Serial adapters/Arduino

How to use an Arduino Uno as a serial adapter for an OLPC XO laptop.

Ingredients

an Arduino Uno, called the Arduino,
a computer running the Arduino development environment, called the host,
a USB cable to connect the Arduino to the host, normally supplied,
an OLPC XO laptop, disassembled to expose the serial connector, called the target,
a way to connect wires between the Arduino and the target (jumper leads, solder, iron, connectors, etc).

disconnect all wires from the Arduino,
connect the USB cable between the Arduino and the host,
download passthrough-serial-adapter.ino and program it into the Arduino,
on the host, start a serial terminal application like screen, and verify that pressing keys causes the D13 LED to flash,
connect a GND pin on the Arduino to the GND pin on the serial connector of the target,
connect the D0 (RX) pin on the Arduino to the RX pin on the target; this carries keys from the host to the target,
connect the D1 (TX) pin on the Arduino to the TX pin on the target; this carries text from the target to the host,
turn on the target, and the boot text should be displayed,
press Escape key, and the Ok prompt should appear.