are only used for convenience in Nokia's proprietary docking station.
The power pads allow powering the device without a battery installed. The + and - pins require a voltage of about 3.8V DC. To externally power the device without a battery installed, a 120k resistor needs to be connected between BSI and -. Otherwise the device will shutdown immediately on boot. The power supply must be able to draw up to 1A of current.
The serial port is located on the tiny 7 pad connector under the battery. It is a 115200 baud UART operating at 3.3V TTL level. You must not connect this directly to your standard RS232 interface on your computer. That will destroy the n810. A TTL level converter is needed between the n810 and your RS232. I use a MAX3232 level converter chip.
The serial console has to be enabled in software before it can be used. This can be done using the 0xFFFF utility. At least version 0.3.2 is needed. Read the documentation of that software before using it! After you read the documentation, you can enable the serial console by connecting the USB cable and entering this command:
sudo 0xFFFF -r 1 -f 0x10
After that switch the n810 device on. 0xFFFF will detect it and change the parameters. You can use
sudo 0xFFFF -R
to reboot the device afterwards.
Nokia doesn't sell the proprietary docking station for the device, so if you want to access it without altering it, you need to build one by yourself. Brass wires can be used to touch the pads.
Here are a few pictures of my docking station: