CODESYS - the IEC 61131-3 automation software

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PostPosted: Thu Oct 18, 2018 3:44 pm 

Joined: Thu Mar 30, 2017 12:13 pm
Posts: 24
Dear reader,

I've implemented a HTTP web client for requesting info from public online services.
As test I'm able to connect to the webserver of a local LAN device and retrieve its webpage by sending a HTTP GET header, works fine.

Then I tried to connect to a public weather service on by setting up a TCP connection to its webserver on port 80.
But on the public service the code times-out after hanging in the "connecting" state for 5 seconds and never gets connected.

Is there a route or firewall rule that I need to set?

The M251 controller interface is configured correctly :
IP :
Subn :
Gateway: (ISP modem)
Controller : M251
Somachine : 4.1 SP2 (codesys 3.5.3)
TCP lib used: TcpUdpCommunication (Schneider electric)

Best regards,

Last edited by tjarcoboerkoel on Wed Nov 07, 2018 8:42 am, edited 1 time in total.

PostPosted: Fri Oct 19, 2018 12:53 pm 
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Joined: Fri Feb 23, 2018 3:41 pm
Posts: 101
DNS settings ?
Gateway allows your PLC to ask for a unknown IP, but not for a name.
If you use a url with a name (ex: then it needs to be resolved by a DNS server.
Use your ISP DNS one or google's one (primary, secondary

Http should correctly passthrough your ISP router.
Maybe you need a proxy configuration ?
Check it from a computer accessing internet on the same router.

Last but not least, start by issuing a ping (from your plc) to check if the service is available.

PostPosted: Mon Oct 22, 2018 4:01 pm 

Joined: Thu Mar 30, 2017 12:13 pm
Posts: 24

Thnx for your reply.
No I connect to the IP directly and setup the HOST name in the HTTP header.
So, I don't need the DNS.

I am able to connect to the IP from a computer using Putty (Raw TCP) and when I send the content of the HTTP header I DO get the response form the webserver.

This does not work form the controller, while when I connect to a local webserver it does work.

I just can;t connect to a public IP-address.

PostPosted: Thu Oct 25, 2018 12:33 pm 
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Joined: Fri Feb 23, 2018 3:41 pm
Posts: 101
tjarcoboerkoel wrote:
[…] and setup the HOST name in the HTTP header.[…]

Could you detail this ? (I'm not familliar with HTTP)

PostPosted: Wed Nov 07, 2018 8:39 am 

Joined: Thu Mar 30, 2017 12:13 pm
Posts: 24

Oke well I found the bug. The HTTP protocol is very strict and needs multiple CR+LF characters.
So, for the people who need an HTTP header for simple GET requests.
You may split up the url into document and arguments, but I didn't need that.

   URL         : STRING[255]    :='/data.xml?arg1=1&arg2=John' ; 
   HOST         : STRING[255]   :='';
   HttpReq       : ARRAY[1..255] OF BYTE;  //Might be changed to a pointer to write to
   HttpReqSize    : UDINT            := 0;            //Size of data in HttpReq       
   sHttpReq   : STRING[255];

StrToLowerA( ADR(URL) );      //convert to lowercase characters
StrToLowerA( ADR(HOST) );      //convert to lowercase characters
sHttpReq := Standard.CONCAT('GET ', URL);
sHttpReq := Standard.CONCAT(sHttpReq , ' HTTP/1.1$0D$0A');
sHttpReq := Standard.CONCAT(sHttpReq , 'Host: ');
sHttpReq := Standard.CONCAT(sHttpReq , HOST);
sHttpReq := Standard.CONCAT(sHttpReq , '$0D$0A');  //first CR+LN
sHttpReq := Standard.CONCAT(sHttpReq , '$0D$0A');  //second CR+LF (indicates end of feed)

HttpReqSize := DINT_TO_UINT(StrLenA( ADR(sHttpReq) ) );
HttpReq      := String_TO_ArrayOfByte(sHttpReq, TRUE);

Just open a TCP connection to the servers IP where it's hosted at port 80, TCP_send the HttpReq and TCP_read back data (may take some time for the server to respond >500ms).
You may read twice, because the server will first send the 400 OK header, followed by the requested data/page/file/...
The Server will shutdown the connection when it's finished (and you know all data has been transferred).

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