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Home > Forums > Electrical / Electronic Engineers > Activities > Programmable logic controllers (PLC) > FAQs

Programmable logic controllers (PLC) FAQ

DDE and PLCs

How do I use Visual Basic and DDE to get data out of an Allen Bradley PLC? by chakorules
faq791-453
Posted: 26 Dec 02 (Edited 27 Dec 02)

You'll need to make sure you have RS Linx running and a DDE topic created. In our example code below, our DDE topic is called "DDE".

Open a new project in Visual Basic.

Use this sample code provided to try some COLD DDE linking to a text box control in Visual Basic.

<<<<<<<<<<<<BEGIN SAMPLE CODE>>>>>>>>>>>>>>>>>>>>>

Public Const DDETOPIC As String = "DDE"

Sub DDEreadStation1()
On Error GoTo MessageRSLinxDead:

    'make sure the DDE Channel is Closed before we define a Topic channel
    txtStat1Force.LinkMode = 0
    txtStat1Status.LinkMode = 0
    
    'set the Application and Topic of the Control Property
    'DDETOPIC is a Public Constant in MainSubs Module
    'I might use a configuration file later and let the user set this
    txtStat1Force.LinkTopic = "RSLinx|" & DDETOPIC
    txtStat1Status.LinkTopic = "RSLinx|" & DDETOPIC
    
    'set the Address of the PLC we want to read
    txtStat1Force.LinkItem = "N7:1"
    txtStat1Status.LinkItem = "B3:1/1"
    
    'set the DDE mode to COLD Link, we will request the data manually
    txtStat1Force.LinkMode = 2
    txtStat1Status.LinkMode = 2
    
    'tell VB to get the data from the PLC NOW!
    txtStat1Force.LinkRequest
    txtStat1Status.LinkRequest
    txtStat1TimeStamp.Text = Now()
    
GoTo SkipMessage

MessageRSLinxDead:
    MsgBox ("RSLINX is not running. Can not continue!")
SkipMessage:

End Sub

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