Power logic or powe meter to Visual basic

This sample application VB for monitoring Diris A20 power meter vendor by SOCOMEC,I use Modbus RTU protocol to communication between Diris A20 and VB.
first make Vb interface below:

write The source code below:
Dim CRCTable(0 To 511) As Byte
Dim CRC_Low, CRC_High As Byte
Private Sub CRC_16(ByVal Data As String, Length As Integer)
Dim i As Integer
Dim Index As Byte
CRC_Low = &HFF
CRC_High = &HFF
For i = 1 To Length