Download MX Component from the Mitsubishi Homepage.
there are COM .dlls included to connect via COM/USB/EtherNet to the PLC
they are very stable to use and much easier as writing that communicating stuff on your own. (done that)
very basic example:
Dim _FX_COM As New ACTPCCOMLib.ActFXCPU
_FX_COM.ActBaudRate = 115200
_FX_COM.ActControl = 7
_FX_COM.ActCpuType = 520
_FX_COM.ActTimeOut = 100
_FX_COM.open
Dim value as short = 123
_FX_COM.w_write("D100", value)