The PLC is a blind device. It does not have any provision of displaying the plant status to the operator, or to enter certain data like set points, or manual mode operation.
An additional device is needed for these provisions, to communicate with PLC, which will have a display to show the status and also the means to enter set points. Such a unit is called Human Machine Interface (HMI).
There are two basic types of HMI:
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PC based system in which the computer acts as HMI
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Industrial panel mounted type HMI
Figure 1: PC based system in which the computer acts as HMI
Figure 2: Industrial panel-mounted type HMI