TwinCAT HMI Symbol Data and Binding
(Video tutorial at the end)

In TwinCAT HMI a variable is called a symbol, in another system, it may be called a tag as well. This symbol can be mapped or linked to the PLC variable. Some symbols can be local to the HMI server. That means it is not read from PLC. The HMI server’s responsibility is to keep up to date on all the symbols that have been mapped.

The following definition is valid for symbols:

Symbol Name/Content:  PLC1.MAIN.varIntPlcVariable

We have a variable in our PLC program, and the name is varIntPlcVariable. We can create a symbol in JavaScript (see our JavaScript section for more).

var symbolString = '%s%PLC1.MAIN.varIntPlcVariable%/s%';
var symbol = new TcHmi.Symbol(symbolString);    
var symbolJSON = JSON.stringify(symbol);

If you observe, symbolJSON with then you see the following content.

The following properties of symbols are clear: type, expression, name, content, etc. When we ask from a symbol, what is symbol expression then the framework returns __expression as a symbol expression.

"__symbol": {
"__diagGUID": "",
"__expression": {
"__path": null,
"__type": 10,
"__options": {},
"__expression": "%s%PLC1.MAIN.varIntPlcVariable%/s%",
"__tag": "s",
"__content": "PLC1.MAIN.varIntPlcVariable",
"__name": "PLC1.MAIN.varIntPlcVariable"
"__state": 10,
"__schema": null,
"__destroyTypeDefinitionChanged": null

Symbol Expression:       %s%PLC1.MAIN.varIntPlcVariable%/s%

var symbolString = '%s%PLC1.MAIN.varIntPlcVariable%/s%';
var symbolExpression = new TcHmi.SymbolExpression(symbolString);
var json = JSON.stringify(symbolExpression);

Again if you see the JSON in then you see the following content:

"__path": null,
"__type": 10,
"__options": {},
"__expression": "%s%PLC1.MAIN.varIntPlcVariable%/s%",
"__tag": "s",
"__content": "PLC1.MAIN.varIntPlcVariable",
"__name": "PLC1.MAIN.varIntPlcVariable"


Mapping is the event of telling the HMI Server where the needed symbols are located in the PLC.

If we map a variable then actually we are updating a file in the TwinCAT environment: 

The file is:


For example, when we map a variable, we shall have the following contents:

Variable in PLC program (for example)


That variable will map to the HMI as


That information will be recorded by the TwinCAT framework in the TcHmiSrv.Config.default.json file as the following (there will be much other information as well):

        "PLC1.MAIN.ArrayGate.0.ArrayMotorControl.0.ArrayROTATIONSENSOR.0.alarm": {
            "ACCESS": 3,
            "CUSTOMERDATA": "",
            "DOMAIN": "ADS",
            "HIDDEN": false,
            "MAPPING": "PLC1::MAIN::ArrayGate[0]::ArrayMotorControl[0]::ArrayROTATIONSENSOR[0]::alarm",
            "OPTIONS": {},
            "SCHEMA": {
                "$ref": "tchmi:general#/definitions/BOOL"
            "USEMAPPING": true


Sometimes, we may have seen a warning symbol besides a mapped variable (see Tips 3 at the bottom), the reason is mismatching in the file.

Download the complete working source code

A binding links any symbol to a property of a control. A binding automatically updates the property of the control when the value of the symbol changes. There are two types of binding:

=>Data binding, also called variable binding, a variable is linked to any property of control then we say the variable is bound to the property (see the image below). If we don’t use any variable then we can’t bind it. See the images below. If we are mapping a variable from the PLC program to the control properties of a UI control, then the framework updates the control UI automatically. See more at

=>Function binding, basically means we can use a function for the control property. Note that when we use function binding then fx is highlighted. A function binding can be created for all control properties if the JavaScript functions return a suitable value for the property of the control. More info at


Figure 01: Data binding and function binding

See the following screenshot to understand the difference between data binding and function binding. We are passing MotorName as function binding (note the highlighted fx). The field returns a javascript object ‘Motor One’ (note the Apostrophe), without the Apostrophe data will not be passed, and the control will not display it.

MotorStartTime will be displayed as it is written as ‘3’

MotorRotationMinute will not be displayed as it is not variable binding nor function binding

MotorStopTime will not be displayed as it expects variable for real but we are passing a text.

If fx is highlighted then we have selected function binding, then it expect a value which has been defined somehow!

Figure 02: passing parameters for function binding and data binding

Some descriptions of the above in Figure 2

Case 1: 

We have a custom JavaScript function that just takes a parameter and returns another number. This works because it returns a number object (primitive wrapper object). Note fx is highlighted! 4 is printed to the Textblock.

            function FunctionJS1(par1) {
                return par1 * 2;

Case 2:  We are directly passing a string 5 as a string. It works and the result is 5, not ‘5’

Case 3: If fx is highlighted then we need to pass an object, 5 is a number, and which is the wrapper object, so it works and the result is 5.

Case 4: fx is not high lighted, still it print 5, the reason 5 is an object, works like data binding.

Case 5: fx is not high lighted and  the parameter is ‘AB’ which is a literal object and it works (like data binding), output ‘AB’ not AB

Case 6: fx is high lighted and  the parameter is AB which is NOT a string type (also not defined anywhere) object and it does not work, output is error as shown in the following image.


Figure 03: passing parameters which are not defined


Case 7: 

We are doing data binding and the result is ‘5’

Case 8: We have data binding to a PLC variable which is ULINT type. The TextBlock expects text and garbage data is printed! such as +RsAAAAAAAA=

PLC_VAR : ULINT := 23;


If we had right data type, the variable would have bound properly (by configuration).


Case 9: We are using our custom function but the fx is not highlighted. This works just like data binding and the output is FunctionJS1(5)  as a string object, it does not evaluate the function at all!



When we add a function by copy-paste then sometimes it shows 404. One solution is  expanding the field and it will be OK,

Also, a good idea to check with the console.log  method. This causes unwanted text to be inserted (newline or CR or similar)  and  it does not work.

Anatomy of binding:


TcHmi.Binding.createEx =>Creates a binding between a symbol and a control attribute using the attribute setter function, takes 3 parameters

Symbol expression (for example, HMI internal variable,  ‘%i%MyInternalSymbol_1%/i%’,  Server variable  ‘%s%PLC1.MAIN.varIntPlcVariable%/s%’, parameter variable %pp%someRealParameter%/pp%)

Name of the control setter function (for example, setText, setLeft  etc setter function for control properties)

Reference to the control instance (  for example,  var myControl = TcHmi.Controls.get(‘TcHmiTextbox_1’);)       

var myControl = TcHmi.Controls.get('TcHmiTextbox_1');
TcHmi.Binding.createEx('%i%MyInternalSymbol_1%/i%', 'setText', myControl); }

CreateEx does not return anything if we want to be sure may be a good idea to check in the following way:

var result = TcHmi.Binding.exists('Text',TcHmiTextbox_1);
console.log('Binding does exists');
} else {
console.log('Binding does not exists');

TcHmi.Binding.exists => Test if there is existing binding between control property and control. The binding can be established by the createEx function of the program or by using the graphical tools. exists takes 2 parameters. If either has not been done then it will return false.

propertyName:  Name of the property, for example, Text, Left, Top, etc. These properties can be set.

Control: (reference to a control,   for example,  var myControl = TcHmi.Controls.get(‘TcHmiTextbox_1’);)

TcHmi.Binding.resolveEx => Returns a symbol expression (for example ‘%i%MyInternalSymbol_1%/i%’,  ‘%s%PLC1.MAIN.varIntPlcVariable%/s%’  etc) if a binding exists between a symbol and the control attribute or null if no binding exists.  This function takes 2 parameters:

propertyName:  Name of the property, for example, Text, Left, Top, etc. These properties can be set.

Control: (reference to a control,   for example,  var myControl = TcHmi.Controls.get(‘TcHmiTextbox_1’);)

var myControl = TcHmi.Controls.get('TcHmiTextbox_1');
    var result = TcHmi.Binding.exists('Text',myControl);
         console.log("Binding exists");
    } else {
         console.log("Binding does not exist");
    var symbolExpression = TcHmi.Binding.resolveEx('Text',myControl);
         console.log(myControl.getId() + '::Text is bound to symbol expression: ' + symbolExpression.toString());
    } else {
console.log(myControl.getId() + '::Text is bound to symbol expression: ' + symbolExpression.toString());

Mapping of Array type in PLC and display in HMI

We have an array declared in PLC like the following,

ArrayInt : ARRAY [1..5] OF INT := [10,20,30,40,50];

We display it in textBlock and it will be shown as the following.


If we execute the following code in any event (for example during attach of textbox)

async function ReadData() 
try {
        var resturnArray ;        
        resturnArray = await ReadSymbol('%s%PLC1.MAIN.ArrayInt%/s%');
        console.log('typeof resturnArray ='+ typeof resturnArray);
for (var key in resturnArray){
    console.log(`key=${key}, resturnArray[key]=${resturnArray[key]}`);            
catch(error) {
    console.log(`Error in ReadData = ${error}`);

The output will be, this will be an object and we can treat the object

typeof resturnArray =object
key=0, resturnArray[key]=10
key=1, resturnArray[key]=20
key=2, resturnArray[key]=30
key=3, resturnArray[key]=40
key=4, resturnArray[key]=50



How to make PLC data available to HMI Server

Figure 04: How to tell HMI from where we can fetch the PLC data

=> First we need to give access to the PLC. The PLC can be in the local network, remote network, or even the local machine (PLC program is running in the same machine). The PLC is identified by the Ams NetId.

=> If the PLC is in the network then the HMI Server computer (where the HMI Server is running) should have the route, for details see how to add a route for searching Beckhoff PLC.

=> If the PLC is running on the local PC, we add the localhost address “” + “1.1”

=> For remote machine, it is wise to set the AmsNetId to  IP address + .1.1,  this help to remember the AmsNet Id, see how to change Ams Net Id

=> You can bring the left-hand side setting dialog by selecting HMI Project | Server and then from the menu  TWINCATHMI | TwinCAT HMI Server Configuration.

=> Set Enabled to true, AmsNetId and the port number and press accept.

Whitelisting and blacklisting

With version 1.12, it is possible to use black or whitelisting for PLC symbols. With black or whitelisting, PLC symbols can be explicitly hidden or shown for use in the HMI. For this, we can tag the corresponding variables in the PLC with an attribute pragma. In the default case, blacklisting is active, which means that we have access to all variables of the PLC that are not explicitly hidden for the HMI. On the configuration page of the ADS extension, we can switch between black and whitelisting (Use whitelisting false is the default case).

If the ‘Use whitelisting’ is true then all datas are hidden including MAIN.

The following attribute pragmas are available for black or whitelisting in the TwinCAT HMI:

{ attribute 'TcHmiSymbol.Show' }
{ attribute 'TcHmiSymbol.ShowRecursively' }
{ attribute 'TcHmiSymbol.Hide' }
{ attribute 'TcHmiSymbol.ReadOnly' }
{ attribute 'TcHmiSymbol.BLOB' }


Now change the program to the following with white listing true

{ attribute 'TcHmiSymbol.ShowRecursively' } //Data can be seen explicitly
bigData : ARRAY[0..dataSize] OF deviceProperty;
initDone : BOOL := FALSE;
index : INT := 0;
innerIndex : INT := 0;
myTimer : TON;
startTimer : BOOL := FALSE;
delayPassed : BOOL := FALSE;
{ attribute 'TcHmiSymbol.Hide' }  //If we try to read this variable it will throw exception.
    dataSize : INT:= 100;



Mapping variables to TwinCAT HMI

Adding Ams Net Id establishes a link between the HMI Server and the PLC. HMI Server does not know what kinds of variables are available in the PLC yet. The mapping process establishes a  connection between HMI Server and the PLC so that HMI Server can read those data over the ADS protocol.

Note: In some smaller controller Symbol name is not supported, in this case the data can be accessed directly by using indexGroup and indexOffset (in that case,  we press on Symbols and then Add)

Figure 05: HMI configuration for making the data available from PLC



=> Select the HMI project and then Server in the solution explorer

=>Now select the HMI Configuration as shown in the left image


Figure 06: Mapping variables

1 => Shows all the variables in the PLC Main

2 =>Mapped Symbol (the variables which have been mapped)

3 => Internal symbol to the HMI server

4 => Localization of this application (see more).

5 => Built-in function provided by TwinCAT framework.


=> Select the HMI project and then Server in the solution explorer while HMI project is selected

=>Now select the HMI Configuration as shown in the left image

=> If we press on the All Symbols then it will show all PLC variables under the MAIN PLC, if this does not show any variables then refresh the configuration

=> To map a variable, we right click on the PLC variable and select and select Map Symbol and press OK. Now this variable will be available in the HMI

=>We can map all the necessary variable one by one or selecting all by in single mapping.

=>Mapped Symbols will show all the mapped variables in the system


Server Symbols

Server symbols can be created centrally in the server under Mapped Symbols. They can also be used offline without a PLC. Clicking on the button Create Server Symbol opens a dialog for adding server symbols. Server symbols can be used to hold some internal variables for processing business logic. This can be mapped to the PLC variable as well. This variable can be persistent meaning if the server is restarted the value is saved.

=> Right click on the Mapped Symbols

=> Select create new server symbol

=> Fill in the necessary information as shown in the following image


Figure 07: New server symbol creation

Internal Symbols

Internal symbols are independent of the server and are valid within a browser instance. The internal symbols thus enable variables to be saved individually for each client.

=> Internal Symbol can be created in same way as server symbols

=> Right-click on the internal symbol and fill in the necessary information as shown in the following image


Figure 08: Internal server symbol creation

Test Application

The test application can be downloaded from and test.

Here is a short description of what the test application does.

1=> An internal symbol is displayed here (the internal symbol has been bounded manually),

2=>If we press this button the internal symbol will increase by one,

3=>Internal symbol will decrease by one,

4=>If we press this button it will read the value of edit box 5 and set it to PLC, this button’s left property is bound with the internal symbol, so if we increase or decrease the internal symbol, this button will move left or right.

5=>Display the current value of the PLC variable if there is binding. If there is no binding then it will be empty

6=>If we press this button the PLC variable (we have a single variable) will be bound to the edit box at 5

7=>If we press this button the PLC variable (we have a single variable) will be unbounded to the edit box at 5 and it will display nothing (because it is not bound anywhere)

8=>Display the binding status if we press on 6 or 7 or even during the initialization of the program.

Figure 09:  UI of the test applications

We embedded JavaScript code for binding and unbinding. For test purpose we have embedded JavaScript for editbox (5). The code can be found in the downloaded zip file.

var myControlInt = TcHmi.Controls.get('TcHmi_Controls_Beckhoff_TcHmiTextbox_IntPLCVariable');
if(myControlInt){     var result = TcHmi.Binding.exists('Text',myControlInt);     if(result){          console.log("Binding exists with IntPLCVariable");     } else {          console.log("Binding does not exist with IntPLCVariable");     } }
if(myControlInt){     var symbolExpression = TcHmi.Binding.resolveEx('Text',myControlInt);     if(symbolExpression){                   console.log(myControlInt.getId() + '::Text is bound to symbol expression: ' + symbolExpression.toString());     } else {          console.log("Binding does not exist with IntPLCVariable, symbol expression is null");     } } 

Figure 10:  Configure the text box onAttached event

We have embedded the following code for button number 6 (to make the binding), for unbinding, we have a little bit of different code that can be check by copying the code.


var myEditBoxControl = TcHmi.Controls.get('TcHmi_Controls_Beckhoff_TcHmiTextbox_IntPLCVariable');
var myMsgBoxControl = TcHmi.Controls.get('TcHmi_Controls_Beckhoff_TcHmiTextblock_Message');
if (myEditBoxControl && myMsgBoxControl) {
    TcHmi.Binding.createEx('%s%PLC1.MAIN.varIntPlcVariable%/s%', 'SetText', myEditBoxControl);
    var result = TcHmi.Binding.exists('Text',myEditBoxControl);
        myMsgBoxControl.setText('Binding Done');
    } else {
        myMsgBoxControl.setText('Binding not Done');

How do we bind control property and variable

We can do it in 2 ways:

=> We can bind a variable by selecting a small square and selecting the mapped variable. If we don’t use these configuration options then we can create by JavaScript code. Whenever the varIntPlcVariable is changed in the PLC, then the Text field will be changed as well.


=> If we don’t have the binding then the field will be empty but we can bind it by the following code.

TcHmi.Binding.createEx('%s%PLC1.MAIN.varIntPlcVariable%/s%', 'SetText', myEditBoxControl);

If we have executed the above code (in any way) then when the variable in PLC is changed then the edit box will 

be updated though we have not configured it by pressing the small rectangle.

Advanced topics about Symbol, Symbol expression and Binding

We have described symbols, compound user control, and how to reduce the number of parameters passing for user control has been described. You need some knowledge about user controls, JavaScript, and subscription mechanisms in TwinCAT.

Here is the link to how you should go through the tutorial.

=> Symbol, Symbol Expression, and bindins  this page

=> Compound user control,

=> How to reduce the number of parameters when using compound user control,

You can watch the video, and go through the source code. If you have an issue then you can comment on the YouTube video or in our Google group


Tips 01: Why my HMI does not updated with initial value set in the server or in server extension?

Data need to changed if we want to display data on the UI. One way to configure the UI with custom events as shown in the following image.

Figure 11:  Custom event configuration (we tell whenever the UI text is changed then it is written to the server variable)

Tips 02: I see warning symbols besides my mapped variables, why it is shown in the following image.

The reason for this you have a running runtime with previous variables mapped (from other PLC programs).

Sol: just unmap these variables (if those variables are not needed in the current program) or check if the PLC is in run mode.


Tips 03: If all variables are marked with the symbol then your route has been lost or the data in the PLC has been changed, if you add the target PLC in TwinCAT route then it will be fine again, or if you refresh mapping it may disappear. If you had an array of 120 and now you change the item numbers to 130, this will create this type of warning. Refresh as shown in figure 13.

Figure 12:  Effect of route changes on PLC variables

Figure 13:  Refresh variables when those are out of sync.

Tips 04: We can see this type of error sometimes in the console.

TcHmiFramework.js:744 [2021-11-08T07:50:48.925Z][Error] [Source=Framework, Module=TcHmi.System.TriggerManager, Event=%i%System_Mode%/i%, ObjectType=WriteToSymbol] Code: 3005/0xbbd, Message: E_SERVER_COMMAND_ERROR

  Reason: %s%PLC1.SystemMode%/s%: Error in command for symbol: ‘PLC1.SystemMode’ in response from server with id: ‘9’.

  Domain: TcHmi.System.Symbol

  as result of: Code: 7/0x7, Message: ADS_API_RETURN_CODE

    Reason: Sum ADS write request of size 13 containing 1 requests failed.


    Domain: ADS

Solution: At least, if our route has been deleted for some reason then we can see this type of message.

Tips 05:

When we add a function by copy-paste then sometimes it shows 404. One solution is  expanding the field and it will be OK,

Also, a good idea to check with the console.log  method. This causes unwanted text to be inserted (newline or CR or similar, not exactly known to me, if you know you can post to our Google group)  and it does not work.

Tips 06:

We can set the color and text of an edit box in the following way. It does not work if we create a hexacode and use it directly.

var deviceColorNormal = {
"color": "rgba(0, 153, 0, 1)"

var NameUpperText = TcHmi.Controls.get('Gate_UpperOne.MyTextBlock');
if (NameUpperText != undefined) {
	var myColorRed = {
	color: "red"
let aa = parseFloat(readData).toFixed(2);

Tips 07: Type mismatch (3005), Invalid type at ´DataField´: ´Expected string, got int.

There are many reasons why we get 3005, quite annoying! Here we list what I see so far.

3005 is defined in the following page:

If we have mapped a data struct and after mapping, we have changed a field then we need to refresh or remap the data from the HMI mapping process (See more in the JavaScript part one for more).

Tips 08: Uncaught (in promise) TypeError: Cannot read property of ‘0’ of undefined

TYPE deviceProperty :
boolData : BOOL;
stringData : STRING;
ArrayDataFirstDays : ARRAY [0..4] OF BOOL;


We changed by mistake ArrayDataFirstDays  to arrayDataFirstDays (Note: A to a).  Adding new field in the PLC struct has no effect on the mapping side.


Download the sample from the link given above.

Next, let’s continue with details of HMI configuration at

Ask questions related to Hemelix sample code and design at Google group