LabVIEW
Good way to connect to Siemens PLC? Alternative to OPC server?
To read/write PLC items I simply use Datasocket functions with a DA OPC server (NI OPC would be ok I guess).
Here is an example of reading a generic item. It can be a variable or even a memory block.
The URL string depends on the OPC server. The trick is to read the item as a variant, then get the OPC Quality as a variant attribute. At last you need to convert the variant to the proper data type; a memory block would be converted to a LabVIEW cluster.
This is a simple example. In a real program, you would open the connection once and use it many times.
One possible alternative is Modbus:
LabVIEW exe with Modbus communication <-> ethernet <-> PLC with Modbus communication.
Modbus API is available here: http://forums.ni.com/t5/NI-Labs-Toolkits/LabVIEW-Modbus-API/ta-p/3524019 , I believe it is also included in newer versions of DSC (I'm not sure about runtime licensing though).
I've done this in one application, I've set up Modbus Master in my LV application and there was Modbus communication enabled on PLC side. I'm not sure about PLC side technical details (someone else programmed it), but I think it's not rocket science 😉 One possible caveat: Modbus communication with PLC can be slow, so be sure to benchmark your application before jumping into full-scale developement.
Also - there is always an option to simply save the whole project to LV 2012... Wouldn't that be the best way for you?
pincpanter, tried your example with NI OPC server, but did not know how to construct a proper opc-URL. In NI-OPC-QC (quick client) one example variable looks like. Tried to use these data with your url structure (and with backslash and dots instead of slash). Can you explain how do this?
There are in fact many different ways to connect to a Siemens S7 PLC from within a LabVIEW application.
1) There is of course OPC, but that requires an OPC Server somewhere either from Siemens or a 3rd party, that knows how to connect to the PLC properly. And accessing OPC from within LabVIEW can either be done with Datasocket, if your requirements are not very demanding and you can live with creating your datasocket URLs correctly, control binding which also uses datasocket internally, or the LabVIEW DSC system which allows a lot more control over how the OPC items are accessed.
2) Modbus/TCP is another solution that does not require much on the LabVIEW side but you need to have a Siemens PLC that supports this protocol or install an according program block in the PLC.
3) There are various 3rd party libraries that access the ISO on TCP protocol that all Siemens S7 PLC with Ethernet interface support. Most of them are either DLLs, ActiveX components or .Net assemblies and require some work to interface, but it's definitely possible. One of them I used in the past is the Accon-AGLink software from Deltalogic. Works like a charm.
Another is a native LabVIEW library from Data Ahead called SP7 Toolkit and you can buy it from the NI 3rd Party Addons here. For the price you can't really do much wrong and for accessing Datablock and IO items it works well. The nice thing about this library is that it is pure LabVIEW code, so it will work on any LabVIEW platform that has a network connection, including the cRIO controllers from NI. With DLLs, ActiveX and .Net based libraries you are usually restricted to Windows only systems, although the Accon AGLink library does support Linux too, but my attempts to get that library to work on a x86 based NI Linux Realtime cRIO where pretty discouraging and the fact that it would anyhow not work on the cheaper ARM based cRIOs was the final knockout for this solution.
4) You can implement your own simple TCP protocol in the Siemens PLC in a programming block and connect to it with the LabVIEW TCP nodes. Siemens provides some samples about implementing your own TCP protocol on their PLCs, a document describing this can be found here. This may be the most reliable solution in terms of controlling the whole protocol.
The ISO on TCP based solutions all have the slightly disturbing problem that they rely on a protocol that is not officially documented by Siemens. Also if you use a third party library that uses the ISO on TCP protocol you usually have to disable the option to compress the datablocks in the Siemens PLC. Most libraries do not support the new TIA extension that can also read compressed data items.
We ended up developing our own VI library that uses the ISO on TCP based protocol since we needed also asynchronous operation for better throughput control than what the library from Data Ahead could provide.
Profibus is a serial interface using on the electrical level a RS-485 circuitry and on the protocol level a protocol that includes also provisions for realtime control and industrial level error control. It's protocol timing is demanding enough that you can't reliably implement it in the user space of an application (for instance in LabVIEW on top of VISA) but either need to have a dedicated controller on the interface board that handles the low level timing aspects autonomously or a kernel device driver that sits on top of the hardware interface and thanks to its higher priority and direct interrupt capability from the hardware can handle the protocol in quasi-realtime.
Profinet is the sibling of Profibus which uses an Ethernet interface instead of an RS-485 circuitry for the physical transport layer.
In order to make a product which implements a Profibus or Profinet interface, one has to be a member of the Profibus Organization. Only members of that organzation get the right to use certain patents that cover the actual implementation of the various Profibus/Profinet protocols.
