Archive for Março 9th, 2009

OPC - OLE (Open?!) Process Control

Segunda-feira, Março 9th, 2009

In my industrial meanderings, I’ve been using some I/O modules for wich I write device drivers (for my data layer abstraction). One of these involves the OPC protocol (OLE for Process Control). It’s actually a pretty good ideia (badly implemented).

The ideia is that all data devices should be easily accessible by a standard communications protocol. There’s the ideia of an OPC server, an application that connects to the devices and exposes their data on the OPC protocol. Then, an OPC client connects to these servers and reads/writes on those devices. The problem is that they based OPC on a Microsoft-only (thus Windows-only) technology, DCOM. DCOM has a history of being difficult and picky to configure remote access and, because of this, almost nobody goes to the length (and risk) of configuring a system remotely (permissions problems, etc), and clients and servers are usually run on the same machine (defeating half the purpose).

Still, the ideia is too good to let go, and there are OPC servers for almost everything on the market. Getting OPC to work was one of my first projects at work, but since I write almost everything in Java (so that it runs on Windows, Mac and Linux) I had to find a way to get access to OPC from Java.

Back then, I developed a small TCP/IP proxy, socket-based, with a simple ASCII protocol, on Visual Basic (using the native OPC DLL). It not only brought along the lost OPC purpose (distributed access), but I could have access from just about anything (Java, Ruby…). Still, it was slow, configuration was local, and it did not implement all of the OPC protocol.

Later, I found almost exactly what I was looking for: a couple of companies were proposing JNI-based Java wrapper libraries, giving access to OPC. The problem: they cost as much as the OPC server (about 600 Euro) and, just like the server, I had to purchase one library license for each deployment.

I never understood this business model; I would hapilly buy the library once and deploy to my hearts content, as this would probably be cheaper than developing the library myself (I do this with JFreeChart, for example). But if I have to pay for every deployment this is simply not economical, since with the money of 2 or 3 deployments I was definitely able to develop the library from scratch. I was seriously thinking of developing the library myself and selling it with JFreeChart’s business model (I had no competition anyway), until I found JEasyOPC.

Antonin Fisher developed an OPC wrapper library based on JNI (and Delphi) and offered it with a LGPL license. I’m testing the library now and it seems to perform very well on Java 5. It crashes on Java 6 though (apparently due to multithreaded issues).

If Antonin ever gets around to fix this issue on Java 6, we’ll have a winner! :)