Hi,

I wrote a small c++ program which uses libnodave to read and write some data with a plc (simatic s-300).

It works really nice.

But now I am wondering how libnodave works so that I can implement it based on simple UDP or TCP/IP sockets myself for a better understanding.

So I made some research on the internet and I found out that I have 3 possibilities:

• TCP/IP Native (RFC 793)
• ISO on TCP (RFC1006)
• UDP (RFC 768)

So I found this PDF which describes the different solutions but I was confused by this statement mentioned at all 3 protocols:

To be able to assign parameters for TCP native and ISO on TSP for
communications connections, create a DB that contains the data structure
UDT 65 "TCON_PAR". This data structure contains the parameters
configuring the connection. You will need such a data structure for
connection. You can assemble this structure in a global DB.

But so far I understand it I would have to change the program of the plc (at least create a datablock with the udt 65 structure) to get a connection working.

But when I used libnodave I did not any extra work on the plc and it works.

So how libnodave exchange data with a plc if I choose the daveProtoISOTCP option and where I can find the specifications for this?

But in case of I understood something completely wrong (I am pretty new to plc's), what are the possibilities of exchanging data based on simple tcp/ip or udp sockets with a pc over ethernet?

Hello,

simply with Libnodave you're using prepared interface with internal defined structure of frames that rightly Exchange data with Device. In this case you don't need make any change on PLC side as communication, as OPC you're looking to memory areas. In case of user / open communication (you want define your exact criteria of frame consistency) you need prepare your communication also on PLC and PC side (simple Ethernet Client/Server via sockets f.ex.). In this case on PLC (with different version of instructions) you need define that exact DB structure with exact type for correct connection.

Duris_I

Hello,

simply with Libnodave you're using prepared interface with internal defined structure of frames that rightly Exchange data with Device. In this case you don't need make any change on PLC side as communication, as OPC you're looking to memory areas. In case of user / open communication (you want define your exact criteria of frame consistency) you need prepare your communication also on PLC and PC side (simple Ethernet Client/Server via sockets f.ex.). In this case on PLC (with different version of instructions) you need define that exact DB structure with exact type for correct connection.

Hi,

thank you for answering my question but I have some troubles to understand it.

So far I understand you are telling me that Open Platform Communications (OPC) is what I am searching for, that way Libnodave archieve it's data exchange?

If not what exactly do you mean with this?

Duris_I

...with Libnodave you're using prepared interface with internal defined structure of frames that rightly Exchange data with Device...

 I mean what's the name of this communication / protocol / and where I can find some specifications / papers describing it?

Hello,

yes, Libnodave is simply some kind of OPC where you have all prepared, you don't need check all frames consistency, type of requests/answers. But I was talking about user/open communication in other side, simply you can omit OPC systems if you need program your own system of messages, in this case you need prepare communication on both sides. Where to find internal structure of frames in different cases of Libnodave/OPC, look to internet or use some tools like WireShark.