Ladder logic is a programming language that represents a program
by a graphical diagram based on the circuit
diagrams of relay logic hardware. It is primarily used to develop
software for programmable logic controllers (PLCs)
used in industrial control applications. The name is based on the observation
that programs in this language resemble ladders, with two vertical rails and a
series of horizontal rungs between them.
Overview
An argument that aided the initial adoption of ladder
logic was that a wide variety of engineers and technicians would be able to
understand and use it without much additional training, because of the
resemblance to familiar hardware systems. This argument has become less
relevant given that most ladder logic programmers have a software background in
more conventional programming languages, and in practice
implementations of ladder logic have characteristics, such as sequential
execution and support for control flow features, that make the analogy to
hardware somewhat inaccurate.
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
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.
Manufacturers of programmable logic controllers generally
also provide associated ladder logic programming systems. Typically the ladder
logic languages from two manufacturers will not be completely compatible;
ladder logic is better thought of as a set of closely related programming
languages rather than one language. (The IEC 61131-3
standard has helped to reduce unnecessary differences, but translating programs
between systems still requires significant work.) Even different models of
programmable controllers within the same family may have different ladder
notation such that programs cannot be seamlessly interchanged between models.
Ladder logic can be thought of as a rule-based language
rather than a procedural language. 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 (scan).
By executing the loop fast enough, typically many times per second, the effect
of simultaneous and immediate execution is relatively achieved to within the
tolerance of the time required to execute every rung in the "loop"
(the "scan time"). It is somewhat similar to other rule-based
languages, like spreadsheets or SQL. However, proper use of programmable controllers requires
understanding the limitations of the execution order of rungs.
Example of a simple ladder logic program
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 electro-mechanical relays is considered
"de-energized". The analogy between logical propositions and relay
contact status is due to Claude Shannon.
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 switches 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.
·
—( )— A regular coil, energized whenever its
rung is closed.
·
—(\)— A "not" coil, energized
whenever its rung is open.
·
—[ ]— A regular contact, closed whenever its
corresponding coil or an input which controls it is energized.
·
—[\]— A "not" contact, open
whenever its corresponding coil or an input which controls it is energized.
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.
Examples
Here is an example of what one rung in a ladder logic
program might look like. In real world applications, there may be hundreds or
thousands of rungs.
For example:
1. ----[ ]---------|--[ ]--|------( )
X | Y | S
| |
|--[ ]--|
Z
The above realizes the
function: S = X AND ( Y OR Z )
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.
1. ----[ ]-----------|---[ ]---|----( )
X | Y | S
| |
|---[ ]---|
Z
2. ----[ ]----[ ]-------------------( )
S X T
2. T = S AND X
This represents a slightly more complex system for rung
2. After the first line has been evaluated, the output coil (S) is fed into
rung 2, which is then evaluated and the output coil T could be fed into an
output device (buzzer, light etc..) or into rung 3 on the ladder. (Note that
the contact X on the second rung serves no useful purpose, as X is already
defined in the 'AND' function of S from the 1st rung.)
This system allows very complex logic designs to be
broken down and evaluated.
For more practical
examples see below: ------[ ]--------------[ ]----------------( )
Key Switch 1 Key Switch 2 Door Motor
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. This is a logical AND.
+-------+
----------------------------+ +----
+-------+
Remote Receiver
--|-------[ ]-------+-----------------( )
| |
|-------[ ]-------|
Interior Unlock
This circuit shows the two things that can trigger a
car's power door locks. The remote receiver is always
powered. The lock solenoid gets power when either set of contacts is closed.
This is a logical
OR.
Often we have a little green "start" button to
turn on a motor, and we want to turn it off with a big red "stop" button.
The stop button itself is wired as a normally closed switch. This means that
when the stop button is in its normal state (not pushed), the PLC input will be
true. When the stop button is pushed, the input will go false. This will make
the rung false and stop the "run" output. A normally-open contact
must be used in the ladder diagram, since this input is normally turned on
through the normally closed pushbutton contact, and turns off when the button
is pressed.
--+----[ ]--+----[ ]----( )
| start | stop run
| |
+----[ ]--+
run
-------[ ]--------------( )
run motor
This latch configuration is a common idiom in ladder
logic. In ladder logic it is referred to as seal-in logic. The key to
understanding the latch is in recognizing that "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"
switch, which latches the solenoid on. The "start" switch opening up
then has no effect.
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.
+-------+
-----[ ]--------------------+ A +----
Remote Unlock +-------+
Remote Counter
+-------+
-----[ ]--------------------+ B +----
Interior Unlock +-------+
Interior Counter
+--------+
--------------------+ A + B +-----------
+ into C +
+--------+
Adder
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. 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. Since execution of rungs is sequential
within a program and may be undefined or obscure within a rung, some logic race
conditions are possible which may produce unexpected results; complex rungs
are best broken into several simpler steps to avoid this problem. Some
manufacturers, e.g. Omron,
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 which
in other languages would call for use of indexed variables.
As microprocessors have become more powerful, notations
such as sequential function charts and function block diagrams can replace ladder
logic for some limited applications. Very large programmable controllers may
have all or part of the programming carried out in a dialect that resembles BASIC or C or other programming language with bindings appropriate
for a real-time application environment.
