Technical Article

The Difference Between HMI and SCADA and How They Work Together

July 06, 2021 by Antonio Armenta

Learn the differences and similarities between human-machine interface (HMI) and supervisory control and data acquisition (SCADA) and how they work together.

Human-machine interface (HMI) and supervisory control and data acquisition (SCADA) are two concepts within the industrial automation field that are often used interchangeably. In many applications, there is often confusion around the boundary between the two components. This is partly caused by users interacting with these systems in a very similar way. Many similarities between HMI and SCADA add many factors to the confusion.


What is an HMI?

HMI stands for human-machine interface. An HMI is a hardware device that runs software that makes it possible for humans to interact with a machine. This feature makes HMIs crucial elements for the operations and maintenance of equipment. Most modern automated systems are equipped with at least one HMI.


Figure 1. A technician using a Siemens HMI to operate machinery. Image used courtesy of Siemens


What is SCADA?

SCADA stands for supervisory control and data acquisition. A SCADA is a system consisting of different hardware components as well as software. Hardware and software in a SCADA system work together to control and monitor equipment in an industrial process. SCADAs increase workflow efficiency by capturing real-time data of the process and allowing changes to the control and setpoints from a remote location.

There are four components considered part of the basic architecture found in nearly every running SCADA system.


RTUs (Remote Terminal Units)

Remote terminal units (RTUs) are compact microprocessor-based control units installed at several different physical locations of the system. These devices share many similarities with programmable logic controllers (PLCs) but have some distinct characteristics. RTUs serve to monitor and control one or more field devices at a time. These field devices can be sensors, valves, actuators, and many others, both analog and digital. In other words, RTUs are the interface between the SCADA and the physical process.


Figure 2. An example of an RTU meant to monitor critical network components. Image used courtesy of DPS Telecom 


Most RTUs in the market support standard protocols, such as Ethernet and RS232. Although Ethernet is the most common today, RS232 and other serial-based protocols are still popular in legacy systems. 

As far as programming languages, there is a large variety in use. While some RTUs are configured through a simple web interface, others need more complex programming using C# or other languages. Some RTUs can be programmed using syntax similar to that of PLC languages, such as ladder logic and structured text.

Comparing the average cost of RTUs versus that of PLCs, contrary to common belief, RTUs have a higher price point. Despite this, one reason why RTUs are preferred in SCADA systems is that they are built to resist harsh environmental conditions. Oil refineries, water treatment plants, and electric power distribution systems are some examples of processes where SCADA systems are commonly used. These are environments with high temperatures and exposure to natural elements, making RTUs the right field controls component. 


Field Instruments (Inputs)

This comprises all the field devices monitored and controlled by the RTUs. The type of instruments, as well as the quantity needed in a SCADA system, is determined by the type and size of the application itself. 


Figure 3. An inclinometer is a common field sensor in offshore drilling platforms and their SCADA. Image used courtesy of Vigor Technology


Temperature sensors, photoelectric presence detection sensors, proximity switches, and pressure sensors are some of the most widely used instruments in SCADA. The most important consideration when selecting field devices is to ensure they are compatible with the RTUs chosen or already in place.


Communications Network

A well-designed network is essential for the efficient and reliable flow of information in a SCADA system. Some of the most common connectivity platforms are Ethernet (wired), optical fiber, and Wi-Fi. The physical size of the SCADA system—as a consequence, the distances between RTUs and instrumentations in the field—determines the best network infrastructure for an application. Just like with any industrial network, it is important to incorporate security guidelines in the design.  



This HMI station may be referred to as Central SCADA. This central computer is responsible for processing the data obtained from the RTUs and displaying it so that operators can analyze it and make system controls decisions accordingly. Machine status, level indicators, alarms, and practically any meaningful information available can be shown in the HMI.


SCADA HMI programming

Figure 4. An example of a SCADA HMI screen. Image used courtesy of Windemuller


What is the Difference Between HMI and SCADA?

After presenting the definitions and basic architecture, the main difference between HMI and SCADA becomes more evident. A SCADA is a full system consisting of physical field devices and a central unit with an HMI. The HMI is a component of the SCADA. Because it is what human operators interact with, the two terms began to be used equally. However, they are two distinct automation concepts. The HMI is a component of the SCADA, but a SCADA cannot be a component of an HMI.