Despatch Protocol Plus Modbus Communications - Manual (Page 48)

Chromalox Instruments and Controls
A-51643 Rev. 6 10/06/03 42
7 Programming Example and General Hints
This section provides an example of Microsoft Visual Basic
TM
code used to communicate with a
Protocol
TM
Plus controller. In addition, some general programming hints are provided.
7.1 Code Example
The following Visual Basic™ code example assumes that an MSComm control named ‗comMain‘ has
been placed on a form named ‗frmComm‘. The form also contains a command button named
‗Command1‘, and a text box named ‗Text1‘. Code is shown below in Bold type.
The example code sends a Modbus message to a Protocol
TM
Plus controller when the command button is
clicked. The text box is cleared when the command button is clicked, and displays ―OK‖ when a good
reply is received from the controller. The received message data byte values are also displayed in the text
box.
The following form declarations are made:
Option Explicit
Dim TxMessage As String
Dim RxMessage As String
TxMessage and RxMessage will be used as message data buffers for sending and receiving, respectively.
The following code procedure is also added to the form to calculate the Modbus CRC value:
Private Function GetCRC(MBuffer As String) As Long
Dim CRCValue As Long
Dim MLength As Long
Dim i As Integer
Dim j As Integer
Dim LsbSet As Boolean
'get message length:
MLength = Len(MBuffer)