Onboard Computer Serial Ports
From MobileRobots Research and Academic Customer Support
Most onboard computers have two or more serial ports.
The first serial port (COM1 on Windows, /dev/ttyS0 on Linux) has been connected directly to the robot microcontroller.
If you have a SICK laser, it has been connected to the computer (typically the third serial port, i.e. COM3 or /dev/ttyS2) via the SICK Laser Integration Board, which also turn the laser on when the serial port is opened and turns power off when closed (this ensures that the laser is reset to a correct state between uses).
For details of configuring serial port settings in the computer operating system see Computer Serial Port Configuration.
The external serial port on the external robot control panel (side or top panel) marked HOST is connected to the internal robot microcontroller; it is not connected to the onboard computer. Since if you have an onboard computer, the robot microcontroller is connected both to the onboard computer's control panel, as well as to the external HOST connector, do not use the external HOST connector while the onboard computer is using its serial port. To be sure that they are not in conflict, you can turn off the onboard computer if using the HOST connector to update firmware etc.
Some robot controllers (Pioneer 3, AmigoBot) also have auxilliary serial ports available for accessories or users. See Robot Aux Serial Ports.