Newer vesions of RSLinx is an OPC Complient Server. You can connect using an OPC Client OCX.

I've used VB.NET as an OPC Client to RSLinx.

.NET has a problem with OPC because COM Interop can't handle the interface.

IMO the best way is to use COM objects that make the OPC calls and then expose an Interop freindly interface to .NET
This is the approach taken by Kineticart and is the one I've used. Their OPC client is extremely good value.  See www.kineticart.co.uk.

There is also a 'pure .NET. method as discussed in the following article:

"OPC and .NET with COM Interoperability" http://www.codeproject.com/dotnet/opcdotnet.asp

Email me if you want to know more

OPCWare Client Developer (http://www.opcware.com/OPCWare.html) includes a VB.NET sample project. That ActiveX packs plenty of power, and is definitely the fastest way to get your OPC client applications developed.

The OPC Data componant and OPC Web Client are .Net componants that have been used in a number of industrial applications.  See http://www.opcdata.net/

They also have lots of example code!

I have used a .ocx from Autoamted Solutions that will talk to the most popular of AB products via Ethernet, KTX or DF-1. I think its a superior product because i can control the timing and programming methodolgy that works best for the project. Otherwise you have of conform to AB gudilines of OPC or DDE.

I've the above referenced OCX control from Automated Solution many times and it's always been very fast and very reliable. I recommend it very highly, not only because of it's abilities but also because of it's price!!

I have read alot about this, one piece of information that seems to be missing is how do you have the plc initialize the communication to the pc.  Such as we want to log to a sql server, but the logging is random.

These are the two ways that i know of but would like to know if there is any better.

1. Timer on pc in vb.net app checking plc for a bit that indicates a log entry needs to be made.

2. IO card in PC interfaces to IO on plc, but you still need a timer to check that IO since there is no real event for IO cards.

Any suggestions?