Uart connector pinout
The rpi_serial and ftdi_serial boards use a 4-pin connector.
The connector is laid out as follows:
|2||RXD||recieve Data (input)|
|3||TXD||transmit data (output)|
|4||VCC||Power 5V (or 3.3V).|
To connect the RPI_SERIAL to an ftdi_serial board, you'll need a null modem (crossover) cable. You can consider leaving the power line unconnected. (on both boards you can power a small project from this pin. For example an AVR processor can easily communicate with the RPI over the serial connection and at the same time be powered from the power pin.)
Connecting the BitWizard boards to an Arduino
|pin||function||arduino pin||Arduino Mega|
|2||RXD||D1||TXD on the arduino|
|3||TXD||D0||RXD on the arduino|
Note that there is also the FT232(*) USB-serial chip connected to these pins. To connect your arduino to the raspberry pi, we recommend you use USB.
On the other hand, the above setup will be useful if you then take the chip out of the arduino and build a project with the bare chip.
(*) It's an at90usb162 on newer arduinos IIRC.