Advosol offers toolkits for the development of OPC servers for the OPC specifications.
The servers consist of generic part that handles the OPC client interface and an application specific part the handles the devices.
The application specific server part can be developed as a .Net assembly using C# or Visual Basic .Net. The same customization .NET plug-in assembly can be used with the different toolkits for Classic OPC (DCOM) or OPC UA.
The same customization plug-in assembly can be used with the server running in 32-bit or 64-bit mode.
Which OPC Specification To Choose?
The XML DA and UA OPC specifications are around since 2003 but still most installed and newly developed OPC application use the DCOM based Classic OPC, mostly OPC DA (current data) and less Alarm&Events (AE) and Historical Data Access (HDA). DCOM has communication limitations but is good enough for most applications. For local connections (server and client on the same machine) DCOM offers best performance and tunneling and bridging products are available for situations that DCOM cannot handle or gets too complicated to configure.
The web services based OPC specifications don't have the communication limitations of DCOM but converter servers are likely necessary because most installed OPC applications are Classic OPC.
Often the OPC specification can be chosen based on personal preferences but there are situations that require a specific OPC specification or exclude some as unusable.
Whatever OPC specification is chosen for an application there probably will be need for converter servers because applications implemented on different specifications need to communicate.
Converter server products are available for most combinations.
It's good practice to implement the OPC specification that minimizes the need for converters. Converters tend to complicate the setup and the configuration because two different communication types are involved.