Smoothieboard has an UART port.
This is a hardware "serial" port independent from the main "USB" serial port.
You can connect to this serial port for example using a FTDI "serial to USB" adapter.
This port is used at boot time to send a lot of debugging information from the smoothieboard to you, if you are running into trouble, this can sometimes be useful as errors and warnings are displayed there.
Once this is done, you can then use the UART port the same way you would use the USB/Serial, or the Telnet port : you send it commands or G-codes, and you get answers.
You configure the baud rate for the UART port in the configuration file by changing the uart0.baud_rate configuration option.
On linux machines the output of the serial debug port is not nicely formated because the smoothieboard sends only a LF but a CRLF is needed.
picocom offers a parameter to fix that:
picocom -b 115200 /dev/ttyUSB0 —imap lfcrlf
The following software will give you a basic terminal interface to communicate with the Smoothieboard over UART :