I find the Wikipedia article and other sources ambiguous and I find that the definitions are so broad that I don't know how and when to use the term "RTU". I would like to figure this out because I have to present a project using Modbus RTU and I would prefer to actually know the definitions of the words I will be using.
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What are the defining characteristics of an RTU?
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Is every slave that allows remote setting and reading of physical parameters an RTU, no matter the protocol?
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One version of the Modbus protocol is called Modbus RTU. Is any slave in a Modbus RTU link by definition an RTU? What if the same device was instead controlled by Modbus ASCII or Modbus TCP, would it still by definition be an RTU?
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As an example, what about a variable frequency drive set up as a Modbus RTU and being controlled via some field bus by a PLC; is that VFD an RTU? Why/why not? What if it was set up with Modbus ASCII or Modbus TCP, would it still by definition be an RTU?
Best Answer
1 & 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.