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The SCADA (Supervisory Control And Data Acquisition) would also be a HMI (Human Machine Interface) and if the HMI system had the additional data acquisition which most are capable of then it would also be SCADA. The advent of the Windows data exchange and client - server model has blurred the picture to the point this this question is bordering on being trivial as the differences are so small. The same questions could be asked or made about the differences in DCS and SCADA and HMI.

In the old days the control systems (PLCs and relay logic) were not capable of handling much in the way of analog signals and control loops. Many systems had standalone controllers that had the capability of reporting over RS232 or RS422/485 to a computer that had many serial ports to communicate with them. Later the controls systems for DCS were built to handle the mix of stand alone (process setpoints and process feedback) and loops controlled by the DCS. SCADA was originally more of a system of monitoring large systems that were more like power systems where the loads were monitored and operators made decisions to control macro elements. Trips could be reset and switches opened and closed. The power houses would use DCS to control the generators etc.

Now a single instance of a modern HMI like WonderWare could handle more that any SCADA 20 years ago. The speed of the computers and the quality of the networks is vastly superior to those systems.
Today the difference is in the "feel" of the terms more than the capability of any of the systems. The amount of security and duplication for any process could be handled, but proving it might be difficult.

As an aside there seems to be a virus aimed at the Siemens HMI/control systems that is coincidently used by Iran for their nuclear systems (I am told). This is currently causing some concern for other users of the system worldwide.
The importance of security of any system cannot be overstated in today's environment and each of the systems has its vulnerabilities.

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