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Very Brief and Incomplete Summary of PLC Evolution

My first PLC programming was right out of college when I was working at an industrial steam plant.  The PLC was an AB 2/30 so, yes, it was a long time ago.   This PLC handled all the discrete control in the plant but the analog control was handled by Toshiba loop controllers.   In those days PLCs did not have the math capability to do complex PID loops with cascade, feedforward and other advanced analog processing.  Now PLCs have bridged the gap and can handle most all analog control necessary.   There is still another class of controllers for heavy process control but for the most part, PLCs can handle run of the mill PID control and other math.  So now you may only need one controller instead of one for analog and one for discrete.

Another advance in the last 20 years has been communication.  Early PLCs relied on push buttons and pilot lights to provide interface to operators.   As things got more advanced, one could add an HMI (Human Machine Interface) by connecting it to a serial port on the PLC.  The HMI provided cost savings over the push buttons on medium to larger sized systems.  Usually the serial protocols were proprietary so that you had to purchase the HMI from the vendor who provided the PLC.  Eventually open standard protocols like Modbus were developed and third party HMI vendors began cracking the code of the proprietary serial communications.  These advances led to a growth in third party HMI manufacturers providing more cost effective solutions.


At some point somebody had a crazy idea to integrate the HMI and the PLC so they were one device.  The early models started with a couple of lines of simple text but advances came quickly.  Unitronics is the leader in this integrated technology.  We started selling Unitronics about 10 years ago.  It was a novel product, very cost effective and fairly powerful.  In recent years Unitronics has added more advanced graphical touchscreens that rival the best HMI’s on the market build right into their PLCs.  The simple 2 line text display PLCs are still offered but so are 10.4″ and 12″ graphics with colors and features that would blow your mind.

The graphic touchscreen integration has been such a big hit that other manufacturers are getting in the game.  Since Unitronics led the wave we have seen other manufactures such as Maple Systems, Toshiba, Exor and Red Lion add combinations IO modules and Logic Processors to their products.   In some cases it was PLC manufactures adding and HMI and in other cases it was HMI vendors adding PLC capability.  Regardless of the roots of the companies their common quest to fill a market segment of integrated HMI and PLC is clear.

Advantages of Integrated PLC and HMI

The most obvious advantage of the integrated HMI/PLC is cost.  You save money mounting the IO and processor in the same enclosure.   Another cost advantage is programming time.   One piece of software is used to program the HMI and PLC features of the device.  This means you work with one common database and don’t have to worry about communication since it is internal to the device.

Another advantage is space.  For small systems the HMI/PLC can be mounted in the door of the enclosure freeing up cabinet space.  Larger systems can utilize expanded IO mounted on DIN rail but it is not necessary for small systems.   This space can be very valuable for OEM users of PLCs.

Redundant processing is an advantage that a couple of our customers realized on their own.   Both of these customers have a primary PLC handling the bulk of the control logic.  The PLC/HMI communicates with the mail PLC and provides a means of interface much like traditional HMIs.  But in addition to this interface the PLC/HMI also has its own logic programmed into it.  In one case it has a shutdown sequence in case the main PLC fails.  In another case it has logic for a first out annunciator that was more easily programmed into the Unitronics than the main processor.

Common Applications of Integrated PLC and HMI

Here are just a few applications our customers have used Integrated HMI/PLC functionality.  The limit is your imagination.

  • OEM Machine Control.  This is by far the most common application since cost and space are important parameters for OEMs
  • Water/Wastewater Control.  Remote sites have very little control logic but it is critical.  These small integrated platforms are cost effective and simple for water pump stations, sewage lift stations and valve stations.  They have Ethernet communications with open protocols for integration into SCADA.
  • Industrial Plant Distributed Control.  A lot of control in manufacturing is small in scale.   One control architecture philosophy is to match the controller to the scale of the job.  A big PLC could handle many smaller control applications under one system but there are disadvantages.  Programming each system exposes the processor to more downloads and possibility for trip or failure. The big central PLC is often more costly.  Lastly, the HMI/PLC is right where the operator needs it so no additional HMI is required.
  • Boiler Control.  Boiler control is mostly analog control of valves, drives and positioners but the Unitronics PLC works well for these applications providing a cost effective solution.

Unitronics V1040