OPC UA, Modbus and MQTT at the level of what each is for and how each fails.
3 questions in Manufacturing & Industrial IoT.
Modbus is a register read/write protocol with no types and no security; OPC UA is an information model with certificate-based security and subscriptions; MQTT is a broker-based transport that says nothing about payload meaning. They sit at different layers rather than competing.
Answering a poll is work the controller does with the processor that runs its logic, so a second master competes with the control function rather than sitting beside it. Take the feed from the existing aggregation point, and treat the tag map as somebody else's change-controlled artefact.
On any protocol that reports by exception, no new value and an unchanged value look identical, so liveness has to be carried deliberately. Status codes, source timestamps, birth and death messages and a source-side counter each establish it at a different layer of the path.