The connected ANSI/VT100 terminal or terminal emulator should be set for 19200 baud, 8 databits, no parity, 1 stop bit, no flow control.
To use these settings in GNU screen, type:
$ screen /dev/ttyUSB0 19200 8N1
/dev/ttyUSB0 is the name of the serial device in this case. You probably have to run that command as root to connect to the serial device.[---ranting below, ignore ---]
Incidentally, the Soekris net5501 manual is an example of how a manual should be written for a real computer. After reading it I am actually reasonably informed about the machine, and have a good idea of what I can do with it. It's great!
A typical consumer device, on the other hand, would come with a half-page color brochure showing happy people standing next to the device with all its lights blinking, and a CD full of adware.