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Human machine interfaces ( hmi) are used to enable the plant operators to interact with the control system. They provide a mimic of the plant or machine and allow the setting and viewing of critical parameters. In more advanced hmis we can have multiple communication ports, trending of values, 21cfrii compliance, alarm summary and grouping etc. They also come in a number of versions like simple text displays, graphic displays in mono and colour with varying number of colours and pixel definitions, led,lcd. The sizes are from 2x20 text display, and 3 to 19 inches diagonal. Proper attention has to be paid to the design of the hmi screens as this has to be intuitive as well as useful as possible for the operator. 

Scada is a substantially more advanced human machine interface system as it is backed by a computer operating systems and interfaces with standard software and databases is possible. Some fo the functions that are available with scada are mimics, trends, alarms,pop up screens, sequence of operation, recipes, reports, interface with databases and erp systems. Significant amount of custom scripting can be done to achieve specific functions, wireless communication with other systems, web connectivity, redundant systems, communication with multiple sources etc can be engineered. VB based front end interfaces are often used.