An EPROM: the Texas Instruments
TMS27C040, a CMOS chip with 4 megabits of storage and 8-bit output
(shown here in a 600-mil ceramic dual-in-line package). The TMS27C040
operates at 5 volts, but must be programmed at 13 volts.
An EPROM (rarely EROM), or erasable programmable read-only memory, is a type of programmable read-only memory (PROM) chip
that retains its data when its power supply is switched off. Computer
memory that can retrieve stored data after a power supply has been
turned off and back on is called non-volatile. It is an array of floating-gate transistors
individually programmed by an electronic device that supplies higher
voltages than those normally used in digital circuits. Once programmed,
an EPROM can be erased by exposing it to strong ultraviolet (UV) light source (such as from a mercury-vapor lamp). EPROMs are easily recognizable by the transparent fused quartz (or on later models, resin) window on the top of the package, through which the silicon chip is visible, and which permits exposure to ultraviolet light during erasing. It was invented by Dov Frohman in 1971.
Operation
An Intel 1702A EPROM, one of the earliest EPROM types (1971), 256 by 8 bit. The small quartz window admits UV light for erasure.
Development of the EPROM memory cell
started with investigation of faulty integrated circuits where the gate
connections of transistors had broken. Stored charge on these isolated
gates changes their threshold voltage.
Following the invention of the MOSFET (metal–oxide–semiconductor field-effect transistor) by Mohamed Atalla and Dawon Kahng at Bell Labs, presented in 1960, Frank Wanlass studied MOSFET structures in the early 1960s. In 1963, he noted the movement of charge through oxide onto a gate. While he did not pursue it, this idea would later become the basis for EPROM technology.
In 1967, Dawon Kahng and Simon Min Sze at Bell Labs proposed that the floating gate of a MOSFET could be used for the cell of a reprogrammable ROM (read-only memory). Building on this concept, Dov Frohman of Intel invented EPROM in 1971, and was awarded U.S. patent 3,660,819 in 1972. Frohman designed the Intel 1702, a 2048-bit EPROM, which was announced by Intel in 1971.
Each storage location of an EPROM consists of a single field-effect transistor.
Each field-effect transistor consists of a channel in the semiconductor
body of the device. Source and drain contacts are made to regions at
the end of the channel. An insulating layer of oxide is grown over the
channel, then a conductive (silicon or aluminum) gate electrode is
deposited, and a further thick layer of oxide is deposited over the gate
electrode. The floating-gate
electrode has no connections to other parts of the integrated circuit
and is completely insulated by the surrounding layers of oxide. A
control gate electrode is deposited and further oxide covers it.
To retrieve data from the EPROM, the address represented by the
values at the address pins of the EPROM is decoded and used to connect
one word (usually an 8-bit byte) of storage to the output buffer amplifiers. Each bit of the word is a 1 or 0, depending on the storage transistor being switched on or off, conducting or non-conducting.
A cross-section of a floating-gate transistor
The switching state of the field-effect transistor is controlled by the voltage
on the control gate of the transistor. Presence of a voltage on this
gate creates a conductive channel in the transistor, switching it on. In
effect, the stored charge on the floating gate allows the threshold
voltage of the transistor to be programmed.
Storing data in the memory requires selecting a given address and
applying a higher voltage to the transistors. This creates an
avalanche discharge of electrons, which have enough energy to pass
through the insulating oxide layer and accumulate on the gate electrode.
When the high voltage is removed, the electrons are trapped on the
electrode. Because of the high insulation value of the silicon oxide surrounding
the gate, the stored charge cannot readily leak away and the data can be
retained for decades.
The programming process is not electrically reversible. To erase
the data stored in the array of transistors, ultraviolet light is
directed onto the die.
Photons of the UV light cause ionization within the silicon oxide,
which allows the stored charge on the floating gate to dissipate. Since
the whole memory array is exposed, all the memory is erased at the same
time. The process takes several minutes for UV lamps of convenient
sizes; sunlight would erase a chip in weeks, and indoor fluorescent lighting over several years. Generally, the EPROMs must be removed from equipment to be erased,
since it is not usually practical to build in a UV lamp to erase parts
in-circuit. Electrically Erasable Programmable Read-Only Memory (EEPROM)
was developed to provide an electrical erase function and has now
mostly displaced ultraviolet-erased parts.
As the quartz window is expensive to make, OTP (one-time
programmable) chips were introduced; here, the die is mounted in an
opaque package so it cannot be erased after programming – this also
eliminates the need to test the erase function, further reducing cost.
OTP versions of both EPROMs and EPROM-based microcontrollers are
manufactured. However, OTP EPROM (whether separate or part of a larger
chip) is being increasingly replaced by EEPROM for small sizes, where the cell cost isn't too important, and flash for larger sizes.
A programmed EPROM retains its data for a minimum of ten to twenty years, with many still retaining data after 35 or more years, and can be read
an unlimited number of times without affecting the lifetime. The
erasing window must be kept covered with an opaque label to prevent
accidental erasure by the UV found in sunlight or camera flashes. Old PC
BIOS
chips were often EPROMs, and the erasing window was often covered with
an adhesive label containing the BIOS publisher's name, the BIOS revision, and a copyright notice. Often this label was foil-backed to ensure its opacity to UV.
Erasure of the EPROM begins to occur with wavelengths shorter than 400 nm.
Exposure time for sunlight of one week or three years for room
fluorescent lighting may cause erasure. The recommended erasure
procedure is exposure to UV light at 253.7 nm of at least 15 Ws/cm2, usually achieved in 20 to 30 minutes with the lamp at a distance of about 2.5 cm.
Erasure, however, has to be
accomplished by non-electrical methods, since the gate electrode is not
accessible electrically. Shining ultraviolet light on any part of an
unpackaged device causes a photocurrent to flow from the floating gate
back to the silicon substrate, thereby discharging the gate to its
initial, uncharged condition (photoelectric effect).
This method of erasure allows complete testing and correction of a
complex memory array before the package is finally sealed. Once the
package is sealed, information can still be erased by exposing it to X
radiation in excess of 5*104rads, a dose which is easily attained with commercial X-ray generators.
In other words, to erase your
EPROM, you would first have to X-ray it and then put it in an oven at
about 600 degrees Celsius (to anneal semiconductor alterations caused by
the X-rays). The effects of this process on the reliability of the part
would have required extensive testing so they decided on the window
instead.
EPROMs have a limited but large number of erase cycles; the silicon
dioxide around the gates accumulates damage from each cycle, making the
chip unreliable after several thousand cycles. EPROM programming is slow
compared to other forms of memory. Because higher-density parts have
little exposed oxide between the layers of interconnects and gate,
ultraviolet erasing becomes less practical for very large memories.
Even dust inside the package can prevent some cells from being erased.
Application
For large volumes of parts (thousands of pieces or more), mask-programmed ROMs
are the lowest cost devices to produce. However, these require many
weeks lead time to make, since the artwork or design in an IC mask layer
or photomask must be altered to store data on the ROMs. Initially, it
was thought that the EPROM would be too expensive for mass production
use and that it would be confined to development only. It was soon found
that small-volume production was economical with EPROM parts,
particularly when the advantage of rapid upgrades of firmware was
considered.
Some microcontrollers, from before the era of EEPROMs and flash memory, use an on-chip EPROM to store their program. Such microcontrollers include some versions of the Intel 8048, the Freescale 68HC11, and the "C" versions of the PIC microcontroller.
Like EPROM chips, such microcontrollers came in windowed (expensive)
versions that were used for debugging and program development. The same
chip came in (somewhat cheaper) opaque OTP packages for production.
Leaving the die of such a chip exposed to light can also change behavior
in unexpected ways when moving from a windowed part used for
development to a non-windowed part for production.
EPROM generations, sizes and types
The first generation 1702 devices were fabricated with the p-MOS technology. They were powered with VCC = VBB = +5 V and VDD = VGG = -9 V in Read mode, and with VDD = VGG = -47 V in Programming mode.
The second generation 2704 / 2708 devices switched to n-MOS technology and to three-rail VCC = +5 V, VBB = -5 V, VDD = +12 V power supply with VPP = 12 V and a +25 V pulse in Programming mode.
The third generation 2716 / 2732 devices upgraded to an evolved n-MOS technology that required only a single-rail VCC = +5 V power supply for read operations, and a single VPP = +25 V programming voltage without pulse. The unneeded VBB and VDD
pins were reused for additional address bits allowing larger capacities
(2716 / 2732) in the same 24-pin package, and even larger capacities
with larger packages.
Later the decreased cost of the CMOS
technology allowed the same devices to be fabricated using it, adding
the letter "C" to the device numbers (27xx(x) are n-MOS and 27Cxx(x) are
CMOS).
While the industry was heading towards a standard pinout and
architecture of the 27xx EPROMs, Texas Instruments (TI) introduced a few
types that are now considered non-standard. While the TMS2716 was a
three-rail device like preceding types, their TMS2516 was a single +5V
EPROM like the 2716 offered by the other chip foundries. TI then
released the TMS2532 and TMS2564, both single +5V devices, but with
pinouts incompatible with the 2732 and 2764.
While parts of the same size from different manufacturers are
compatible in read mode, different manufacturers added different and
sometimes multiple programming modes leading to subtle differences in
the programming process. This prompted larger capacity devices to
introduce a "signature mode", allowing the manufacturer and device to be
identified by the EPROM programmer. It was implemented by forcing +12 V
on pin A9 and reading out two bytes of data. However, as this was not
universal, programmer software also would allow manual setting of the
manufacturer and device type of the chip to ensure proper programming.
PLCs for a monitoring system in the pharmaceutical industry
A programmable logic controller (PLC) or programmable controller is an industrial computer that has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, machines, robotic devices, or any activity that requires high reliability, ease of programming, and process fault diagnosis.
PLCs can range from small modular devices with tens of inputs and outputs
(I/O), in a housing integral with the processor, to large rack-mounted
modular devices with thousands of I/O, and which are often networked to
other PLC and SCADA systems. They can be designed for many arrangements of digital and analog I/O, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact.
PLCs were first developed in the automobile manufacturing
industry to provide flexible, rugged and easily programmable
controllers to replace hard-wired relay logic systems. Dick Morley, who invented the first PLC, the Modicon 084, for General Motors in 1968, is considered the father of PLC.
A PLC is an example of a hard real-time
system since output results must be produced in response to input
conditions within a limited time, otherwise unintended operation may
result. Programs to control machine operation are typically stored in
battery-backed-up or non-volatile memory.
Invention and early development
The PLC originated in the late 1960s in the automotive industry in the US and was designed to replace relay logic systems. Before, control logic for manufacturing was mainly composed of relays, cam timers, drum sequencers, and dedicated closed-loop controllers.
The hard-wired nature of these components made it difficult for
design engineers to alter the automation process. Changes would require
rewiring and careful updating of the documentation. Troubleshooting was a
tedious process. When general-purpose computers became available, they were soon applied
to control logic in industrial processes. These early computers were
unreliable and required specialist programmers and strict control of working
conditions, such as temperature, cleanliness, and power quality.
The PLC provided several advantages over earlier automation
systems. It was designed to tolerate the industrial environment better
than systems intended for office use, and was more reliable, compact,
and required less maintenance than relay systems. It was easily
expandable with additional I/O modules. While relay systems required
tedious and sometimes complicated hardware changes in case of
reconfiguration, a PLC can be reconfigured by loading new or modified
code. This allowed for easier iteration over manufacturing process
design. With a simple programming language focused on logic and
switching operations, it was more user-friendly than computers using general-purpose programming languages. Early PLCs were programmed in ladder logic, which strongly resembled a schematic diagram of relay logic. It also permitted its operation to be monitored.
In recent years, the introduction of virtual PLCs has expanded the
scope of programmable logic controllers. Virtual PLCs are software-based
controllers that simulate the functions of traditional PLCs but are
executed on general-purpose hardware, offering a more cost-effective and
flexible alternative. These controllers enable automation systems to be
managed without the need for dedicated hardware, making them ideal for
applications requiring simulation, remote control, or cloud-based
systems.
Modicon
Logo as of 1997
In 1968, GM Hydramatic, the automatic transmission division of General Motors, issued a request for proposals
for an electronic replacement for hard-wired relay systems based on a
white paper written by engineer Edward R. Clark. The winning proposal
came from Bedford Associates from Bedford, Massachusetts. The result, built in 1969, was the first PLC and designated the 084, because it was Bedford Associates' eighty-fourth project.
Bedford Associates started a company, Modicon, Inc., dedicated to developing, manufacturing, selling, and servicing this new product, which they named Modicon (standing for "modular digital controller"). One of the people who worked on that project was Dick Morley, who is considered to be the father of the PLC. The Modicon brand was sold in 1977 to Gould Electronics and later to Schneider Electric, its current owner. About this same time, Modicon created Modbus,
a data communications protocol to be used with its PLCs. Modbus has
since become a standard open protocol commonly used to connect many
industrial electrical devices.
One of the first Modicon 084 models built is now on display at Schneider Electric's facility in North Andover, Massachusetts. It was presented to Modicon by GM,
when the unit was retired after nearly twenty years of uninterrupted
service. Modicon used the 84 moniker at the end of its product range
like Modicon Micro 84 and Modicon TSX CSY 84 until after the 984 made
its appearance.
Allen-Bradley
In a parallel development, Odo Josef Struger is sometimes known as the "father of the programmable logic controller" as well. He was involved in the invention of the Allen-Bradley programmable logic controller. Prior to the release of the IBM "Personal Computer" in 1981, these
devices were commonly known simply as "Programmable Controllers", or
"PC's. But rapid expansion of the personal computer industry in the
early 1980's, the abbreviation "PC" was rapidly and universally accepted
as a reference to "Personal Computers". Odo was later credited with
coining the "PLC" acronym to distinguish these two technologies, And the inclusion of the word
"Logic" was an easy choice as "Ladder Logic" had already been accepted
as the name of the programming language. Allen-Bradley (now a brand
owned by Rockwell Automation) went on to become the dominant PLC manufacturer in the United States during his tenure. Struger played a leadership role in developing IEC 61131-3 PLC programming language standards. Allen-Bradley's PLC-5 family, preceded by the 1774-PLC (released in the
1980's, was the first modular design found in modern-day systems.
While Modicon was the most popular of the first full featured
PLC's through the early 1980's, Allen Bradley overtook them in the
mid-1980's and grew to own the largest market share of PLC hardware and
software in North America. A primary reason for this rapid growth and market share was the
pioneering use of a "limited distribution" model for Rockwell PLC's and
other products throughout the US. As of 2025 there are more than 200
authorized distributors in the US. Rockwell assigns just one authorized
distributor to each geographic territory and limits support and
resources to customers who do not purchase from their local authorized
distributor (or from an OEM/SI who used an authorized distributor). In
addition, each distributor is required to have a staff of multiple
product "Specialists" for PLC's, Industrial Controls (such as buttons,
relays, contactors, etc), network security, Variable Speed Drives,
Motion products, Safety solutions, Life-Cycle services and other
value-Add services and other categories. This mix of "protected"
territory and multiple local experts made it easier for consumers to get
local assistance with design, selection and configuration of complex
systems.
Early methods of programming
Many early PLC programming applications were not capable of graphical
representation of the logic, and so it was instead represented as a
series of logic expressions in some kind of Boolean format, similar to Boolean algebra.
As programming terminals evolved, because ladder logic was a familiar
format used for electro-mechanical control panels, it became more
commonly used. Newer formats, such as state logic, function block diagrams, and structured text
exist. Ladder logic remains popular because PLCs solve the logic in a
predictable and repeating sequence, and ladder logic allows the person
writing the logic to see any issues with the timing of the logic
sequence more easily than would be possible in other formats.
Up to the mid-1990s, PLCs were programmed using proprietary programming panels or special-purpose programming terminals, which often had dedicated function keys representing the various logical elements of PLC programs. Some proprietary programming terminals displayed the elements of PLC programs as graphic symbols, but plain ASCII character representations of contacts, coils, and wires were common. Programs were stored on cassette tape cartridges. Facilities for printing and documentation were minimal due to a lack of memory capacity. The oldest PLCs used magnetic-core memory.
Architecture
A PLC is an industrial microprocessor-based controller with
programmable memory used to store program instructions and various
functions. It consists of:
A processor unit (CPU) which interprets inputs, executes the control program stored in memory and sends output signals,
A power supply unit which converts AC voltage to DC,
A memory unit storing data from inputs and program to be executed by the processor,
An input and output interface, where the controller receives and sends data from and to external devices,
A communications interface to receive and transmit data on communication networks from and to remote PLCs.
PLCs require a programming device that is used to develop and later
download the created program into the memory of the controller.
Compact PLC with 8 inputs and 4 outputsModular PLC with EtherNet/IP module, discrete and analog I/O, with some slots being empty
There are two types of mechanical design for PLC systems. A single box (also called a brick)
is a small programmable controller that fits all units and interfaces
into one compact casing, although, typically, additional expansion
modules for inputs and outputs are available. The second design type – a
modular PLC – has a chassis (also called a rack) that
provides space for modules with different functions, such as power
supply, processor, selection of I/O modules and communication
interfaces – which all can be customized for the particular
application. Several racks can be administered by a single processor and may have
thousands of inputs and outputs. Either a special high-speed serial I/O
link or comparable communication method is used so that racks can be
distributed away from the processor, reducing the wiring costs for large
plants.
Analog signals
can use voltage or current that is analogous to the monitored variable
and can take any value within their scale. Pressure, temperature, flow,
and weight are often represented by analog signals. These are typically
interpreted as integer values with various ranges of accuracy depending
on the device and the number of bits available to store the data. For example, an analog 0 to 10 V or 4-20 mA current loop input would be converted
into an integer value of 0 to 32,767. The PLC will take this value and
translate it into the desired units of the process so the operator or
program can read it.
Redundancy
Some special processes need to work permanently with minimum unwanted
downtime. Therefore, it is necessary to design a system that is fault tolerant. In such cases, to increase the system availability in the event of hardware component failure, redundant CPU or I/O modules with the same functionality can be added to a hardware configuration to prevent a total or partial process shutdown
due to hardware failure. Other redundancy scenarios could be related
to safety-critical processes, for example, large hydraulic presses could
require that two PLCs turn on an output before the press can come down
in case one PLC does not behave properly.
Programming
Example of a ladder diagram logic
Programmable logic controllers are intended to be used by engineers
without a programming background. For this reason, a graphical
programming language called ladder logic
was first developed. It resembles the schematic diagram of a system
built with electromechanical relays and was adopted by many
manufacturers and later standardized in the IEC 61131-3 control systems programming standard. As of 2015, it is still widely used, thanks to its simplicity.
Modern PLCs can be programmed in a variety of ways, from the
relay-derived ladder logic to programming languages such as specially
adapted dialects of BASIC and C.
While the fundamental concepts of PLC programming are common to all manufacturers, differences in I/O addressing, memory organization, and instruction sets
mean that PLC programs are never perfectly interchangeable between
different makers. Even within the same product line of a single
manufacturer, different models may not be directly compatible.
Programming device
Manufacturers develop programming software for their PLCs. In
addition to being able to program PLCs in multiple languages, they
provide common features like hardware diagnostics and maintenance,
software debugging, and offline simulation.
PLC programs are typically written in a programming device, which can take the form of a desktop console, special software on a personal computer, or a handheld device. The program is then downloaded to the PLC through a cable connection or over a network. It is stored either in non-volatile flash memory or battery-backed-up RAM
on the PLC. In some PLCs, the program is transferred from the
programming device using a programming board that writes the program
into a removable chip, such as EPROM, that is then inserted into the PLC.
Simulation
An incorrectly programmed PLC can result in lost productivity and
dangerous conditions for programmed equipment. PLC simulation is a
feature often found in PLC programming software. It allows for testing
and debugging
early in a project's development. Testing the project in simulation
improves its quality, increases the level of safety associated with
equipment and can save time during the installation and commissioning of
automated control applications since many scenarios can be tried and
tested before the system is activated.
Functionality
PLC system in a rack, left-to-right: power supply (PS), CPU, interface module (IM) and communication processor (CP)Control
panel with PLC (gray elements in the center). The unit consists of
separate elements, from left to right: power supply, controller, relay
units for input and output.
The main difference compared to most other computing devices is that
PLCs are intended for and therefore tolerant of more severe
environmental conditions (such as dust, moisture, heat, cold), while
offering extensive input/output(I/O) to connect the PLC to sensors and actuators. PLC input can include simple digital elements such as limit switches,
analog variables from process sensors (such as temperature and
pressure), and more complex data such as that from positioning or machine vision systems. PLC output can include elements such as indicator lamps, sirens, electric motors, pneumatic or hydraulic cylinders, magnetic relays, solenoids,
or analog outputs. The input/output arrangements may be built into a
simple PLC, or the PLC may have external I/O modules attached to a
fieldbus or computer network that plugs into the PLC.
The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems, and networking.
The data handling, storage, processing power, and communication
capabilities of some modern PLCs are approximately equivalent to desktop computers.
PLC-like programming combined with remote I/O hardware allows a
general-purpose desktop computer to serve as a PLC in certain
applications. Desktop computer controllers have not been generally accepted in heavy industry because desktop computers run on less stable operating systems
than PLCs, and because the desktop computer hardware is typically not
designed to the same levels of tolerance to temperature, humidity,
vibration, and longevity as PLCs.
Basic functions
The most basic function of a programmable logic controller is to
emulate the functions of electromechanical relays. Within the PLC
programming environment, discrete inputs are given a unique address, and
a PLC instruction can test if the input state is on or off. Just as a
series of relay contacts performs a logical AND function, not allowing current to pass unless all the contacts are closed, so a series of examine if on
instructions will energize its output storage bit if all the input bits
are on. Similarly, a parallel set of instructions will perform a logical OR. In an electromechanical relay wiring diagram, a group of contacts controlling one coil is called a rung of a ladder diagram, and this concept is also used to describe PLC logic. The output of each rung sets or clears a storage bit, which may be associated with a discrete output or which may be an internal coil with no physical connection. Such internal coils can be used, for example, as a common element in multiple separate rungs.
More advanced instructions of the PLC may be implemented as
functional blocks, which carry out some operation, such as manipulating
an internal variable, when enabled by a logical input and which produce
outputs to signal, for example, completion or errors.
Communication
PLCs use built-in ports, such as USB, Ethernet, RS-232, RS-485, or RS-422 to communicate with external devices (sensors, actuators) and systems (programming tools, SCADA and other user interfaces). Communication is carried over various industrial network protocols, like Modbus, or EtherNet/IP.
Many of these protocols are vendor specific. Formerly, some
manufacturers offered dedicated communication modules as an add-on
function where the processor had no network connection built in.
PLCs used in larger I/O systems may have peer-to-peer
(P2P) communication between processors. This allows separate parts of a
complex process to have individual control while allowing the
subsystems to coordinate over the communication link.
User interface
Control panel with a PLC user interface for thermal oxidizer regulation
PLCs may need to interact with people for the purpose of configuration, alarm reporting, or everyday control. A human-machine interface
(HMI) is employed for this purpose. HMIs are also referred to as
man-machine interfaces (MMIs) and graphical user interfaces (GUIs). A
simple system may use buttons and lights to interact with the user. Text
displays are available as well as graphical touch screens. More complex
systems use programming and monitoring software installed on a
computer, with the PLC connected via a communication interface.
Process of a scan cycle
A PLC works in a program scan cycle, where it executes its program repeatedly. The simplest scan cycle consists of 3 steps:
Read inputs
Execute the program
Write outputs
The program follows the sequence of instructions. It typically takes a
time span of tens of milliseconds for the processor to evaluate all the
instructions and update the status of all outputs. If the system contains remote I/O—for example, an external rack with
I/O modules—then that introduces additional uncertainty in the response
time of the PLC system.
Special-purpose I/O modules may be used where the scan time of
the PLC is too long to allow predictable performance. Precision timing
modules, or counter modules for use with shaft encoders,
are used where the scan time would be too long to reliably count pulses
or detect the sense of rotation of an encoder. This allows even a
relatively slow PLC to still interpret the counted values to control a
machine, as the accumulation of pulses is done by a dedicated module
that is unaffected by the speed of program execution.
As PLCs became more advanced, methods were developed to change
the sequence of ladder execution, and subroutines were implemented.
Security
In his book from 1998, E. A. Parr pointed out that even though most
programmable controllers require physical keys and passwords, the lack
of strict access control and version control systems, as well as an
easy-to-understand programming language make it likely that unauthorized
changes to programs will happen and remain unnoticed.
Prior to the discovery of the Stuxnet computer worm
in June 2010, the security of PLCs received little attention. Modern
programmable controllers generally contain real-time operating systems,
which can be vulnerable to exploits in a similar way as desktop
operating systems. PLCs can also be attacked by gaining control of a
computer they communicate with. Since 2011,
these concerns have grown as networking is becoming more commonplace in
the PLC environment, connecting the previously separated plant floor
networks and office networks.
In February 2021, Rockwell Automation publicly disclosed a critical vulnerability affecting its Logix controller family. The secret cryptographic key used to verify communication
between the PLC and workstation could be extracted from the programming
software (Studio 5000 Logix Designer) and used to remotely change
program code and configuration of a connected controller. The
vulnerability was given a severity score of 10 out of 10 on the Common Vulnerability Scoring System.
Safety PLCs
Safety PLCs can be either a standalone device or safety-rated
hardware and functionality added to existing controller architectures (Allen-Bradley GuardLogix, Siemens
F-series, etc.). These differ from conventional PLC types by being
suitable for safety-critical applications for which PLCs have
traditionally been supplemented with hard-wired safety relays and areas of the memory dedicated to the safety instructions. The standard of safety level is the safety integrity level (SIL).
A safety PLC might be used to control access to a robot cell with trapped-key access,
or to manage the shutdown response to an emergency stop button on a
conveyor production line. Such PLCs typically have a restricted regular
instruction set augmented with safety-specific instructions designed to
interface with emergency stop buttons, light screens, and other
safety-related devices.
Comparison to other control systems
PLC installed in a control panelControl center with a PLC for a RTO
PLCs are well adapted to a range of automation
tasks. These are typically industrial processes in manufacturing where
the cost of developing and maintaining the automation system is high
relative to the total cost of the automation system, and where changes
to the system would be expected during its operational life. PLCs
contain input and output devices compatible with industrial devices and
controls; little electrical design is required, and the design problem
centers on expressing the desired sequence of operations. PLC
applications are typically highly customized systems, so the cost of a
packaged PLC is low compared to the cost of a specific custom-built
controller design. On the other hand, in the case of mass-produced
goods, customized control systems are economical. This is due to the
lower cost of the components, which can be optimally chosen instead of a
generic solution, and where the non-recurring engineering charges are spread over thousands or millions of units.
PLC chip or embedded controller
Small Embedded PLC integrated in small mobile machine
These are for small machines and systems with low or medium volume. They can execute PLC languages such as ladder logic, function block diagram and sequential function chart.
They are similar to traditional PLCs, but their small size allows
developers to design them into custom printed circuit boards like a microcontroller,
without computer programming knowledge, but with a language that is
easy to use, modify and maintain. Architecturally, they sit between the
classic PLC or micro-PLC and microcontrollers.
Microcontrollers
A microcontroller-based
design would be appropriate where hundreds or thousands of units will
be produced, and so the development cost (design of power supplies,
input/output hardware, and necessary testing and certification) can be
spread over many sales, and where the end-user would not need to alter
the control. Automotive applications are an example; millions of units
are built each year, and very few end-users alter the programming of
these controllers. However, some specialty vehicles, such as transit
buses, economically use PLCs instead of custom-designed controls,
because the volumes are low and the development cost would be
uneconomical.
Single-board computers
Very complex process control, such as that used in the chemical
industry, may require algorithms and performance beyond the capability
of even high-performance PLCs. Very high-speed or precision controls may
also require customized solutions; for example, aircraft flight
controls. Single-board computers
using semi-customized or fully proprietary hardware may be chosen for
very demanding control applications where the high development and
maintenance cost can be supported. "Soft PLCs" running on desktop-type
computers can interface with industrial I/O hardware while executing
programs within a version of commercial operating systems adapted for
process control needs.
The rising popularity of single board computers has also had an influence on the development of PLCs. Traditional PLCs are generally closed platforms, but some newer PLCs (e.g., groov EPIC from Opto 22, ctrlX from Bosch Rexroth, PFC200 from Wago, PLCnext from Phoenix Contact, and Revolution Pi from Kunbus) provide the features of traditional PLCs on an open platform.
Programmable logic relays (PLR)
In more recent years, small products called programmable logic relays (PLRs) or smart relays
have become more common and accepted. These are similar to PLCs and are
used in light industries where only a few points of I/O are needed, and
low cost is desired. These small devices are typically made in a common
physical size and shape by several manufacturers and branded by the
makers of larger PLCs to fill their low-end product range. Most of these
have 8 to 12 discrete inputs, 4 to 8 discrete outputs, and up to 2
analog inputs. Most such devices include a tiny postage stamp-sized
LCD screen for viewing simplified ladder logic (only a very small
portion of the program being visible at a given time) and status of I/O
points, and typically these screens are accompanied by a 4-way rocker
push-button plus four more separate push-buttons, similar to the key
buttons on a VCR remote control, and used to navigate and edit the logic. Most have an RS-232 or RS-485
port for connecting to a PC so that programmers can use user-friendly
software for programming instead of the small LCD and push-button set
for this purpose. Unlike regular PLCs that are usually modular and
greatly expandable, the PLRs are usually not modular or expandable, but
their cost can be significantly lower than that a PLC, and they still
offer robust design and deterministic execution of the logic.
A variant of PLCs, used in remote locations, is the remote terminal unit
or RTU. An RTU is typically a low power, ruggedized PLC whose key
function is to manage the communications links between the site and the
central control system (typically SCADA) or in some modern systems, "The Cloud". Unlike factory automation using wired communication protocols such as Ethernet,
communications links to remote sites are often radio-based and are less
reliable. To account for the reduced reliability, RTU will buffer
messages or switch to alternate communications paths. When buffering
messages, the RTU will timestamp each message so that a full history of
site events can be reconstructed. RTUs, being PLCs, have a wide range of
I/O and are fully programmable, typically with languages from the IEC 61131-3
standard that is common to many PLCs, RTUs and DCSs. In remote
locations, it is common to use an RTU as a gateway for a PLC, where the
PLC is performing all site control and the RTU is managing
communications, time-stamping events and monitoring ancillary equipment.
On sites with only a handful of I/O, the RTU may also be the site PLC
and will perform both communications and control functions.
Environmental personhood or juridic personhood is a legal concept which designates certain environmental entities the status of a legal person. This assigns to these entities, the rights, protections, privileges, responsibilities and legal liability
of a legal personality. Because environmental entities such as rivers
and plants can not represent themselves in court, a "guardian" can act
on the entity's behalf to protect it. Environmental personhood emerged from the evolution of legal focus in pursuit of the protection of nature.
Over time, focus has evolved from human interests in exploiting nature,
to protecting nature for future human generations, to conceptions that
allow for nature to be protected as intrinsically valuable. This concept can be used as a vehicle for recognising Indigenous peoples' relationships to natural entities, such as rivers. Environmental personhood, which assigns nature (or aspects of it)
certain rights, concurrently provides a means to individuals or groups
such as Indigenous peoples to fulfill their human rights.
Background
The United States
Professor Christopher D. Stone first discussed the idea of attributing
legal personality to natural objects in the 1970s, in his article
"Should trees have standing? Towards legal rights for natural objects". A legal person cannot be owned; therefore, no ownership can be
attributed to an environmental entity with established legal
personality. Standing is directly related to legal personality. Entities with standing, or locus standi,
have the right or capacity to bring action or appear in court.
Environmental entities cannot themselves bring action or appear in
court. However, this action or standing can be achieved on behalf of the
entity by a representing legal guardian.
Representation could increase protection of culturally significant
aspects of the natural environment, or areas vulnerable to exploitation
and pollution.
Although there is no federal law in the United States
implementing environmental personhood, the idea has been advocated for
by a US Supreme Court Justice. In the decision of the 1972 US Supreme
Court case Sierra Club v. Morton, Justice William Douglas wrote a dissenting opinion arguing that certain "environmental elements" should have locus standi,
and that people with a meaningful relationship to that environmental
element should be able to act on its behalf for its protection. As of June 2021, at least 53 initiatives in 12 countries have used the concept of 'person' in their legal text.
The Sierra Club,
an environmental advocacy group, brought this suit against then
Secretary of the Interior of the United States, Roger C. B. Morton stating that the federal government, according to the Administrative Procedure Act,
could not grant permits for developers to build infrastructure –
specifically a highway, powerlines, and a ski resort – in the Mineral
King Valley, part of the Sequoia National Forest. The Sierra Club aimed to protect this undeveloped land within the national forest, but the U.S. Court of Appeals for the Ninth Circuit
had stated that because the members of the Sierra Club would not be
directly affected they could not sue under the Administrative Procedure
Act, which "provides standards for judicial review" for instances where a
person is negatively impacted by an agency action, such as granting a
permit. The Supreme Court agreed that the Sierra Club could not sue under the
Administrative Procedure Act, as it could not show that the actions of
the defendant caused or would cause injury to its members. This ruling led Supreme Court Justice William Douglas to write his
dissenting opinion, arguing that people should be allowed to sue on
behalf of non-living things writing, "[t]hose who have that intimate
relation with the inanimate object about to be injured, polluted, or
otherwise despoiled are its legitimate spokesmen." This opinion is shared by those who continue to argue for environmental personhood in the United States and around the world.
In 2014, Te Urewera National Park was declared Te Urewera, an environmental legal entity. The area encompassed by Te Urewera ceased to be a government-owned national park and was transformed into freehold, inalienable land owned by itself.
Following the same trend, New Zealand's Whanganui River was declared to be a legal person in 2017. This new legal entity was named Te Awa Tupua
and is now recognised as "an indivisible and living whole from the
mountains to the sea, incorporating the Whanganui River and all of its
physical and metaphysical elements." The river would be represented by two guardians, one from the Whanganui iwi and the other from the Crown.
Also in 2017, the New Zealand government signed an agreement granting similar legal personality to Mount Taranaki and pledging a name change for Egmont National Park, which surrounds the mountain.
India
The Ganges and Yamuna Rivers are now considered legal persons in an effort to combat pollution. The rivers are sacred to Hindu culture for their healing powers and attraction of pilgrims who bathe and scatter the ashes of their dead. The rivers have been heavily polluted by 1.5 billion litres of
untreated sewage and 500 million litres of industrial waste entering the
rivers daily.
The High Court in the northern Indian state of Uttarakhand
ordered in March 2017 that the Ganges and its main tributary, the
Yamuna, be assigned the status of legal entities. The rivers would gain
"all corresponding rights, duties and liabilities of a living person."
This decision meant that polluting or damaging the rivers is equivalent
to harming a person. The court cited the example of the New Zealand
Whanganui River, which was also declared to possess full rights of a
legal person.
This development of environmental personhood has been met with
scepticism as merely announcing that the Ganges and Yamuna are living
entities will not save them from significant, ongoing pollution. There
is a possible need to change long-held cultural attitudes towards the
Ganges, which hold that the river has self-purifying properties.
There is further criticism that the guardianship of the rivers
was only granted to Uttarakhand, a region in northern India which houses
a small part of the rivers' full extent. The Ganges flows for 2,525 km
through Uttarakhand, Uttar Pradesh, Bihar, Jharkhand and West Bengal,
with only a 96 km stretch running through Uttarakhand. Only a small
section of the 1,376 km Yamuna tributary runs through Uttarakhand –
which also crosses through the states of Haryana, Himachal Pradesh, Delhi and Uttar Pradesh.
Regardless of scepticism surrounding the decision of the
Uttarakhand High Court, proclaiming these vulnerable rivers as legal
entities invokes a movement of change towards environmental and cultural
rights protection. The decisions may be built upon as a foundation for
future environmental legislative change.
United States
In 2006, the borough of Tamaqua, Pennsylvania, worked with a rights of nature group called the Community Environmental Legal Defense Fund (CELDF). Together, the groups drafted legislation to protect the community and its environment from the dumping of toxic sewage. Since 2006, CELDF has assisted with over 30 communities in ten states
across the United States to develop local laws codifying the rights of
nature. CELDF also assisted in the drafting of Ecuador's 2008
constitution following a national referendum.
Besides Tamaqua, several other towns throughout the United States
have drafted legislation that would, in effect, give nature natural
rights. In 2008, residents in a town by the name of Shapleigh,
Maine, added new provisions to the town's legal code. The new sections
granted rights to the nature and natural bodies of water that surrounded
Shapleigh, and purported to strip the rights of corporations granted by the United States Constitution. What prompted the change to Shapleigh's legal code was a plan by the Nestle
Corporation, which owns several water bottle brands such as Poland
Spring, to pump truckloads of groundwater from Shapleigh to a water
bottling facility. As of 2019, no lawsuits have been filed against Shapleigh, Maine for
the change in the town's legal code, and the Nestle Corporation has not
chosen to challenge the code either. In this case the CELDF did not
assist the residents of Shapleigh in drafting sections 99-11 and 99-12
of their legal code, they were instead assisted by lawyers from Vermont.
In April 2013, the CELDF assisted officials in Mora County,
New Mexico, in creating an ordinance that limited the ability of
corporations to extract gas and oil, and gave rights to the natural
ecosystems and bodies of water that resided within Mora County. This ordinance made Mora County the very first place within the United
States to ban the production of gas and oil, within a certain area, in
an official statement. A lawsuit was filed against Mora County on November 12, 2013, which
asserted that Mora County's ordinance infringed on corporations rights,
especially the first, fifth, and fourteenth amendments. In January 2015, Mora County's ordinance was overthrown by U.S. District Judge James O. Browning as he viewed the ordinance to violate the first amendment rights of corporations.
In early 2014, Grant Township, Indiana, Pennsylvania,
enlisted the CELDF's help in drafting an ordinance that would give the
natural bodies of water surrounding Grant Township natural rights. A
company named Pennsylvania General Energy (PGE) had converted an old oil
and gas well into a "wastewater injection well," and residents became
concerned for what that could mean for the natural ecosystems
surrounding their township. The water in a wastewater injection well is waste that is left over from a process called fracking.
This water can contain harmful pollutants and chemicals that can poison
groundwater. In Grant Township, most residents rely on the Little
Mahoning Creek for their water needs. If the wastewater injection well
were to leak, there is a possibility it could contaminate the Little
Mahoning. The risk of contamination is what prompted Grant Township
residents to ask the CELDF for assistance in drafting an ordinance.
Grant Township's ordinance gave natural rights to the ecosystems and
bodies of water that were within the borders of Grant Township. Grant Township's ordinance also stripped corporations of their rights
deeming that corporations would not be seen as "persons" within the
borders of Grant Township. In August 2014, PGE sued Grant Township which began a legal battle that would last for almost five years. Grant Township lost the lawsuit against PGE in April 2019, and Judge Susan Baxter
ordered Grant Township to pay PGE's legal expenses which were over
$100,000. In addition, Grant Township's ordinance was declared invalid.
On 26 February 2019, voters in Toledo, Ohio passed the Lake Erie Bill of Rights. The main point of the Lake Erie Bill of Rights is that Lake Erie has the right to "flourish." Residents of Toledo, and surrounding areas, have suffered times where
the tap water, which comes from Lake Erie, was not safe to drink, or
use, due to pollution. Cases of unsafe water conditions, amongst other pollution problems, is
what prompted residents of Toledo to ask the CELDF for help. On 27 February 2019, the day after the Lake Erie Bill of Rights was passed by voters, a lawsuit was filed by an Ohio farmer. On 27 February 2020, U.S. District Judge Jack Zouhary invalidated the bill, ruling it was "unconstitutionally vague" and beyond "the power of municipal government in Ohio."
The rights of nature "to exist, persist, maintain and regenerate its vital cycles" have been proclaimed under Ecuador's 2008 constitution. This occurred after a national referendum in 2008, allowing the Ecuador
constitution to reflect rights for nature, a world first. Every person and community has the right to advocate on nature's behalf. The Constitution proclaims that the "State shall give incentives to
natural persons and legal entities and to communities to protect nature
and to promote respect for all the elements comprising an ecosystem."
The first successful case of the rights of nature implementation
under Ecuador constitutional law was presented before the Provincial
Court of Justice of Loja in 2011. This case involved the Vilcabamba
River as the plaintiff, representing itself with its own rights to
'exist' and 'maintain itself' – as it attempted to halt construction of a
government highway project interfering with the natural health of the
river. This case was brought before court by two individuals, Richard
Frederick Wheeler and Eleanor Geer Huddle, as legal guardians acting in
favour of nature – specifically the Vilcabamba River. A constitutional
injunction was granted in favour of the Vilcabamba River and against the
Provincial government of Loja, attempting to conduct the
environmentally-harmful project. The project was forced to be halted and
the area was to be rehabilitated.
Bolivia
The constitutional change in Ecuador was followed legislatively by Bolivia in 2010, passing the 'Law of the Rights of Mother Earth' (Ley de Derechos de la Madre Tierra). This legislation designates Mother Earth the character of 'a collective subject of public interest' with inherent rights specified in the law. The Law of the Rights of Mother Earth give aspects of legal personhood
to the natural environment. Judicial action can be taken for
infringements against individuals and groups as part of Mother Earth as
'a collective subject of public interest'. The legislation states that "Mother Earth is the dynamic living
system made up of the indivisible community of all living systems,
living, interrelated, interdependent and complementary, sharing a common
destiny."
Colombia
The ColombiaConstitutional Court found in November 2016 that the Atrato River
basin possesses rights to "protection, conservation, maintenance, and
restoration." This ruling came about as a result of degradation to the
river basin from mining, impacting nature and harming of Indigenous
peoples and their culture. The court referred to the New Zealand
declaration of the Whanganui River as a legal person holding
environmental personhood. The court ordered that joint guardianship
would be undertaken in the representation of the Atrato River basin.
Similarly to the New Zealand declaration, the representatives would come
from the national government and the Indigenous people living in the
basin.
The court stated:
(I)t
is the human populations that are interdependent of the natural world –
and not the opposite – and that they must assume the consequences of
their actions and omissions with the nature. It is a question of
understanding this new sociopolitical reality with the aim of achieving a
respectful transformation with the natural world and its environment,
as has happened before with civil and political rights…Now is the time
to begin taking the first steps to effectively protect the planet and
its resources before it is too late...
In April 2018
the Supreme Court of Colombia has issued a decision recognizing the
Amazon River ecosystem as a subject of rights and beneficiary of
protection.
Canada
The Magpie river in the Côte-Nord region of Quebec was given a set of rights, including the right to take legal action, by the Innu Council of Ekanitshit and Minganie county. Representatives can be appointed by the regional municipality and the
Innu to act on behalf of the river and take legal action to protect its
rights which they define as: "the right to flow; the right to respect
for its cycles; the right for its natural evolution to be protected and
preserved; the right to maintain its natural biodiversity; the right to
fulfil its essential functions within its ecosystem; the right to
maintain its integrity; the right to be safe from pollution; the right
to regenerate and be restored; and finally, the right to sue." This aligns with the belief that the river is an independent, living entity separate from human activity.
Spain
In Spain, the Law recognizes environmental personhood to Mar Menor.
Arguments for and against
The concept of environmental personhood is controversial, even among
environmentalists. One can advocate for a legal framework that
acknowledges rights of nature, but may not believe that environmental
personhood is the right way to implement it. Proponents of environmental
personhood argue that it is valuable to be able to sue on behalf of the
environment, because it would allow for environmental protection that
does not rely on harm being done to human beings. Environmental
personhood also better honors the significant relationships of
Indigenous peoples to their environment.
However, there are arguments against the concept of environmental
personhood. One concern is that the status of legal personhood implies a
right not only to sue but to be sued. For example, it has been questioned how legal liability applies to a natural entity (e.g., in natural disasters),
and whether legal representatives of such an entity would or should be
held liable for environmental persons. Community Environmental Defense
Fund lawyer Lindsey Schromen-Wawrin writes that this concern is "one of
the things that could derail in my opinion the ability for rights in
nature to be a check on destructive activities and instead could set up
kind of like natural resource trustees for ecosystems where there's a
flood and now the ecosystem has to pay out of the fund that would
otherwise have gone to restoring habitat that had been destroyed."
Another concern is that even with a legal right to sue on behalf of a natural entity, lawsuits are expensive. There are issues of environmental justice
if the cost to exercise the right to sue is inaccessible. Other issues
arise when environmental entities exist beyond the bounds of the
jurisdiction that decided on environmental personhood, which was the
case with a river which held rights as a legal person in Uttarakhand,
India. According to reporting by National Public Radio, there are also cases
where the rights of environmental entities may be at odds with the
rights of human beings, "Many of the [environmental personhood] laws
have also been met with resistance from industry, farmers and river
communities, who argue that giving nature personhood infringes on their
rights and livelihoods."
Significance for cultural human rights
The recognition of the Whanganui River as a legal entity in New Zealand (Te Awa Tupua) encompassed a vivid sense of cultural "inalienable connection" to the local iwi and hapū
of the river. Māori culture considers natural features such as the
Whanganui River as ancestors and iwi hold deep connections with them as
living entities. This inalienable connection of indigenous culture to their natural
surroundings is apparent in other parts of the world such as Colombia
where a similar environmental personhood declaration was made for the
Atrato River basin.
The lead negotiator for the Whanganui iwi, Gerrard Albert, said
"we consider the river an ancestor and always have...treating the river
as a living entity is the correct way to approach it, as an indivisible
whole, instead of the traditional model for the last 100 years of
treating it from a perspective of ownership and management." James D K Morris and Jacinta Ruru suggest that giving "legal
personality to rivers is one way in which the law could develop to
provide a lasting commitment to reconciling with Maori." This was the
longest-running legal dispute in New Zealand. The Whanganui iwi had been fighting to assert their rights in harmony with the river since the 1870s.
The concept of environmental protection on behalf of the environment
is not new, and widespread harm to the environment has a name: ecocide.
The Independent Expert Panel for the Legal Definition of Ecocide
defines ecocide as "unlawful or wanton acts committed with knowledge
that there is a substantial likelihood of severe and either widespread
or long-term damage to the environment being caused by those acts." There are advocates of making ecocide an international crime, like the
crimes dealt with by the Rome Statute of the International Criminal
Court (ICC). This would place ecocide alongside currently recognized international
crimes like genocide, war crimes, and crimes against humanity. If added, ecocide would be the only crime "in which human harm is not a prerequisite for prosecution." This protection of nature for nature's sake is central to the advocacy
behind environmental personhood. Do human beings need to be harmed to
warrant legal action? The concept of ecocide is not new, nor is the
advocacy for adding it to the Rome Statute of the ICC.
Extraterrestrial
With new increased interest in extraterrestrial spaceflight in the 2020s planetary personhood has been discussed, for Mars (including Martian meteorites), but particularly for the Moon, recognizing the Moon as having memory and agency, with its surface interacting, changing and remembering.
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