(This complex set up is no longer needed, newer instructions here)
The RPi has a serial interface called ttyAMA0 and this can be used as a general purpose serial port. On a 'vanilla' system though there is some setting up to do. This is described in the serial section of the RPi software installation page.
Important: One thing to be aware of is that the serial interface uses 3.3V logic and some 5V devices will output on their TX line 0 to 5V. This is taken care of by the devices on this site that have a SWS (Sideways Stackable Connector).
Minicom is okay to use but there is a miniterm.py which is more useful. It has been modified so that the defaults work with the sws devices and so it is simple to use. Also it is just a small Python file.
wget http://www.pin1.org/download/py/swsterm.py python swsterm.py
This is a screen shot of downloading swsterm.py and then running it, putty can be used as it is not a GUI.
Assuming that a relay is connected to the serial port typing fH<CR> brings back an ACK (6) result and fD returns the device ID followed by the ACK. 'f' is the default address of the relay (BV4601)
Sideways Stackable Serial Software
This software has been specifically designed for the RPi and can be installed following the instructions on the PRi software installation page.
To get the software working in an LX terminal type
This is not an error, the software at the moment cannot detect the serial port but it is there, simply type it in at the drop down.
Here after entering /dev/ttyAMA0 and pressing the red icon to turn it green a connection is established.
Here is an example of a typical connection to the RPi, the +V may not be needed but the +L should always go to the 3V3 pin
Pressing the Get Attached Devices button will relveal all of the devices on the bus. The text line needs to be double clicked to open the specific dialog for that device.
For further information see the specific device.