bunun için mscomm.dll diye bir kiatplık var onu yüklemen lazım
sana örnek bir port okuma yazma kodu


porta bilgi gönderme


Private Sub Command1_Click()
With MSComm1
'make sure the serial port is open
If .PortOpen = False Then .PortOpen = True
'send the data (including a tailing carriage return as often needed)
.Output = Text2.Text & vbCr
End With 'MSComm1
With Text2
'place the focus back to the textbox
.SetFocus
'select the current text to be overwritten
.SelStart = 0
.SelLength = Len(.Text)
End With 'Text1
End Sub

portu açma

Private Sub Form_Load()
With MSComm1
'make sure the serial port is not open (by this program)
If .PortOpen Then .PortOpen = False
'set the active serial port
.CommPort = 2
'set the badurate,parity,databits,stopbits for the connection
.Settings = "9600,N,8,1"
'set the DRT and RTS flags
.DTREnable = True
.RTSEnable = True
'enable the oncomm event for every reveived character
.RThreshold = 1
'disable the oncomm event for send characters
.SThreshold = 0
'open the serial port
.PortOpen = True
End With 'MSComm1

End Sub

porttan gelen bilgiyi okuma


Private Sub MSComm1_OnComm()
Dim strInput As String
With MSComm1
'test for incoming event
Select Case .CommEvent
Case comEvReceive
'display incoming event data to displaying textbox
strInput = .Input
Text1.SelText = strInput
End Select
End With 'MSComm1
End Sub