Eclipse Milo, OPC-UA : where to start?

Question

I am entirely new to the OPC-UA world.

I need to establish a proof of concept of how to allow our ERP to communicate with PLCs. I am evaluating software that acts as an OPC-UA server (it is working). I found about Milo and got the code working in Eclipse. Looking at the examples, I got a toy program connected to the OPC-UA server and doing... something.

The basic plumbing works, but I can't figure out where to start with OPC-UA and Milo. I have not been able to find introductory material to help me make sense of it all.

Is there documentation about the architecture of OPC-UA and how Milo implements that? Is there a better way to go about it than looking at the examples to figure out how things should be done?

Watercourse

I think the answer to your two questions is NO. But playing around with the examples helps a lot. – Hatbox

Answer 1

Documentation for Milo is one of the next major things to tackle now that we've got a release published to Maven central. Even once that documentation exists, though, it will assume some knowledge of OPC UA.

There's a somewhat expensive green book called "OPC Unified Architecture" available on Amazon. I haven't read it myself, but I've heard other people mentioned they have. It's probably a lot easier and cheaper than trying to learn OPC UA from the specifications.

edit: Seems there's a couple other cheaper alternatives now too.

The Milo project has a gitter channel and a mailing list you can ask questions on as well.

Intervocalic

Here is my situation. I have a PLC connected to an OPC-UA server, and I know the tag names I am interested in. Can you point me to examples of (1) reading a specific, known tag, (2) writing a tag and (3) getting notified of changes in a specific tag. Going through the examples, I am not able to figure out if this is what the code is indented to do or not, so I am missing some knowledge to "bootstrap" my understanding. Thanks for suggestion introductory reading on OPC-UA, I will surely buy one. – Watercourse

ReadExample, WriteExample, and SubscriptionExample in milo-examples/client-examples module cover these 3 cases pretty directly. The only thing of note would be that the read example reads from one of the well-known NodeIds rather than a user defined one like the write example. The subscription example does the same thing - but you would just replace the NodeId with the ones you're interested in. – Intervocalic