What is the difference between Human Machine Interface (HMI), and Operator Interface Terminal (OIT)? 

All OITs are HMIs, but not all HMIs are OITs. OIT usually means a dedicated terminal with graphics and/or text that provides data/control of a local (rather than remote) piece of equipment or a process by a person acting as an operator of said equipment. HMIs can also mean the same thing, but they can also be something that provides remote monitoring and/or operational control of an entire system.

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