1

PLCs

Ladder diagram (LD)

programming language

CPU

Origins of Ladder Diagram

The Ladder Diagram (LD) programming

language originated from the graphical

representation used to design an electrical

control system

Control decisions were made using relays

After a while Relays were replaced by logic

circuits

Logic gates used to make control decisions

Finally CPUs were added to take over the

function of the logic circuits

I/O Devices wired to buffer transistors

Control decisions accomplished through

programming

OR AND

The origins of LD

Ladder Diagram (LD) was developed from the graphical representation used

for the electrical wiring diagrams

LD language has been developed to

facilitate the creation and maintenance

of programs in an easy way

LD uses computer graphic

representation that are similar with

electric diagrams which are easy to

understand

this reduces the costs of learning the

language and technical support

What is a Rung?

A rung of ladder diagram code can contain both input and

output instructions

Input instructions perform a comparison or test and set the rung

state based on the outcome

Normally left justified on the rung

Output instructions examine the rung state and execute some

operation or function

In some cases output instructions can set the rung state

Normally right justified on the rung

Input Instruction Output Instruction

Series Vs Parallel Operations

Ladder Diagram input instructions perform logical AND and OR

operations in and easy to understand format

If all Input Instructions in series must all be true for outputs to execute

(AND)

If any input instruction in parallel is true, the outputs will execute (OR)

Paralleling outputs allows multiple operations to occur based on the

same input criteria

OR

AND

A

B

C D

IF ((A OR B) AND (NOT C) AND D) THEN E=1; F=1 END_IF

E

F

Branches

Writing program in LD

Writing a program involves drawing a diagram (LD diagram) similar with electrical diagram.

LD diagram components model the operation of electrical diagrams based on contact elements.

Interpretation of operation is similar to the interpretation of electrical diagrams.

The basic elements used for writing a program in LD language are: contacts,

coils,

timers,

counters and

functional blocks (functions).

Contacts

Contacts are programming elements that

model the electrical contacts of switching

devices.

As in the case of real contact, these can be

normaly open NO- (a) or normaly closed NC (b).

a) b)

IN001 IN002 IN003

Contacts

In a LD program, contacts may be associated with PLC inputs, PLC outputs or internal variables.

At the input we can connect two state devices such as:

auxiliary contacts of the contactor and relay,

NC contacts or NO contacts of the control buttons,

NC contacts or NO contacts of the limit switches,

NC contacts or NO contacts of detectors of physical quantities,

NC contacts or NO contacts of protection elements,

digital outputs of measuring, protection or control devices,

digital outputs of PLC of other control systems etc.

Contacts

In addition to regular contacts, some manufacturers

make available to programmers other programming

elements corresponding to PLC inputs encountered,

especially in digital circuits, such as:

latched inputs

inputs active on the the rising edge

inputs active on the falling edge.

Coils

The coils are programming elements that model the

operation of the coils of contactors and

electromagnetic relays.

As with real coils, the coils of LD programs can have

two states: powered or unpowered.

Coils can be associated with PLC outputs and

internal variables modeling auxiliary contacts of

relays.

a) b)

OUT 001 OUT 002 OUT 003

Coils

Each output is uniquely identified, in a different way from one manufacturer to another.

Each output is associated with a single coil and one or more contacts that can be used similarly to auxiliary contacts of contactors and relays.

At these outputs can be connected devices that have two operation states such as:

coils of contactors or relays,

signaling and light elements,

small power loads,

digital inputs of some meters, protection and control devices,

digital inputs of other PLCs or control systems etc.

Timers

Timers are programming elements that model the operation of timer relays and timer contacts.

They are used to perform actions delayed or lasting a certain interval of time.

TON IN

T#200ms

Pump_Tmr

PT ET 178

Q

Timers

PLC manufacturers provide both basic and complex timing functions.

Timers used in LD programs have more flexibility and functionality than individual timers.

Simple timers allow an action to be delayed by a certain time that can be programmed.

Complex timers can offer a variable timing based on certain conditions occurring at a time.

Timers

Each timer in the diagram is uniquely identified, this identification being different from one manufacturer to another.

If the time base is the same for all the timers, specified in the PLC programming manual, the time based is omitted.

The default value can be expressed in units of time (s).

Timers have at least one initialization input, its activation starts timing, and an output.

In some implementations, the timers have a an enable input and one output which is the negated of the first output.

Counters

Counters are programming elements that can receive a series of pulses that are analyzed in the LD program to detect the number of occurrences of events such as:

number of steps performed by a stepper motor,

number of connection-disconnection of an appliance.

number of bottles that have been filled in a filling station, etc.

CTU

200

Load_Cnt

PV CV 178

Q

IN ENO

Load_Cnt_DN R

Counters

The number of events can be compared to certain default values and the outcome of these comparisons can result in certain appropriate given decisions and orders.

There are several types of counters among the most common being:

unidirectional counters up counter

down counter

bidirectional counters which can count up and down.

Each counter in the diagram is uniquely identified, how is this done differes from one manufacturer to another.

Counters

For each counter one must specifies the default value, this being the maximum number counted by the counter before the ouput is activated.

The counter has at least two inputs, one for counting and one for the initialization (following the initialization, the counter starts to count incoming pulses to the counting input) and an output.

Other options can also be provided such as a count enable input and output which is the negated of the first output.

Functional blocks

For materialization of complex functions needed to

facilitate writing programs in LD language functional

blocks (FB) are used.

FBs model various categories of the most used

functions such as:

Load functions of numerical constants,

arithmetic functions,

logic functions of 8 or 16 bits,

conversion functions of the various formats of information

(binary, BCD, Gray, etc.),

interrupt handling functions

Function blocks

edge detecting,

functions implementing complex controllers and

sequencers,

functions handling high-speed counters.

The design and operation of function blocks differ

from a PLC to another and is specific to each

manufacturer if not compying to IEC 61131 standard.

Restrictions in writing LD programs

When writing a LD program for a specific

PLC one must take into account the

limitations of the software package which

comes with the PLC:

limitations regarding the size of LD diagram

limitations on LD diagram execution

Limitations concerning the size of LD diagram

Limitations related to:

intrinsic properties of language;

Specific implementations of various commercial

software packages

Are due to technical solutions adopted by

producers to implement various elements of

the language.

Limitations concerning the size of LD diagram

Some of these limitations are as follows:

a coil must always be fed through a contact;

coil must always be introduced at the right end of a rung;

all contacts have to be in the horizontal direction;

number of contacts on a rung is limited by the software;

Limitations concerning the size of LD diagram

a contact group can feed a single coil;

the achievement of loops can be carried out in one way or

may not be permitted;

the direction of flow through the circuit is from left to the

right of the diagram.

Usually the PLC programming manuals contain all

information necessary for the user to write the

program in the format accepted by the PLC.

Limitations on how the program is executed

PLC operation is based on the repeated execution of the program that it stores into memory. Each cycle of execution of the program includes three separate stages:

read entries

execution of program instructions

updating outputs

The duration of such a cycle depends on the speed of the processor that comes with PLC and the length of the user program.

Limitations on how the program is executed

Reading of input stage: inputs states are and stored in memory in the input table.

Execution of program instructions stage: input values are used for instruction execution, the result of which is wirtten in the output table.

Updating the output stage: information from the output table is used to activate/deactivate PLC outputs.

The three steps mentioned above are executed separately. PLC input changes during second stage have no effect on input values used for the instruction execution. They will be used only after they have will be read in the input read stage of the next execution cycle.

Limitations on how the program is executed

If the second stage, after the execution of one or more instructions the value of an ouput is modified, this change will not actually appear in the corresponding output terminal, but only on the third stage when the outputs are updated based on the output table.

In writing an LD program one should consider also the way the PLC executes the written program.

Interpretation of LD diagrams

There are two ways of interpreting LD

diagram:

line evaluation - contacts are read along the line,

from left to right, line by line, from the first line and

ending with the last;

column evaluation - read contacts in column one,

from top to bottom, column by column from the

first column on the left and ending with the last on

the right.

Interpretation of LD diagrams

In both cases one must consider the difference from the operation of electric diagrams:

Relays: A change in the status of a contact in the power supply circuit of the coil of a contactor can lead to immediate change of its state whether or not there are other elements connected in series or parallel with the contact.

LD Program: Coil state will not be changed until it will be read the status of all elements by which it is fed.

Due to the high speed of execution of the processor, this does not generally pose a problem. It should be considered in cases of critical applications where it might appear a different operation from the desired one.

Interpretation LD diagram - Reading on line

Interpretation LD diagram - Reading column

LD languagein IEC 61131-3

By introducing this language in IEC 61131-3

it was aimed to standardize a whole set of

languages that were declared to be of Ladder Diagram type.

Program execution

Ladder rungs are

evaluated from left to

right and from top to

bottom

Branches within a line

are evaluated from top

left to bottom right

P S

R

The

B

D E

F G H

I J K

Phase

Branch

Null

Line program

Non Retentive Coils

The referenced bit is reset when processor power is cycled

Coil -( )-

Sets a bit when the rung is true(1) and resets the bit when the rung is false (0)

PLC5 calls this an OTE Output Enable

Negative coil -( / )-

Sets a bit when the rung is false(0) and resets the bit when the rung is True(1)

Not commonly supported because of potential for confusion

Set (Latch) coil -(S)-

Sets a bit (1) when the rung is true and does nothing when the rung is false

Reset (Unlatch) Coil -(R)-

Resets a bit (0) when the rung is true and does nothing when the rung is false

Contacts

Normally Open Contact -| |-

Enables the rung to the right of the instruction if the rung to the left

is enabled and underlining bit is set (1)

Normally Closed Contact -|/|-

Enables the rung to the right of the instruction if the rung to the left

is enabled and underlining bit is reset (0)

Positive transition contact -|P|-

Enables the right side of the rung for one scan when the rung on

left side of the instruction is true

Negative transition contact -|N|-

Enables the right side of the rung for one scan when the rung on

left side of the instruction is false

Retentive Vs Non-retentive Operation

Definitions

Retentive values or instructions maintain their last state

during a power cycle

Non-retentive values or instructions are reset to some

default state (usually 0) after a power cycle

IEC1131 permits values to be defined as retentive

A contradiction to this is ladder diagram where 3

instructions are classified as retentive

In most PLCs only timer and coil instructions operate as

non-retentive

Retentive Coils

The referenced bit is unchanged when processor

power is cycled

Retentive coil -(M)-

Sets a bit when the rung is true(1) and resets the bit when the

rung is false (0)

Set Retentive (Latch) coil -(SM)-

Sets a bit (1) when the rung is true and does nothing when the

rung is false

Reset Retentive (Unlatch) Coil -(RM)-

Resets a bit (0) when the rung is true and does nothing when

the rung is false

Transition Sensing Coils

Positive transition-sensing coil -(P)-

Sets the bit bit (1) when rung to the left of the

instruction transitions from off(0) to on(1)

The bit is left in this state

Negative transition-sensing coil -(N)-

Resets the bit (0) when rung to the left of the

instruction transitions from on(1) to off(0)

The bit is left in this state

Timers in Ladder Diagram

There three timer instructions in

IEC1131

TP - Pulse timer

TON - Timer On Delay

TOF - Timer Off Delay

Time values

Time base is 1msec (1/1000 of a sec)

Values entered using duration literal

format

Two possible visualizations Depending

on use of EN/ENO

1st method requires extra programming

if timer done status needs to be

referenced on other rungs

2nd method sets a bit with Q which can

be referenced by other logic, ENO=EN

TON IN

T#200ms

Pump_Tmr

PT ET 178

Q

TON

T#200ms

Pump_Tmr

PT ET 178

Q

IN ENO

Pump_Tmr_DN

Timer Operation IN

Q

ET PT

|

0

Pulse (TP) Timing

IN

Q

ET PT

|

0

On-Delay (TON) Timing

IN

Q

ET PT

|

0

Off-Delay (TOF) Timing

IN = Rung input condition

Q = Comparison output

results

Varies with timer types

PT = Preset Time

ET = Elapse Time

Counters in Ladder Diagram

There three counter instructions in

IEC1131

CTU - Count Up Counter

CTD - Count Down Counter

CTUD - Count Up/Down Counter

All three count rung transitions

Two possible visualizations

Depending on use of EN/ENO

1st method requires extra

programming if timer done status

needs to be referenced on other

rungs

2nd method sets a bit with Q which

can be referenced by other logic,

ENO=EN

CTU

200

Load_Cnt

PV CV 178

Q

IN ENO

Load_Cnt_DN R

CTU

200

Load_Cnt

PV CV 178

Q IN

R

Counter Operation Parameters

CU/CD = Count up/Down

Q/QU/QD = Comparison Output

R = Reset to Zero

LD = Load CV with PV

PV = Preset Value

CV = Count Value

...

...

CV PV

|

0

CU

QU

CD

QD

LD

R

Count Up/Down (CTUD) Counter

... IN

Q

CV PV

|

0

LD

... Count Down (CTD) Counter

... IN

Q

CV PV

|

0

R

Count Up (CTU) Counter

...

CAL

RET RET

CAL

Execution Control Elements

Jump / Label Instructions

Jump to a label skips a block of

code without it being scanned

LBL - Named target for a jump

operation

JMP - Performs a jump when

the rung conditions are true

CALL / RETURN Instructions

Used to encapsulate logic and call

it as a subroutine

Causes execution to change

between functions or subroutines

CAL - Passes control to another

named function

PLC5 uses JSR

RET - Exits a function and returns

control back to the calling routine

| Skip_Calc | |-| |-------------(JMP)--| | ... | | Skip_Calc | |---[LBL]---...

The look and feel of IEC 1131-3 is somewhat different from the

1Million+ PLCs that Allen Bradley has running in factories throughout the world

IEC places the input parameters on the outside of the instruction block

vs the PLC5 where they are presented inside of the block

TON

Timer

Preset

Pump_Tmr

200.000

Accum 178.251

(EN)

(DN)

ADD

Source A

Source B

Tank1_In

Offsetr

Destination Tank_Level

178.251

78.251

100.000

+ EN

100.000 178.251

ENO

78.251 Offsetr

Tank1_In Tank_Level

Different Instruction Presentations

TON

T#200ms

Pump_Tmr

PT ET 178

Q

IN ENO

Pump_Tmr_DN

Extending the IEC1131-3 Instruction Set

IEC1131-3 Provides a very basic set of instructions to do simple operations (81 Ladder Diagram Instructions)

Data Type Conversion - Trunc, Int_to_Sint, Dint_to_Real, Bcd_To_Int

Boolean Operations - Bit Test, Bit Set, One Shot, Semaphores

Timers / Counters - Ton, Tp, Ctu, Ctd, Ctud

Simple Math - Add, Sub, Mul, Div, Mod, Move, Expt

Misc. Math - Abs, Sqrt, Ln, Log, Exp, Sin, Cos, Tan, Asin, Acos, Atan

Bit Shift - Shl, Shr, Ror, Rol

Logic - And, Or, Xor, Not

Selection - Sel, Max, Min, Limit, Mux

Compare - GT, GE, EQ, LE, LT, NE

String - Len, Left, Right, Mid, Concat, Insert, Delete, Replace, Find

Control - JMP, LBL, JSR, RET

All complex operations are left to the user or vendor to define

File Operations, PID, Diagnostic, For/Nxt Loop, Search, Sort are not in IEC1131-3

Extensions to the instruction set are permitted so that vendors can add instructions that their customers need

All vendors have defined their own set of extensions

Bibliography

Ron Bliss, Introduction to IEC1131-3 Ladder

Diagram, Allen-Bradley

SR EN 61131-3, PLC. Part 3. Programming

Languages

CGHaba, Sisteme de comanda a masinilor

electrice, Ed. Gh.Asachi, Iai