Reading and writing of string variables with Visual Basic

Task

A .Net application should read a string from the PLC and write a string to the PLC.

Description

The PLC contains the string: MAIN.text.

In the MainForm_Load event method a new instance of the class TcAdsClient is created. Then the method TcAdsClient.Connect of the TcAdsClient object is called to establish a connection to the port 801. Finally the method  TcAdsClient.CreateVariableHandle is used to fetch the handle of the PLC variable . When the program finishes, the Dispose method of the TcAdsClient object is called. When the user clicks the "Read" button on the form, the string is read by means of the TcAdsClient.Read method and is displayed in the text box. When the user clicks the "Write" button ont the form, the string is wirtten to PLC and is displayed in the text box.

TwinCAT.Ads.NET version < 1.0.0.10:

The length of the AdsStream must be equal to size of the string in the PLC plus 1 (inclusive the terminating zero character). The parameter System.Text.Encoding.ASCII has to be passed to constructor of the BinaryReader class. This is necessary for the BinaryReader to be able to convert the ANSI characters to the char type.

TwinCAT.Ads.NET version >= 1.0.0.10:

The classes AdsBinaryReader and AdsBinaryWriter can be used to read and write strings ( see commented section in sample program ). These classes derive from the BinaryReader/Writer classes. To read a string from the stream one has to call the method AdsBinaryReader.ReadPlcString. To write a string to the stream one has to call the method AdsBinaryWriter.WritePLCString. The length of the AdsStream must be equal to size of the string in the PLC (without terminating zero character). The length is passed to these method and have to be equal to the length of the string in the PLC (without the terminating zero character).

Visual Basic (for .NET framework) program

Imports TwinCAT.Ads
Imports System.IO

Public Class Form1
    Inherits System.Windows.Forms.Form

    Private adsClient As TcAdsClient
    Private varHandle As Integer

#Region " Windows Form Designer generated code "

...

#End Region
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    Try
        adsClient = New TcAdsClient
        adsClient.Connect(801)
        varHandle = adsClient.CreateVariableHandle("MAIN.text")
    Catch err As Exception
        MessageBox.Show(err.Message)
    End Try
    End Sub

    Private Sub Form1_Closing(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs) Handles MyBase.Closing
    adsClient.Dispose()
    End Sub


    Private Sub btnRead_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnRead.Click
    Dim length As Integer
    Dim text As String
    Dim dataStream As AdsStream
    Dim reader As BinaryReader
    Try
        'length of the stream = length of string in sps + 1
        dataStream = New AdsStream(31)
        reader = New BinaryReader(dataStream, System.Text.Encoding.ASCII)
        length = adsClient.Read(varHandle, dataStream)        
        text = New String(reader.ReadChars(length))
        'necessary if you want to compare the string to other strings
        'text = text.Substring(0, text.IndexOf(Chr(0)))
        textBox1.Text = text
    Catch err As Exception
        MessageBox.Show(err.Message)
    End Try
    End Sub

    Private Sub btnWrite_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnWrite.Click
    Dim dataStream As AdsStream
    Dim writer As BinaryWriter
    Dim c As Char
    Try
        'length of the stream = length of string + 1
        dataStream = New AdsStream(textBox1.Text.Length + 1)
        writer = New BinaryWriter(dataStream, System.Text.Encoding.ASCII)
        writer.Write(textBox1.Text.ToCharArray())
        'add terminating zero
        writer.Write(Chr(0))
        adsClient.Write(varHandle, dataStream)
    Catch err As Exception
        MessageBox.Show(err.Message)
    End Try

    End Sub

End Class

PLC program

PROGRAM MAIN
VAR
    text : STRING[30] := 'hello';
END_VAR

;

Requirements