Applications that use the serial port should provide the user with a means to select the name and unit number of the driver. The defaults will be "serial.device" and unit number 0. Typically unit 0 refers to the user-selected default. Unit 1 refers to the built-in serial port. Numbers above 1 are for extended units. The physically lowest connector on a board will always have the lowest unit number. Careful attention to error handling is required to survive in a multiple port environment. Differing serial hardware will have different capabilities. The device will refuse to open non-existent unit numbers (symbolic name mapping of unit numbers is not provided at the device level). The SDCMD_SETPARAMS command will fail if the underlying hardware cannot support your parameters. Some devices may use quick I/O for read or write requests, others will not. Watch out for partially completed read requests; io_Actual may not match your requested read length. If the Tool Types mechanism is used for selecting the device and unit, the defaults of "DEVICE=serial.device" and "UNIT=0" should be provided. The user should be able to permanently set the device and unit in a configuration file.