Vol. Control Systems
Chapter Basics of Programmable Logic Controllers (PLCs)

Examples of PLC Systems

PLC specs and performance may change over the years, but the core functions of processing, I/O, and network communication remain, making PLC technology reliable and upgradeable for generations.

Programmable logic controllers are essentially nothing more than special-purpose, industrial computers. As such, they are built far more ruggedly than an ordinary personal computer (PC), and designed to run extremely reliable operating system software. PLCs, as a rule, do not contain rotating disk drives, cooling fans, or any other moving parts. This is an intentional design decision, intended to maximize the reliability of the hardware in harsh industrial environments where the PLC chassis may be subjected to temperature extremes, vibration, humidity, and airborne particulates (dust, fibers, and/or fumes).

Large PLC systems consist of a rack into which circuit “cards” are plugged. These cards include processors, input and output (I/O) points, communications ports, and other functions necessary to the operation of a complete PLC system. Such “modular” PLCs may be configured differently according to the specific needs of the application. Individual card failures are also easier to repair in a modular system since only the failed card need be replaced, not all the cards or the whole card rack.

Small PLC systems consist of a monolithic “brick” containing all processor, I/O, and communication functions. These PLCs are typically far less expensive than their modular cousins, but are also more limited in I/O capability and must be replaced as a whole in the event of failure.

Legacy PLCs and Systems Still In Use Today

The following photographs show several examples of real PLC systems, some modular and some monolithic. These selections are not comprehensive by any means, as there are many more manufacturers and models of PLC. These first few examples represent older PLC systems that might still be seen in use today.

It’s important to realize that many PLCs in use today are not brand new models, but have been installed and exhibited very reliable performance for decades. The concepts of PLC wiring, diagnostics and troubleshooting apply to all generations of PLC technology.

The first photograph is of an older style Siemens/TI 505 series PLC (end of life announcement was in early 2016), installed in a control panel of a municipal wastewater treatment plant. This is an example of a modular PLC, with an individual processor, I/O, and communication cards plugged into a rack. Three racks appear in this photograph (two completely filled with cards, and the third only partially filled):

The power supply and processor card for each rack is located on the left-hand end, with I/O cards plugged into slots in the rest of the rack. Input devices such as switches and sensors connect by wire to terminals on input cards, while output devices such as lamps, solenoids, and motor contactor coils connect by wire to terminals on output cards.

One of the benefits of modular PLC construction is that I/O cards may be changed out as desired, altering the I/O configuration of the PLC as needed. If, for example, the PLC needs to be configured to monitor a greater number of sensors, more input cards may be plugged into the rack and subsequently wired to those sensors. Or, if the type of sensor needs to be changed – perhaps from a 24 volt DC sensor to one operating on 120 volts AC – a different type of input card may be substituted to match the new sensor(s).

In this particular application, the PLC is used to sequence the operation of self-cleaning “trash racks” used to screen large debris such as rags, sticks, and other non-degradable items from municipal wastewater prior to treatment. These trash racks are actuated by electric motors, the captured debris scraped off and transported to a solid waste handling system. The motion of the trash racks, the sensing of wastewater levels and pressures, and the monitoring of any human-operated override controls are all managed by these PLCs. The programming of these PLCs involves timers, counters, sequencers, and other functions to properly manage the continuous operation of the trash racks.

The next photograph shows an Allen-Bradley/Rockwell Automation PLC-5 system (end of life also announced fairly recently in 2017), used to monitor and control the operation of a large natural gas compressor. Two racks appear in this first photograph, with different types of I/O cards plugged into each rack:

Like the Siemens 505 PLC seen previously, this Allen-Bradley PLC-5 system is fully modular and configurable. The types and locations of the I/O cards inserted into the rack may be altered by appropriately skilled technicians to suit any desired application. The programming of the PLC’s processor card may also be altered if a change in the control strategy is desired for any reason.

In this particular application, the PLC is tasked with monitoring certain variables on the gas compressor unit, and taking corrective action if needed to keep the machine productive and safe. The automatic control afforded by the PLC ensures safe and efficient start-ups, shut-downs, and handling of emergency events. The networking and data-logging capability of the PLC ensure that critical data on the compressor unit may be viewed by the appropriate personnel. For this particular compressor station, the data gets communicated from Washington state where the compressor is located all the way to Utah state where the main operations center is located. Human operators in Utah are able to monitor the compressor’s operating conditions and issue commands to the compressor over digital networks.

Both the Siemens (formerly Texas Instruments) 505 and Allen-Bradley (Rockwell) PLC-5 systems are considered “legacy” PLC systems by modern standards, the two systems in the previous photographs being about 30 years old each. It is not uncommon to find these “obsolete” PLCs still in operation, though. Given their extremely rugged construction and reliable design, these control systems may continue to operate without significant trouble for decades.

A somewhat newer, yet still legacy model of PLC manufactured by Allen-Bradley is the SLC 500 series, often verbally referred to as the “Slick 500”. This PLC is also modular in design like the older PLC-5 system, although the racks and modules of the SLC 500 design are more compact. The SLC 500 rack shown in the next photograph has 7 “slots” for processor and I/O cards to plug in to, numbered 0 through 6 (left to right):

The first three slots of this particular SLC 500 rack (0, 1, and 2) are occupied by the processor card, an analog input card, and a discrete input card, respectively. The slots 3 and 4 are empty (revealing the backplane circuit board and connectors for accepting new cards). The slots 5 and 6 hold discrete output and analog output cards, respectively.

A feature visible on all cards in this system are numerous LED indicators, designed to show the status of each card. The processor card has LED indicators for “Run” mode, “Fault” conditions, “Force” conditions (when either input or output bits have been forced into certain states by the human programmer for testing purposes), and communication network indicators. Each discrete I/O card has indicator LEDs showing the on/off status of each I/O bit, and the analog card has a single LED showing that the card is powered.

A nine-slot SLC 500 system is shown in the next photograph, controlling a high-purity water treatment system for a biopharmaceutical manufacturing facility. As you can see in this photograph, not all slots in this particular rack are occupied by I/O cards either:

Some of the inputs to this PLC include water level switches, pressure switches, water flow meters, and conductivity meters (to measure the purity of the water, greater electrical conductivity indicating the presence of more dissolved minerals, which is undesirable in this particular process application). In turn, the PLC controls the starting and stopping of water pumps and the switching of water valves to manage the water purification and storage processes.

A final legacy PLC system manufactured by Siemens appears in this next photograph, an S7-300, which is a different design of modular PLC. Instead of individual cards plugging into a rack, this modular PLC design uses individual modules plugging into each other on their sides to form a wider unit:

Modern PLC Models

A modern PLC manufactured by Allen-Bradley (Rockwell) is this ControlLogix 5000 system, shown in this photograph used to control a cereal manufacturing process. The modular design of the ControlLogix 5000 system follows the more traditional scheme of individual cards plugged into a chassis, or rack, of fixed size. This chassis contains 13 slots, while models are commonly available for all modular PLCs with as few as 4 and up to 20 or more slots.

In this PLC system, colors are used to easily identify the module varieties. From a close inspection of this Rockwell PLC, the power supply and CPU reside at the far left, followed by modules for communication (purple) analog (brown/yellow), AC signals (red), and relay signals (orange). Not all manufacturers follow the same coloring conventions, but it can help to isolate and fix problems when it is immediately clear which modules types may be affected by a system failure.

Another popular model of PLC recently released by a major manufacturer is the S7-1500. A selection of the local and remote CPU modules with some I/O module examples is shown below, courtesy of the manufacturer’s website. This is the latest evolution in the Siemens S7 family which includes the S7-300 shown earlier. This PLC is designed to include all of the standard I/O and communication abilities of any PLC, but is also designed for connections to virtual data processing spaces, referred to as ‘the cloud’.

Another important aspect of all PLCs is their communication ability. The earliest forms of digital communication were serial point-to-point interfaces, usually RS-232 connections (known by many users as simply a ‘serial port’). This point-to-point connection is still useful for programming, but it will more likely be seen as a USB connector on a modern PLC.

For interfacing with the endless array of external devices, a network adapter is an absolute necessity. A few main network protocols dominate the industrial market. The most familiar is Ethernet, where an 8-pin RJ or circular M12 connector provide an internet-enabled connection medium. Many older communication protocols (notably Modbus and controller area net [CAN] systems) ride along on the Ethernet interface, giving an easy method of using old equipment with a brand new controller.

Other common network interfaces include Profinet, ControlNet, EtherCAT, RS-485, and more. Most PLCs can accept downloaded programs through any of these interfaces, provided the programming PC has an adapter that interfaces with that protocol. In the image below, you can see a CPU from the Automation Direct ‘Productivity’ series with many of these ports on the front: RS-485 (the green header at the top), USB, RS-232, Ethernet, and even an SD card slot for logging of data and programs.

Cloud-connected PLCs and the Internet of Things (IoT)

In these modern PLCs with internet and cloud-connected systems, the local controller still receives input signals from the field devices, sensors, switches, and even local human-machine interface (HMI) screens. The PLC also still handles logical decisions and causes output devices to activate, such as solenoids, valves, and relays. 

The unique feature of these cloud systems is the long-term processing of data for improvement. A large amount of the process data handled by the PLC (both inputs and outputs) is sent through a messaging system to a remote server hard drive. In this location, vast amounts of data can be stored and sifted with complex algorithms. 

But to what end? Why are we storing data and what exactly are we calculating with this data?

Long-term process operation and machine reliability can be tied to various parameters associated with the operating environment. If a machine runs too hot, it will fail, and the process will report increased downtime. Excess vibration, moisture, and even light exposure can also deteriorate key components at varying rates. There are other outlying variables that affect the system, perhaps the summer has been exceptionally warm, or a nearby town opened a new facility and introduced new occasional power fluctuations into the local grid. These pieces of data are unavailable to the local PLC.

When data is collected in the cloud, all of these seemingly unrelated data points may be used to create correlations. In the future, by measuring all of this information, predicting failures and downtimes is far more accurate. This concept of predictive maintenance is one of the most common uses of cloud computing. 

When a computer is used to process information, even at a local shop-floor level, and figure out new connections between variables to reach process improvement conclusions that often escape us human operators on a day-to-day basis… This concept is the foundation of artificial intelligence, which is a common term throughout industrial processes in recent times.

Edge Computing

Sometimes, companies are hesitant to submit massive amounts of data to a server in another location, worried about possible security breaches, which can be a legitimate concern. Other times, the urgency of safety measures creates a roadblock for collecting sensor data, transmitting it to the cloud, waiting for a reply, then disabling the power to a machine in an emergency.

For these cases, a middle ground between PLC logic and cloud computing, called ‘edge computing’ still collects the most critical and time-sensitive data from the PLC. This computer down on the shop floor makes the immediate, important decisions and calculations, but then will still provide a measure for handing the long-term trending data off to a remote server for the more long-term processing. 

The edge computing data may be stored and collected in small quantities for a relatively short time, but the cloud storage is far more expansive when machine trends over days, weeks, and even years are important to monitor process health. But for those ultra-sensitive scenarios that require extra security or a rapid response time, the edge computing machine is a great candidate.

Smaller PLC Systems

While the Siemens S7 and Rockwell ControlLogix PLC platforms represent large-scale, modular PLC systems, there exist much smaller PLCs available for a fraction of the cost. As mentioned previously, these small systems can provide a disadvantage since the number of I/O points is limited. However, across most manufacturers, you can expect to find plugs and headers on the sides and fronts of these small PLCs that still allow expansion through add-on I/O modules.

A good example of this small, yet module PLC is the Siemens S7-1200, shown below. It has embedded digital and analog input and digital output terminals, but also includes a slot for an additional add-on module at the top. The current model below features an analog output signal board. A small plastic flap removed from the right side also reveals a small pin header which can accept side-mounted I/O modules.

Rockwell Automation also offers a popular line of smaller, more compact PLCs, reasonably named the CompactLogix series, which can be seen in the image below. This family of PLCs includes some modular versions, just like a scaled-down version of the ControlLogix series described previously. Another version features embedded I/O terminals on the front of the controller, which allows customers to purchase a single unit rather than a chassis, power supply, CPU., and various I/O modules. 

These CompactLogix PLCs, are also able to accept add-on modules to provide extra analog and digital inputs and outputs. These add-ons are from the Point I/O family of remote I/O modules from Rockwell Automation.

Perhaps the least expensive PLC on the market is the Koyo “CLICK” PLC series, sold by Automation Direct. The processor module, along with eight discrete input and six discrete output channels all embedded, is shown in my hand (this is being sold for 85 US dollars currently, and with free programming software!):

This Click PLC is another example of a semi-modular design, with a minimum of input/output (I/O) channels built into the processor module, but having the capacity to accept multiple I/O modules plugged into the side, much like the Siemens S7-1200 PLC. Down at the bottom near my fingers, you can see a small plastic cover that, once removed, will reveal a multi-pin connection header.

Other semi-modular PLCs expand using I/O cards that plug into the base unit, not unlike traditional rack-based PLC systems. The Koyo DirectLogic DL06 is a good example of this type of semi-modular PLC, the following photograph showing a model DL06 accepting a thermocouple input card in one of its four available card slots:

This photograph shows the PLC base unit with 20 discrete input channels and 16 discrete output channels, accepting an analog input card (this particular card is designed to input signals from thermocouples to measure up to four channels of temperature).

Some low-end PLCs are strictly monolithic, with no ability to accept additional I/O modules. This General Electric Series One PLC (used to monitor a small-scale hydroelectric power generating station) is an example of a purely monolithic design, having no “expansion” slots to accept I/O cards:

A disadvantage of monolithic PLC construction is that damaged I/O cannot be independently replaced. If an I/O channel on one of these PLCs becomes damaged, the entire PLC must be replaced to fix the problem. In a modular system, the damaged I/O card may simply be unplugged from the rack and replaced with a new I/O card. Another disadvantage of monolithic PLCs is the inherently fixed nature of the I/O: the end-user cannot customize the I/O configuration to match the application. For these reasons, monolithic PLCs are usually found on small-scale processes with few I/O channels and limited potential for expansion. The good news for users is that most small PLCs in production today allow some sort of expansion, providing opportunities to upgrade and improve the system as the needs advance.


  • PLCs can work reliably for decades, and many technicians are quite likely to encounter a discontinued model working away reliably in an industrial process.
  • Larger, modular PLCs contain chassis which contain the modules as pluggable ‘cards’ that can be replaced individually in case of upgrade or failure.
  • Small PLCs can be more cost-effective, but limited in the number of I/O. Fortunately, most of them allow add-on modules for expansion.


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