A Glossary of IIoT Terms

IIoT Alphabet Soup

This post is Part 2, please see: Industrial Internet of Things for Part 1.

The infusion of Internet protocol professionals into Industrial Machine communications has presented a new alphabet soup of acronyms and terms. Even some seasoned IT professionals are left scratching their heads when presented with the closed systems that have existed for years in industrial machine to machine communications.

Below are a few key acronyms for newcomers to industrial automation. While this list is far from exhaustive, and the explanations are basic, these common terms are a good starting place when diving into IIoT. These terms and acronyms are intertwined, and while this information is presented as a glossary, any chance at understanding requires reading through the entire list rather than expecting to fully understand any one definition in isolation.

ICS – Industrial Control System
ICS a general term for different types of control systems and their components used to manage industrial processes. They can range from a few modular panel-mounted controllers to large interconnected Distributed Control Systems with thousands of connections to process sensors and monitors. The systems receive data from remote sensors measuring process variables (PVs). The measured values are compared to benchmarks or set points (SPs) and commands are sent back to the controllers or Final Control Element FCEs, like valves and switches, to make adjustments. A Distributed Control System is an ICS, but not all ICSs are distributed.

DCS – Distributed Control System
A DCS is distinguished by autonomous control loops, where the remote control is decentralized and distributed throughout the system. The system still has central operator supervisory control, but individual processes have localized control. This is in contrast to non-distributed control systems where all control happens at the top levels. The DCS concept increases efficiency by localizing control functions near the processes themselves, with only monitoring and supervision centralized.

Distributed control systems were used first in applications with critical processes where interruption isn’t acceptable. DCSs allowed task level controllers to continue to function even if central control was interrupted. The manufacturers of Distributed Control Systems sold all the equipment as a package, with central control and task specific modules included, significantly reducing integration risk. Today the functionality of SCADA and DCS systems are very similar.

PLC – Programmable Logic Controller
A PLC is a ruggedized, industrial computer adapted for the control of a manufacturing process, such as assembly lines, or robotic devices, or any activity that requires high reliability control, site level programming, and problem or fault diagnosis. They were first developed for the automobile industry to replace mechanical devices like relays, timers, and sequencers. A PLC reports to a SCADA supervisor, but is still not the unit that completes a task. PLCs are usually used localy, where control is wired. They usually communicate with the Modbus protocol. There is significant functional overlap between PLCs and RTUs.

RTU – Remote Terminal Unit
RTUs are programmable, microprocessor-controlled electronic devices that interface sensors and other input objects in the physical world to a distributed control system or SCADA. Like a PLC, the RTU doesn’t do the work, but translates data to the SCADA supervisor. An RTU is usually capable of running programs and processing data before it reports to supervisory control. RTUs are more suitable to remote monitoring and wireless communications. RTU also may have proprietary tools for programming but will use common protocols, like Modbus, for communications. There is significant functional overlap between PLCs and RTUs.

FCE – Final Control Elements
An industrial process has several components, and the FCE is the part that actually does the work at the end of the line. FCEs include valves, dampers, couplings, gates and many others that are adjusted by the system to achieve or maintain a Set Point (SP).

PV – Process Variable

An FCE will have a process variable, an end value that is monitored or measured in an industrial system. For example, temperature or pressure could be Process Variables that are monitored. Each PV will have a Set Point or SP.

SP – Set Point
A sensor measuring a Process Variable (PV) will have a Set Point. The SP is a goal for the value. A process may have a temperature range with a Set Point of 38°C. The FCE will report the Process Variable to the PLC or RTU which will make adjustments to the FCE to achieve the Set Point.

HMI – Human Machine Interface
also MMI (man–machine interface) or HCI (human–computer interface)
While strictly speaking, a keyboard and mouse on any computer is an HMI, the term is generally used for the human control panel of an Industrial Control System. It is usually the human input panel at the local level for the PLC or the RTU, but it could be the control at supervisor level, although some would reserve the term Operator Interface Terminal for the main control. An HMI can be buttons, touch screens or a keyboard and monitor. Plain old computer keyboards and monitors are called UIs (User Interfaces) or GUI (Graphical User Interfaces) even if they are human-machine interfaces.

SCADA – Supervisory Control And Data Acquisition

To over simplify, a SCADA system is like a DCS but in a more modular form. The SCADA Control system architecture is above PLCs or RTUs (both local controllers) and the FCE, the things that do the work. The PLC/RTU will run the operation, but the SCADA sets the control points and levels for the PLC/RTU. Pretty much anywhere you look you will find a SCADA system. Even the multiple refrigerators at your local supermarket are probably SCADA controlled.

Modbus is a serial communications protocol for use with programmable logic controllers (PLCs). While there is no document proclaiming it such, Modbus has become the go-to protocol for connecting SCADA systems. It is the common language for connecting industrial electronic devices. It has taken this role because it was developed for industrial applications, it is openly published, and it is royalty-free. Modbus is easy to deploy and maintain, and moves raw bits or words without placing many restrictions on vendors. Modbus allows mixing of sensors, RTUs, PLC, and FCEs from various sources.

DNP3 – Distributed Network Protocol
DNP3 is a communications protocol. It is more complex than Modbus. According to Wikipedia, it is used almost exclusively in utilities management, which is confirmed by the DNP3 Users Group, which has taken ownership of the protocol and assumes responsibility for its evolution. DNP3 was originally developed for the electric utility industry but is being used in water, wastewater, and oil and gas. Like Modbus, DNP3 is an open and public protocol.

In summary: A SCADA system is like a DCS (which is an ICS) that connects PCLs and/or RTUs and their corresponding FCEs to monitor PVs in order to maintain the SPs. The systems mostly use the Modbus protocols except in the utility sector where DNP3 is favored.

If the previous two sentences made any sense to you at all, take a bow. You are ready for the Industrial Internet of Things.