Well you have to have somthing that can communicate to the PLC so you would need a dll that you can use to write code against. There are a few vendoes that do ActiveX controlls or dll's for communication. Some protocols are simple enough that you could actually impliment your own code to talk to the device. If y ou do a google search for ActiveX Automation device communicaitons you will find them.

Keep in mind that you are coing to have to all the communicaitons management that the communications portion of the OPC server would have done and you are going to have have your own error handling.

Depending on the type of PLC you are using, one option would be using an HMI to act as a OPC Driver. It is possible to use an HMI Tag and Microsoft Excel DDE, with a little bit of help from VBA. I doesn't work as well as an OPC Server and much more limited.

One of the ways that I've suggested to do this is to use Modbus TCP/IP, you can implement a Modbus Master in a PC Application pretty quickly using standard sample code found on the net. Of course, this also requires the PLC to support Modbus, but most are capable (I even saw some sample code the other day which allows Siemens to talk Modbus...crazy.)

I'd second Nick's vote for Modbus/TCP.

We use it all the time with our custom controllers.

There's plenty of Master (client) code around on the net.

I've personally written several Modbus/TCP clients in Python using the Twisted network library.

There is always OpenSCADA if you are on the Java platform. The website is a bit of a mess at the moment (we are working on it), but if you are interested, just drop us line (info AT inavare DOT net)

There is a standalone project to talk to a OPC server written in pure java (Utgard project) which is already in use for quite some time.

And we have a brand new implementation of the dave protocol (S7) in pure Java (no dlls required, so it works just fine on linux!) which will be used in some of our projects (although we are not sure at the moment under which kind of license it will be available, if you are interested just let us know).

Full disclosure: I'm one of the developers of OpenSCADA

There are several ways of communicating with a PLC. Depending on your PLC of course ;-)

Possible solutions should be:

• Modbus (both RTU and TCP)
• Dave (for Siemens PLC)
• IEC60870
• A custom protocol developed for both PLC and host system

I justed finished an application that had these exact requirements with an Omron PLC.

Check out www.plclibrary.com

I believe it is exactly what you are looking for ... and relatively cheap!