Master terminal units (MTU) in SCADA system is a device that issues the commands to the Remote Terminal Unit (RTUs) which are located at remote places from the control, gathers the required data, stores the information, and process the information and display the information in the form of pictures, curves and tables to human interface and helps to take control decisions. This is the operation of the Master Terminal Unit (MTU) located in the control center.
Communication between the MTU and RTU is bidirectional, however the major difference is RTU cannot initiate the conversation, an RTU simply collects the data from the field and stores the data. Communication between the MTU and RTUs are initiated by the programs with in the MTU which are triggered either by operator instructions or automatically triggered.99% of the instructions and messages to the RTUs from MTUs are automatically triggered.
When Master Terminal Unit (MTU) asks the desired information RTU sends it. So MTU is considered as master and RTU is the Slave. After receiving the data required MTU communicates to the printers and CRTs which are operator interface through necessary protocols. At this level of communication it will be of the form peer-to-peer communication rather than master slave communication unlike communication between MTUs and RTUs. Thus in SCADA system Master Terminal Unit (MTU) acts as heart of the system.