interface object "USB_RD_N" of mode out cannot be read

In theory a buffer is different from an out port, in practice the compiler may treat them alike (although our friends at Xilinx warn us : http://www.xilinx.com/itp/xilinx4/data/docs/sim/coding4.html (http://www.xilinx.com/itp/xilinx4/data/docs/sim/coding4.html)). Peter Ashenden and Jim Lewis explain the subtle difference in their book: "VHDL-2000-8 Just The New Stuff", pages 162 to 165.