OPC Unified Architecture (OPC UA) is a machine to machine communication protocol for industrial automation developed by the OPC Foundation. Refer to https://en.wikipedia.org/wiki/OPC_Unified_Architecture for a detailed overview.
A proof-of-concept extension called topcua provides a Tcl binding to a C based OPC UA implementation from https://open62541.org/ and can be found in https://www.androwish.org/index.html/dir?name=jni/topcua
The code is very portable and might run on all Tcl supported platforms provided that the platform's C compiler supports C99.