The version 1.08 board used a USB interface chip for serial communications with a PC. This interface was dropped as the IC was unavailable in a DIP package and the USB headers were very fine pitch and deemed not rugged enough for hobby use.
Version 2.0 adopted the use of a MAX232 IC. This chip converts the 5 volt signals from the microcontroller to the voltage levels required for RS232 communication with a PC. Since most laptops no longer have a serial port, it in necessary to use a USB to Serial converter cable.
Headers are included on the sides of the MAX232 IC for debugging purposes.