PLCs, HMIs, and SCADA: How They All Work?

Process automation and advanced system design is highly beneficial for a variety of applications, and they rely on HMIs, PLCs, and SCADA. While each device or system has its own unique role in overall process automation, all work together to perform various overlapping functions. To help you better understand such complex apparatuses and how they work together, we will provide a brief overview of each.

SCADA Systems

SCADA is an acronym that stands for Supervisory Control and Data Acquisition. SCADA systems comprise a mix of hardware and software, and they can conduct various operations ranging from the remote control of processes to the monitoring and processing of real-time data. With their basic role, SCADA systems find use in countless modern organizations as they promote efficiency, higher accuracy, smoother system communication, and so much more. Across public and private sectors, SCADA Systems may be found in applications simple or complex, regularly aided operations related to manufacturing, energy, transportation, recycling, oil, gas, and more.


Programmable logic controller (PLC) components are crucial elements of industrial automation as a form of industrial computer. PLCs often range in their size and capabilities, and designs are often based on the particular needs and requirements of a given application. When a PLC is in operation, it will monitor the state of input devices, using garnered information to determine the best way to control the state of output devices. PLCs can control anything from a chunk of a production line to the entire line itself, meaning that it can greatly improve standard operations and collect information.

To conduct their operations, PLCs first conduct an Input Scan to determine the state of all attached input devices. Then, the Program Scan is executed where the PLC will scan the created program before carrying it out. Once that is done, the Output Scan is carried out as all output devices are scanned before the device determines which to energize. The final step of the PLC process is to conduct communications for internal diagnostics, and once that is finalized, the PLC repeats the entire process again in a repetitive fashion.


The Human-Machine Interface (HMI) is the device that communicates with Programmable Logic Controllers during standard operations, and HMIs differ from SCADAs and other systems in the fact that they are a local machine. Despite this, they still perform a very similar role to the SCADA, collecting data and communicating. Furthermore, HMIs are less of a system and more of an interactive screen that permits control over a device. Generally, HMIs are used to manage processes and may range from a simple computer monitor to a robust industrial system.

Alongside their similar role to SCADA systems, HMIs also work together with PLCs to promote industrial automation. As discussed before, their biggest difference comes down to the fact that SCADA systems are for remote operations while the HMI is a local machine. Nevertheless, both are advantageous options where the choice boils down to the particular needs of the user and their operations.


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