I have a scx-21 gps compass. I dont have gp39 but I want to connect it on pc by a usb to rs422/485 converter name is moxa uport 1130. I use to connect several times but I couldnt. so I take a warning windows that says ^^ failed to connect to the antenna unit^^ I try with connect td1-a, td1-b and rd1-c, rd1-h cables