In this article, we will use the Beckhoff TF6421 to read an XML File and transfer the result to a PLC variable. Let’s get started!

Reference Video

Beckhoff.Let’s read data from XML File via TF6421

TF6421?

The TwinCAT XML Server TF6421 provides a simple PLC library for writing to and reading from XML files; the XML Server also enables implementations such as loading initialisation data, which is often required when starting up a machine, for example.

PLC variables can also be formatted and stored in XML files containing PLC comments; the variable structure of the XML document matches the structure of the PLC variables.

System Requirment

Structure

This is the structure of the TF6421 XML Server.

TwinCAT XML Server

The TwinCAT XML Server is a service that starts/stops together with TwinCAT and is the link between TwinCAT and the XML file.

PLC Library

PLC library The PLC library has four Function Blocks, which allow data to be written from the PLC library to an XML file or read from an XML file. Variables can be formatted and saved in an XML file and variables can be initialised in TwinCAT from the XML file.

XML File

This is an XML File stored in TwinCAT Runtime.

Principle of operation

The XML server communicates with the TwinCAT PLC via ADS All variables written by the XML server are converted to text and written to an XML file via the MSXML DOM parser The XML file does not contain any information about data types, only the variable names and values as tag names only as tag names.

<variable name> value </variable name>

Conversely, during a read operation, the data type of the variable being read is transferred to the XML Server via ADS and the text from the XML file is converted accordingly.

Data Type

This is an example of the correspondence between TwinCAT PLC Runtime variables and XML File.

Function Block

FB_XmlSrvRead

The Function Block FB_XmlSrvRead here can be used to initialize PLC variables with data from an XML file. In this case, the input variable sXPath must point to a valid node in the XML file specified by sFilePath.

VAR_INPUT

Variable	Data Type	Description
sNetId	T_AmsNet_Id	Network address of the TwinCAT 3 XML server, empty string for Local PC.
ePath	E_OpenPath	Specify the TwinCAT system path on the target device where the file will be opened
nMode	WORD	You can control how XML files are evaluated, currently only XMLSRV_SKIPMISSING and XmlSrvRead are supported.
pSymAddr	DWORD	Address Offset of the PLC variable to which the data from the XML file is written.
cbSymSize	UDINT	Size of the PLC variable to which data from the XML file is written.
sFilePath	T_MaxString	Specify the path and filename of the file that FB will open. Note that the path is only for the file system of the local computer, the path cannot be used on a network.
sXPath	T_MaxString	This will be the address of the tag in the XML document from which the data is to be accessed.
bExecute	BOOL	Execute FB with start-up signal.
tTimeout	TIME	Maximum running time of FB

VAR_OUTPUT

Variable	Data Type	Description
bBusy	BOOL	True=FB in progress
bError	BOOL	True= FB error exists
nErrId	UDINT	Error code returned by TC3 XML Server

FB_XmlSrvReadByName

You can use FB_XmlSrvReadByName here to initialize PLC variables with data from an XML file. In this case, the input variable sXPath must point to a valid node in the XML file specified by sFilePath.

VAR_INPUT

Variable	Data Type	Description
sNetId	T_AmsNet_Id	The network address of the TwinCAT 3 XML server, which must be an empty string on the Local PC.
ePath	E_OpenPath	Specify the TwinCAT system path on the target device to open the file
nMode	WORD	You can control how XML files are evaluated, currently only XMLSRV_SKIPMISSING and XmlSrvRead are supported.
sSymName	T_MaxString	This will be the name of the PLC symbol to which data from the XML file will be written.
sFilePath	T_MaxString	Specify the path and file name of the file that FB will open. Note that the path is only for the local computer’s file system; the path cannot be used on a network.
sXPath	T_MaxString	This will be the address of the tag in the XML document from which the data will be accessed.
bExecute	BOOL	立ち上げ信号でFBを実行する
tTimeout	TIME	Maximum execution time of FB

VAR_OUTPUT

Variable	Data Type	Description
bBusy	BOOL	True=FB in progress
bError	BOOL	True= FB error exists
nErrId	UDINT	Error code returned by TC3 XML Server

Installation

Download the XML Server Installation File for TF6421 from the Beckhoff website.

https://www.beckhoff.com/en-en/products/automation/twincat/tfxxxx-twincat-3-functions/tf6xxx-connectivity/tf6421.html

Start the TF6421 installer, select English >Ok to proceed.

Click Next> to continue.

Agree to the license and proceed with Next>.

Enter your personal information and proceed with Next>.

Select Complete and proceed with Next>.

Proceed with Next>.

Get started with Install.

Just a second..

Done!

Implementation

We will now use the TF6421 API from TwinCAT3 to read the data in the XML File.

Add Library

Add the Tc2_XMLDataSrv library to your project.

DUT

The next step is to create a structure that matches the structure of the XML File.

DUT_MyXML

TYPE DUT_MyXML :
STRUCT
COMMON:STRING(30);
BOTANICAL:STRING(30);
ZONE:INT;
LIGHT:STRING(30);
PRICE:REAL;
AVAILABILITY:BOOL;
END_STRUCT
END_TYPE

DUT_MyXML_Books

TYPE DUT_MyXML_Books :
STRUCT
title:STRING(30);
author:STRING(30);
year:DINT;
price:REAL;
END_STRUCT
END_TYPE

DUT_MyXML and DUT_MyXML_Books do not need to worry about variable names, as long as they actually match the XML File, data type, and order.

Program

VAR

This one defines variables to be used in the program.

PROGRAM MAIN
VAR
fbXMLSrvRead:FB_XmlSrvRead;
fbXmlSrvReadByName:FB_XmlSrvReadByName;
bExecute:BOOL;
bReset :BOOL;
sFilePath:T_MaxString:=’C:\Users\threespc02\OneDrive\Desktop\plant_catalog-1.xml’;
sXMLPath:T_MaxString:=’/CATALOG/PLANT’;
sXMLPath2:T_MaxString:=’/CATALOG/book’;
iState:INT:=0;
myXMLValue:DUT_MyXML;
myXMLValueReadByName:DUT_MyXML_Books;
ErrorCode:UDINT;
END_VAR

This is the XML File used in this article, and the setting corresponds to a variable such as sFilePath.

Code

This program reads an XML File and reads the data in it using fbXmlSrvReadByName and fbXMLSrvRead Function Block.

CASE iState OF

0:
fbXMLSrvRead(
pSymAddr:=ADR(myXMLValue)
,cbSymSize:=SIZEOF(myXMLValue)
,sFilePath:=sFilePath
,sXPath:=sXMLPath
,bExecute:=FALSE
);
fbXmlSrvReadByName(
sSymName:=’MAIN.myXMLValueReadByName’
,sFilePath:=sFilePath
,sXPath:=sXMLPath2
,bExecute:=FALSE
);
ErrorCode:=0;
IF NOT fbXMLSrvRead.bBusy
AND NOT fbXMLSrvRead.bError
AND NOT fbXmlSrvReadByName.bBusy
AND NOT fbXmlSrvReadByName.bError
AND bExecute THEN
iState:=10;
END_IF
10:
fbXMLSrvRead(
bExecute:=TRUE
);
IF fbXMLSrvRead.bBusy
AND NOT fbXMLSrvRead.bError THEN
iState:=20;
END_IF;
IF fbXMLSrvRead.bError THEN
iState:=900;
END_IF
20:
fbXMLSrvRead(
bExecute:=FALSE
);
IF NOT fbXMLSrvRead.bBusy THEN
iState:=100;
END_IF
;
100:
fbXmlSrvReadByName(
bExecute:=TRUE
);
IF fbXmlSrvReadByName.bBusy
AND NOT fbXmlSrvReadByName.bError THEN
iState:=110;
END_IF;
IF fbXmlSrvReadByName.bError THEN
iState:=910;
END_IF

110:
fbXmlSrvReadByName(bExecute:=FALSE);
IF NOT fbXmlSrvReadByName.bBusy THEN
iState:=120;
END_IF

120:
bExecute:=FALSE;
iState:=0;

900:
ErrorCode:=fbXMLSrvRead.nErrId;
bExecute:=FALSE;
IF bReset THEN
iState:=0;
END_IF

910:
ErrorCode:=fbXmlSrvReadByName.nErrId;
bExecute:=FALSE;
IF bReset THEN
iState:=0;
END_IF
;
END_CASE