How to access an array from an OPC-UA client

[SudhakarKuma]

Hello everyone!

I am working on a server-client model for my project. In this project, I have a plant/ system running in OpenModelica (an open-source tool for simulating complex systems), which has a built-in OPC-UA server. I invoke this server for my model and then try to connect it with an OPC-UA client. For the client, I am making use of the Python opcua package.

In my server-client model, I have an array of elements on the server. For my project, I need to access this array on the client-side. Accordingly, I need to modify some of its elements and write them back to the server. In the past, I have worked on accessing single values from the client. However, I am not able to figure out how to access an array using the OPC-UA client. Could you please help me with this?

Regards,
Sudhakar

[AndreasHeine]

https://reference.opcfoundation.org/v104/Core/docs/Part6/5.1.6/

python-opcua/opcua/ua/uatypes.py

Line 708 in 8582747

class Variant(FrozenClass):

from opcua import ua

# ua.Variant(value, varianttype)

ua.Variant([1,2,3], ua.VariantType.UInt16)

[AndreasHeine]

just google "opc ua simulation server" or "opc ua demo server" ;)

[SudhakarKuma]

Hi, @AndreasHeine Thanks for your response and constant support. I have come across a few OPC UA simulation servers and will try these out shortly. Meanwhile. I thought of using the Server module of the Python OPC UA package. First, I wanted to see how an array is handled and how to modify the array elements. So, I have a server (having an array [1, 2, 6]) as given below:

from opcua import Server
import time

server = Server()

url = "opc.tcp://192.168.0.3:4840"
server.set_endpoint(url)

name = "OPCUA_Simulation_Server"
addspace = server.register_namespace(name)

node = server.get_objects_node()
params = node.add_object(addspace, "Parameters")

arr = params.add_variable(addspace, "myArray", [1, 2, 6])
arr.set_writable()

server.start()
print("Server started at {}".format(url))

while True:
    print("Array is: {}".format(arr.get_value()))
    time.sleep(1)

Now, suppose we wish to have a client that can modify the array to [1, 5.6, 6]. To do so, one way to write the client code is as given below:

from opcua import Client
import time

url = "opc.tcp://192.168.0.3:4840"

if __name__ == "__main__":
    client = Client(url)

    try:
        client.connect()
        print("Client connected!")

        while True:
            arr = client.get_node("ns = 2; i = 2")
            arrVal = arr.get_value()
            print(arrVal)

            arr.set_value([1, 5.6, 6])

            time.sleep(1)

    except KeyboardInterrupt:
        print("Stopping Sequence!")
    
    finally:
        print("Done!")
        client.disconnect()

Could you please suggest some other ways, which would help to modify the values at a particular index (or a group of indices)? It would eliminate the need to pass all the values in the set_value method.

[AndreasHeine]

https://reference.opcfoundation.org/v104/Core/docs/Part4/5.10.4/
the feature you are looking for is called "indexrange" in the write parameters, but i dont think we support that in python-opcua!

or do we? @oroulet

[GrantEdwards]

Does python-opcua support indexed writes?

[AndreasHeine]

Does python-opcua support indexed writes?

it is better to open own issues or discussions rather then post on old ones with a different topic!

if you use the lowlevel-client you can set the request params manually (if the server supports that feature)

[SudhakarKuma]

Hi @AndreasHeine With your advice, I have been able to work with 1D arrays on the OPC-UA server. Now, I am facing issues with models having 2D arrays. When I connect to these models using a Python OPC-UA client, it works fine during the simulation. However, at the end of the simulation, I get an array of invalid pointer as shown below:

The same model works perfectly fine without an OPC-UA server. But when I simulate it on the OPC-UA server, I get this error, as shown above. That's why it makes me believe that the simulation has something to do with OPC-UA only. Could you please provide me with some insights into this?

[swamper123]

I get an array of invalid pointer as shown below [...]free(): invalid pointer

Well this neither looks like a python error nor a Client error. The opc-ua server should tell you what is wrong and should throw typical error messages like BadAttributeError etc. Otherwise the server is not stable.

[SudhakarKuma]

@swamper123 Thank you for your prompt response. On the client side, the exchange of data happens properly. So, should I assume that my OPC-UA server is having some issues?

[swamper123]

If a request is faulty, the server should respond properly with a common status code and not crash, like this.
And if you say yourself, that the Client is doing his job propperly, it has to be the server.