The purpose of OPC generally is to allow interconnection of different brands of field devices (controllers/PLC's) to HMI/SCADA software.
An OPC server (usually a software package on a Windows PC, but sometimes embedded in a field device) is responsible for communicating to the field device (PLC) and getting data that the OPC client asks for from (or to) the field device. The 'driver' for the field device is part of an OPC server.
Software products in the generic category of HMI/SCADA are almost always have (optional) OPC client capability.
OPC clients automatically scan the host PC and the network for OPC servers and connect to them.
OPC (before the latest UA version) used a Windows component known as DCOM (distributed oomponent object model) to communicate from OPC server to OPC client.
DDE is an older form of Windows based communication.
OPC was an outgrowth of the lack of DDE robustness.
OPC servers are licensed software.
OPC clients are licensed software options in HMI/SCADA software
OPC test clients are generally free for purposes of testing OPC server configurations, but are limited to test type tasks.
> compact logix l32e processor and RSVIEW32
I'm not an Allen Bradley guy, but I recognize these as A-B model names.
You can probably get these devices communicating without using OPC, because they are both A-B products, but you'll need an A-B guy to tell you what is needed to do so.