Quoting the Lego Mindstorm with Linux Mini-HOWTO by Luis Villa:" In case you don't know, the Lego Mindstorms Kit is a robotics kit from The Lego Group that retails for about 200 US dollars. For that, you get a lot of Lego pieces, a large brick containing a CPU, an LCD, and some connectors (known as the RCX), a couple of motors, and some light and touch sensors that allow you to interact with the outside world. ..."
"All communication to the RCX is done via the IR tower, which is connected to the machine via a serial port. As a result, if you have no serial port connection, you will be unable to use the RCX unless you can buy an adapter. Furthermore, under certain circumstances, there may be problems with IRQs or serial port conflicts. This is particularly likely if your modem uses /dev/ttyS0."