As AlfredoQuintero noted, connections from an HMI are USUALLY explicit connections, in effect peer-to-peer connections. There really is no master/slave relationship. I don't know what your system comes with for native drivers. Often, the connection is run through a tag server of some type, especially when the HMI is "third party" and needs to support multiple protocols from multiple manufacturers. My guess is this is the way that the Codesys HMI is designed to interface with a non-Codesys plc. I just don't know if the install you are using comes with a stand-alone com server, if the functionality is built in to the install or if no com server is provided at all.
Keith