yes I am connecting the board with the 8051 chip to the computer using the USB port and then connect the other side of the board to the external device using the serial port.
The 8051 chip I am using is called the CY7C64613-128NC EZ-USB microcontroller which is made by cypress.
I actually need to program the chip to control an external device that has 8 toggle switches and 8 LEDs, which means the chip will read the inputs from the toggle switches and then send out digital out to the LEDs.

1). computer--USB to burn data into 8051 chip-->8051 chip
2). toggle switches of external device (digital in)--serial port-->8051 chip--serial port--> LEDs of external device (digital out).

I hope this is clear enough.

thanks