0) both RD+ and C51 write 0xFF to P1
1) RD+ clears D0 line:  11111110b   ; 0xFE
2) C51 clears D7 line:  01111110b   ; 0x7E
3) RD+ realeases D0:    01111111b
      and clears D1:    01111101b   ; 0x7D
4) C51 releases D7:     11111101b   ; 0xFD
5) RD+ releases D1:     11111111b   ; 0xFF

now goto 1)