Exactly what is an RTU (Remote Terminal Unit)?
1 &amp; 2. An RTU is any device that collects data for and performs controls for a master device irregardless of how the data and controls are communicated between it and the master.


ModBus RTU is a protocol where the data is encoded as a series of 8 bit binary values.  ModBus ASCII is instead encoded into ASCII representations of 4 bit Hex values (it is readable as text on a standard TTY style terminal).  ModBus TCP is ModBus RTU optimized for transmission over ethernet via TCP.  There is also ModBus over TCP which is ModBus RTU over TCP (like Telnet) and is not compatible with ModBus TCP.
In general an RTU only does SCADA (data to the master and controls from the master).  If it does something else (like a VFD controller with a communication link) it is more properly referred to as an IED (Intelligent Electronic Device).  There are very few real RTUs any more, although the term is as common as it ever was.


So yes there is a technical definition of what is and isn't an RTU, but modern usage mostly ignores it.