Ladder logic was originally a written method to document the design and construction of
relay racks as used in manufacturing and
process control. Each device in the relay rack would be represented by a symbol on the ladder diagram with connections between those devices shown. In addition, other items external to the relay rack such as pumps, heaters, and so forth would also be shown on the ladder diagram.
Ladder logic has evolved into a
programming language
A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language.
The description of a programming ...
that represents a program by a graphical diagram based on the
circuit diagram
A circuit diagram (wiring diagram, electrical diagram, elementary diagram, electronic schematic) is a graphical representation of an electrical circuit. A pictorial circuit diagram uses simple images of components, while a schematic diagram s ...
s of
relay logic
Relay logic is a method of implementing combinational logic in electrical control circuits by using several electrical relays wired in a particular configuration.
Ladder logic
The schematic diagrams for relay logic circuits are often calle ...
hardware. Ladder logic is used to develop software for
programmable logic controller
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 tha ...
s (PLCs) used in industrial control applications. The name is based on the observation that programs in this language resemble
ladder
A ladder is a vertical or inclined set of rungs or steps used for climbing or descending. There are two types: rigid ladders that are self-supporting or that may be leaned against a vertical surface such as a wall, and rollable ladders, such ...
s, with two vertical rails and a series of horizontal rungs between them. While ladder diagrams were once the only available notation for recording programmable controller programs, today other forms are standardized in
IEC 61131-3
IEC 61131-3 is the third part (of 10) of the open international standard IEC 61131 for programmable logic controllers. It was first published in December 1993 by the IEC; the current (third) edition was published in February 2013.
Part 3 of ''IEC ...
(For example, as an alternative to the graphical ladder logic form, there is also a language more like C called
Structured text
Structured text, abbreviated as ST or STX, is one of the five languages supported by the IEC 61131-3 standard, designed for programmable logic controllers (PLCs). It is a high level language that is block structured and syntactically resembles ...
within the
IEC 61131-3
IEC 61131-3 is the third part (of 10) of the open international standard IEC 61131 for programmable logic controllers. It was first published in December 1993 by the IEC; the current (third) edition was published in February 2013.
Part 3 of ''IEC ...
standard).
Overview
Ladder logic is widely used to program
PLCs, where sequential control of a process or manufacturing operation is required. Ladder logic is useful for simple but critical control systems or for reworking old
hardwired
Hardwire or hardwired may refer to:
* Electrical wiring
*Hardwired control unit, a part of a computer's central processing unit
*In computer programming, a kludge to temporarily or quickly fix a problem
*Wired communication
In arts and entertainm ...
relay circuits. As programmable logic controllers became more sophisticated it has also been used in very complex automation systems. Often the ladder logic program is used in conjunction with an
HMI program operating on a computer workstation.
The motivation for representing
sequential
In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is called th ...
control logic Control logic is a key part of a software program that controls the operations of the program. The control logic responds to commands from the user, and it also acts on its own to perform automated tasks that have been structured into the program.
...
in a ladder diagram was to allow factory engineers and technicians to develop software without additional training to learn a language such as
FORTRAN or other general-purpose computer language. Development and maintenance were simplified because of the resemblance to familiar relay hardware systems.
[Edward W. Kamen ''Industrial Controls and Manufacturing'', (Academic Press, 1999) ,
Chapter 8 ''Ladder Logic Diagrams and PLC Implementations''] Implementations of ladder logic may have characteristics, such as sequential execution and support for control flow features, that make the analogy to hardware somewhat inaccurate.
Ladder logic can be thought of as a
rule-based language
This is a list of notable programming languages, grouped by type.
There is no overarching classification scheme for programming languages. Thus, in many cases, a language is listed under multiple headings (in this regard, see " Multiparadigm lan ...
rather than a
procedural language
Procedural programming is a programming paradigm, derived from imperative programming, based on the concept of the ''procedure call''. Procedures (a type of routine or subroutine) simply contain a series of computational steps to be carried ...
. A "rung" in the ladder represents a rule. When implemented with relays and other electromechanical devices, the various rules execute simultaneously and immediately. When implemented in a programmable logic controller, the rules are typically executed sequentially by software in a continuous loop, or "scan". By executing the loop fast enough, typically many times per second, the effect of simultaneous and immediate execution is achieved. Proper use of programmable controllers requires an understanding of the limitations of the execution order of rungs.
Syntax and examples
The language itself can be seen as a set of connections between logical checkers (contacts) and actuators (coils). If a path can be traced between the left side of the rung and the output, through asserted (true or "closed") contacts, the rung is true and the output coil storage bit is asserted (1) or true. If no path can be traced, then the output is false (0) and the "coil" by analogy to electromechanical
relay
A relay
Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts
An automotive-style miniature relay with the dust cover taken off
A relay is an electrically operated switch ...
s is considered "de-energized". The analogy between logical propositions and relay contact status is due to
Claude Shannon
Claude Elwood Shannon (April 30, 1916 – February 24, 2001) was an American people, American mathematician, electrical engineering, electrical engineer, and cryptography, cryptographer known as a "father of information theory".
As a 21-year-o ...
.
Ladder logic has contacts that make or break circuits to control coils. Each coil or contact corresponds to the status of a single bit in the programmable controller's memory. Unlike electromechanical relays, a ladder program can refer any number of times to the status of a single bit, equivalent to a relay with an indefinitely large number of contacts.
So-called "contacts" may refer to physical ("hard") inputs to the programmable controller from physical devices such as pushbuttons and
limit switch In electrical engineering, a limit switch is a switch operated by the motion of a machine part or the presence of an object. A limit switch can be used for controlling machinery as part of a control system, as a safety interlock, or as a counter en ...
es via an integrated or external input module, or may represent the status of internal storage bits which may be generated elsewhere in the program.
Each rung of ladder language typically has one coil at the far right. Some manufacturers may allow more than one output coil on a rung.
; Rung input: Checkers (contacts)
; Rung output: Actuators (coils)
The "coil" (output of a rung) may represent a physical output which operates some device connected to the programmable controller, or may represent an internal storage bit for use elsewhere in the program.
A way to recall these is to imagine the checkers (contacts) as a push button input, and the actuators (coils) as a light bulb output. The presence of a slash within the checkers or actuators would indicate the default state of the device at rest.
Logical AND
The above realizes the function: Door motor = Key switch 1
AND
or AND may refer to:
Logic, grammar, and computing
* Conjunction (grammar), connecting two words, phrases, or clauses
* Logical conjunction in mathematical logic, notated as "∧", "⋅", "&", or simple juxtaposition
* Bitwise AND, a boolea ...
Key switch 2
This circuit shows two key switches that security guards might use to activate an electric motor on a bank vault door. When the normally open contacts of both switches close, electricity is able to flow to the motor which opens the door.
Logical AND with NOT
The above realizes the function: Door motor = Close door
AND
or AND may refer to:
Logic, grammar, and computing
* Conjunction (grammar), connecting two words, phrases, or clauses
* Logical conjunction in mathematical logic, notated as "∧", "⋅", "&", or simple juxtaposition
* Bitwise AND, a boolea ...
NOT(Obstruction).
This circuit shows a push button that closes a door and an obstruction detector that senses if something is in the way of the closing door. When the normally open push button contact closes and the normally closed obstruction detector is closed (no obstruction detected), electricity is able to flow to the motor which closes the door.
Logical OR
The above realizes the function: Unlock = Interior unlock
OR Exterior unlock
This circuit shows the two things that can trigger a car's
power door locks
Power door locks (also known as electric door locks or central locking) allow the driver or front passenger to
simultaneously lock or unlock all the doors of an automobile or truck, by pressing a button or flipping a switch.
Power door locks w ...
. The remote receiver is always powered. The unlock
solenoid
upright=1.20, An illustration of a solenoid
upright=1.20, Magnetic field created by a seven-loop solenoid (cross-sectional view) described using field lines
A solenoid () is a type of electromagnet formed by a helix, helical coil of wire whose ...
gets power when either set of contacts is closed.
Industrial STOP/START
In common industrial latching start/stop logic we have a "Start" button to turn on a motor contactor, and a "Stop" button to turn off the contactor.
When the "Start" button is pushed the input goes true, via the "Stop" button NC contact. When the "Run" input becomes true the seal-in "Run" NO contact in parallel with the "Start" NO contact will close maintaining the input logic true (latched or sealed-in). After the circuit is latched the "Stop" button may be pushed causing its NC contact to open and consequently the input to go false. The "Run" NO contact then opens and the circuit logic returns to its inactive state.
The above realizes the function: Run = (Start
OR Run)
AND
or AND may refer to:
Logic, grammar, and computing
* Conjunction (grammar), connecting two words, phrases, or clauses
* Logical conjunction in mathematical logic, notated as "∧", "⋅", "&", or simple juxtaposition
* Bitwise AND, a boolea ...
(
NOT Stop)
This
latch
A latch or catch (called sneck in Northern England and Scotland) is a type of mechanical fastener that joins two (or more) objects or surfaces while allowing for their regular separation. A latch typically engages another piece of hardware on t ...
configuration is a common
idiom
An idiom is a phrase or expression that typically presents a figurative, non-literal meaning attached to the phrase; but some phrases become figurative idioms while retaining the literal meaning of the phrase. Categorized as formulaic language, ...
in ladder logic. It may also be referred to as ''seal-in logic''. The key to understanding the latch is in recognizing that the "Start" switch is a momentary switch (once the user releases the button, the switch is open again). As soon as the "Run" solenoid engages, it closes the "Run" NO contact, which latches the solenoid on. The "Start" switch opening up then has no effect.
: Note: In this example, "Run" represents the status of a
bit
The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...
in the
PLC, while "Motor" represents the actual output to the real-world
relay
A relay
Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts
An automotive-style miniature relay with the dust cover taken off
A relay is an electrically operated switch ...
that closes the motor's real-world
circuit.
For safety reasons, an emergency stop ("ES") may be hardwired in series with the "Start" switch, and the relay logic should reflect this.
Complex logic
Here is an example of what two rungs in a ladder logic program might look like. In real-world applications, there may be hundreds or thousands of rungs.
Typically, complex ladder logic is "read" left to right and top to bottom. As each of the lines (or rungs) are evaluated the output coil of a rung may feed into the next stage of the ladder as an input. In a complex system there will be many "rungs" on a ladder, which are numbered in order of evaluation:
This represents a slightly more complex system for rung 2. After the first line has been evaluated, the output coil "A/C" is fed into rung 2, which is then evaluated and the output coil "Cooling" could be fed into an output device "Compressor" or into rung 3 on the ladder. This system allows very complex logic designs to be broken down and evaluated.
Additional functionality
Additional functionality can be added to a ladder logic implementation by the PLC manufacturer as a special block. When the special block is powered, it executes code on predetermined arguments. These arguments may be displayed within the special block.
In this example, the system will count the number of times that the interior and remote unlock buttons are pressed. This information will be stored in memory locations A and B. Memory location C will hold the total number of times that the door has been unlocked electronically.
PLCs have many types of special blocks. They include timers, arithmetic operators and comparisons, table lookups, text processing,
PID control, and filtering functions. More powerful PLCs can operate on a group of internal memory locations and execute an operation on a range of addresses, for example, to simulate a physical sequential drum controller or a
finite-state machine
A finite-state machine (FSM) or finite-state automaton (FSA, plural: ''automata''), finite automaton, or simply a state machine, is a mathematical model of computation. It is an abstract machine that can be in exactly one of a finite number o ...
. In some cases, users can define their own special blocks, which effectively are subroutines or macros. The large library of special blocks along with high-speed execution has allowed use of PLCs to implement very complex automation systems.
Limitations and successor languages
Ladder notation is best suited to control problems where only
binary variables are required and where interlocking and sequencing of binary is the primary control problem. Like all
parallel programming language
Parallel computing is a type of computation in which many calculations or processes are carried out simultaneously. Large problems can often be divided into smaller ones, which can then be solved at the same time. There are several different fo ...
s, the sequential order of operations may be undefined or obscure; logic
race condition
A race condition or race hazard is the condition of an electronics, software, or other system where the system's substantive behavior is Sequential logic, dependent on the sequence or timing of other uncontrollable events. It becomes a software ...
s are possible which may produce unexpected results. Complex rungs are best broken into several simpler steps to avoid this problem. Some manufacturers avoid this problem by explicitly and completely defining the execution order of a rung, however, programmers may still have problems fully grasping the resulting complex semantics.
Analog quantities and arithmetical operations are clumsy to express in ladder logic and each manufacturer has different ways of extending the notation for these problems. There is usually limited support for arrays and loops, often resulting in duplication of code to express cases that in other languages would call for use of indexed variables.
As
microprocessors
A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...
have become more powerful, notations such as
sequential function chart Sequential function chart (SFC) is a visual programming language used for programmable logic controllers (PLCs). It is one of the five languages defined by IEC 61131-3 standard. The SFC standard is defined as ''Preparation of function charts for c ...
s and
function block diagram
The Function Block Diagram (FBD) is a graphical language for programmable logic controller design, that can describe the function between input variables and output variables. A function is described as a set of elementary blocks. Input and outp ...
s can replace ladder logic for some limited applications. Some newer PLCs may have all or part of the programming carried out in a dialect that resembles
BASIC
BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College ...
,
C, or other
programming language
A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language.
The description of a programming ...
with bindings appropriate for a real-time application environment.
Popularity
In 2019, IEEE Spectrum ranked ladder logic as number 50 out of 52 in a list of popular programming languages.
See also
*
Digital circuit
*
IEC 61131
*
Structured text
Structured text, abbreviated as ST or STX, is one of the five languages supported by the IEC 61131-3 standard, designed for programmable logic controllers (PLCs). It is a high level language that is block structured and syntactically resembles ...
References
Further reading
* {{cite book , title=The Programmable Logic Controller: its prehistory, emergence and application , author-first=Mark John , author-last=Walker , date=2012-09-08 , type=PhD thesis , location=Department of Communication and Systems Faculty of Mathematics, Computing and Technology , publisher=
The Open University
The Open University (OU) is a British public research university and the largest university in the United Kingdom by number of students. The majority of the OU's undergraduate students are based in the United Kingdom and principally study o ...
, url=http://oro.open.ac.uk/54687/1/594090.pdf , access-date=2018-06-20 , url-status=live , archive-url=https://web.archive.org/web/20180620115412/http://oro.open.ac.uk/54687/1/594090.pdf , archive-date=2018-06-20
External links
"Chapter 6: ladder logic"by Tony R. Kuphaldt
Electronic design automation
Programmable logic controllers
Visual programming languages