In this tutorial I will show you how to use beckhoff twincat TF6760 to send a http request to the server and change the json object to the plc variable.

Hope you like this.

System Requirement

installation

There is no additional installation for this function.

Driver: TcIotDrivers.sys(included in TwinCAT3 XAE・XAR)

PLC library: Tc3_IotBase(included in TwinCAT3 XAE)

TF6760 Iot HTTPS/REST

By using TF6760, we can implement the http command (put/get/etc..) and exchange the data between server and client ( twincat) with the Tc3_IotBase library.

HTTP?

Http is a standard protocol to implement the communication between client and server.

The most common case is your web browser.

The request is sent to google.jp if you type in the web address inside the browser.

And this time we will use the REST-Web services.

Please be careful that the commands of each http command are separated.

It means that the server will determine the connection after the data is sent to the client side.

(But there is some options to keep the connection alive)

Methods

Please refer to your server API documentation – because not all commands are supported on the server side.

Status Code

Header and body are replied from the server separated. It is very important to check the status code that returns from the server.

( Did you remember some error codes like 404/501?)

insert the library

Tc3_IotBase

Tc3_JsonXml

Function Block

I will only explain the methods and function blocks that are used in this tutorial.

And there is a very powerful tool in chrome that will help you to find out the parameters/reply from the server.

Please open your tools and right click “Inspect”.

This below screen is shown.

Click the network tab.

The request that was sent from the client (chrome ) to the server is shown and you can check all inside.

FB_IotHttpClient

This function block can implement a connection to the http server.you need to call the executive () method cycling and use another FB_IotHttpRequest function block to send the request to the server.

VAR_INPUT
sHostName	STRING	Default=127.0.0.1, server IP
nHostPort	UINT	Default=80 , the using port.
bKeepAlive	BOOL	Default=True, the connection to server will check until client side cut the connection *Not all server had this function
tConnectionTimeout	TIME	Timeout
stTLS	ST_IotSocketTls	The configuration that if TLS is used.

VAR_OUTPUT
bError	STRING	True=error
hrErrorCode	HRESULT	Error code
bConfigured	BOOL	True=Block configuration is finished
bConnected	BOOL	True=connection is established

Method

Disconnect	ServerのConnectionを切断する
Execute	Background通信で使われてるメソッドです。そのメソッドは毎周期で呼び出す必要があります。

FB_IotHttpRequest

This function block is used to send the request to sever , and while the signal of FB_IotHttpRequest.bBusy is changed from true to false – it means response is received from the server, we can encode and get the field inside the header field with json format.

VAR_INPUT
sContentType	STRING	Request Contect Type Setting
eCompressionMode	E_IotHttpCompressionMode	compression Mode

VAR_OUTPUT
bError	BOOL	True=Error
bBusy	BOOL	True=FB is running
eErrorId	ETcIotHttpRequestError	Error ID
nStatusCode	UINT	Status Codes from Server
nContentSize	UDINT	The size of HTTP Server response

The sContentType can be checked by the Inspect tools in chrome.

The eCompressionMode can be checked by the Inspect tools in chrome also.

Method

GetContent

A method to get the content from HTTP Response.Please execute this method after the FB_IotHttpRequest.bBusy signal is changed from True to False.

frp
ntSize	UDINT	The size of Read Data size
bSetNullTermination	BOOL
Return
GetContent	BOOL	True=Method is called without error

GetHeaderField

A method to get the http response header.Please execute this method after the FB_IotHttpRequest.bBusy signal is changed from True to False.

sField	STRING	The Header Field that you want to take
pValue	PVOID	ReadData memory offset
nValueSize	UDINT	ReadData memory size
Return
GetHeaderField	BOOL	True=Method is called without error

We can also check the header by inspection tools of chrome.

GetJsonDomContent

A method to change the response to a json object.Please execute this method after the FB_IotHttpRequest.bBusy signal is changed from True to False.

fbJson	REFERENCE TO FB_JsonDomParser
Return
GetJsonDomContent	SJsonValue	JSON Root Node

SendRequest

A method to send the request to the http server.

sUri	STRING	HTTP Request
fbClient	REFERENCE TO FB_IotHttpClient	FB_IotHttpClient FB Instance
eRequestType	ETcIotHttpRequestType	Http Command
pContent	PVOID
nContentSize	UDINT
fbHeader	REFERENCE TO FB_IotHttpHeaderFieldMap
Return
SendRequest	BOOL	True=Methodi is called without error

FB_JsonDomParser

We can use this function block to change the json object to another object.

Method

GetJson

Get the length of the Json object.

If the length of the string is more than 255, null is returned .

v	SJsonValue	Json Obejct
Return
GetJson	STRING(255)	String that changed from Json

FindMember

Search and return a specific property inside the json object.

0 is returned if that property is not found.

v	SJsonValue	Json Obejct
member	STRING	Property
Return
FindMember	SJsonValue	The member inside that json object

ParseDocument

A method to change from string to json object.

sJson	STRING	The string that would like to change
Return
ParseDocument	SJsonValue	Json Object

MemberBegin

Get the first property from the json object.

This method is used with MethodEnd() and NextMember() to implement the looping.

	SJsonValue	Proprety that would like to take
Return
MemberBegin	SJsonIterator	The first property inside a json object.

MemberEnd

Get the last property inside a json object.

This method is used with MemberBegin() and NextMemver() to loop all the property inside the json object.

v	SJsonValue	Proprety that would like to take
Return
MemberEnd	SJsonIterator	The last property inside a json object

NextMember

Get the next member inside the json object.

This method is used with MemberBegin() and MemberEnd() to loop all the properties inside the json object.

v	SJsonValue	Proprety that would like to take
Return
NextMember	SJsonIterator	The next property inside a json object

GetMemberName

Get the property name of the current json object.

i	SJsonIterator
Return
GetMemberName	STRING	The current proptery name

GetMemberValue

Get the property value of the current json object.

i	SJsonIterator
Return
GetMemberValue	STRING	The property value

TEST API

Now we can try to configure your Test API.

You can find thousands of API testing servers from google.

https://gorest.co.in/

Get request is used in this tutorial.

Enter this link into your browser.

https://gorest.co.in/public/v1/users

Response is came from the server.

String process is not an easy task of the plc program , and I will choose only the response that is redlined out.

Add a /1666 to the end of the request, the information of user id 1666 is return .

https://gorest.co.in/public/v1/users/1666

Example

Good! Test API is confirmed and we can make the twincat program.we will send the http rest request to that test API server and convert the response into json format, finally take some specific fields inside the content.

_CallRestAPI

This _CallRestAPI POU can create a client to establish a connection to server and send the request, process the response.

VAR

VAR

//Client
IotHttpClient:FB_IotHttpClient;

//Request
IotHttpRequest:FB_IotHttpRequest;
JsonDomParser:FB_JsonDomParser;
R_TRIG :R_TRIG;
bSendRequest:BOOL;
bGetContentResult :BOOL;
sContent:STRING(511);
//Json
jsonDoc:SJsonValue;
jsonVal:SJsonValue;
JsonSaxWriter:FB_JsonReadWriteDataType;
jsonDocWithSpeicificMember:SJsonValue;
jsonCurrentIterator:SJsonIterator;
jsonLastIterator :SJsonIterator;
sProperty:STRING;
jsonLoopValue:SJsonValue;
sHeaderValues:STRING;
jsonDataWithSpecificMember:STRING(511);
sJsonPropertyValue:STRING;
//Unitiy
nReqCount:UDINT;
iState:INT;
nVaildCount:INT:=0;
bConfigurated:BOOL;
iVaildHeaderCount:INT;
bReset:BOOL;
bError:BOOL;
nError:INT;

END_VAR

VAR CONSTANT

VAR CONSTANT
ciStepInit :INT:=0;
ciStepSendRequest:INT:=10;
ciStepGetContent:INT:=20;
ciStepError :INT:=8000;
ciHTTPSPort :UINT:=443;
END_VAR

Code

//Start to Send the Request
R_TRIG(
CLK:=bSendRequest
);

CASE iState OF

ciStepInit:
IF NOT IotHttpClient.bConfigured THEN
IotHttpClient.nHostPort:=ciHTTPSPort;
IotHttpClient.sHostName:=’gorest.co.in’;
IotHttpClient.bKeepAlive:=TRUE;
IotHttpClient.tConnectionTimeout:=T#10S;
IotHttpClient.stTLS.bNoServerCertCheck:=TRUE;
nReqCount:=0;
nVaildCount:=0;
bError:=FALSE;
bReset:=FALSE;
nError:=DUTE_HTTPRequest_ERRORCode.E_ERR_Noraml;
ELSE
iState:=ciStepSendRequest;
END_IF

ciStepSendRequest:
IF R_TRIG.Q THEN
IF IotHttpRequest.SendRequest(
sUri:=’/public/v1/users/62′
,fbClient:=IotHttpClient
,eRequestType:=ETcIotHttpRequestType.HTTP_GET
,pContent:=0
,nContentSize:=0
,fbHeader:=0
)
THEN
nReqCount:=nReqCount+1;
bSendRequest:=FALSE;
iState:=ciStepGetContent;
END_IF;
END_IF
ciStepGetContent:
bGetContentResult:=IotHttpRequest.GetContent(
pContent:=ADR(sContent)
,nContentSize:=SIZEOF(sContent)
,bSetNullTermination:=TRUE
);
IF IotHttpClient.bError THEN
iState:=ciStepError;
nError:=DUTE_HTTPRequest_ERRORCode.E_ERR_FB_TioHttpRequest;

ELSE
CASE IotHttpRequest.nStatusCode OF
200..299:
//get the Json Dom from Response
jsonDoc:=IotHttpRequest.GetJsonDomContent(
fbJson:=JsonDomParser
);
//get the Header
IF IotHttpRequest.GetHeaderField(
sField:=’Server’
,pValue:=ADR(sHeaderValues)
,nValueSize:=SIZEOF(sHeaderValues)
)
THEN
iVaildHeaderCount:=iVaildHeaderCount+1;
END_IF;

//get ‘data’ property from JSON DOM
jsonDataWithSpecificMember:=JsonDomParser.GetJson(
v:=JsonDomParser.FindMember(
jsonDoc
,’data’
)
);

//Convert it to Dom again
jsonDocWithSpeicificMember:=JsonDomParser.ParseDocument(
jsonDataWithSpecificMember
);

//While loop + NextMember(),MemberBegin(),MemberEnd
jsonCurrentIterator:=JsonDomParser.MemberBegin(jsonDoc);
jsonLastIterator:=JsonDomParser.MemberEnd(jsonDoc);
WHILE jsonCurrentIterator <> jsonLastIterator DO
sProperty:=JsonDomParser.GetMemberName(
jsonCurrentIterator
);
jsonLoopValue:=JsonDomParser.GetMemberValue(
jsonCurrentIterator
);

IF sProperty =’email’ THEN
JsonDomParser.CopyString(
jsonLoopValue
,sJsonPropertyValue
,SIZEOF(sJsonPropertyValue)
);
END_IF
jsonCurrentIterator:=JsonDomParser.NextMember(
jsonCurrentIterator
);

END_WHILE
iState:=ciStepSendRequest;

//if json Dom from Response is 0, somethings error in your program
IF jsonDoc <> 0 THEN
nVaildCount:=nVaildCount+1;
ELSE
iState:=ciStepError;
nError:=DUTE_HTTPRequest_ERRORCode.E_ERR_JsonDOM_IsZero;
END_IF

ciStepError:
IF bReset THEN
iState:=ciStepInit;
END_IF
END_CASE
END_IF;

END_CASE

IotHttpClient.Execute();