Problem to program ATxmega256 whith command line


I want to program my µc ATxmega256A3 whith line command using STK600, so i write "STK600.EXE -dATxmega256A3 -cUSB -e -ifRC1.hex -pf -vf" But i have always an error : "Could not connect to STK600 on USB". Idon' know what's the problem. If you have an idea please to answer me .