Eee3420 lecture03 rev2011

1Week © Vocational Training Council, Hong Kong.

│ Lecture 3 │

Basic Programmable Logic Controller (PLC) Programming

EEC3420 Industrial ControlDepartment of Electrical Engineering

2© Vocational Training Council, Hong Kong.

EEE3420 Industrial Control

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Learning Objectives Know different types of programming languages used in

PLC Understand the basic programming techniques of PLC



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IEC 61131 standards for PLC the use of PLC has been standardized by the IEC developed with the input of vendors, end-users and

academics, IEC 61131 consists of five parts: General information Equipment and test requirements PLC programming languages User guidelines Communications



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The IEC 61131-3 programming language the international standard for programmable controller

programming languages specifies the syntax, semantics and display for the

following suite: Ladder diagram (LD) Sequential Function Charts (SFC) Function Block Diagram (FBD) Structured Text (ST) Instruction List (IL)



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Ladder diagram (LD) widely spread graphical programming language for

PLCs a programmatically replacement for relays and coils

which can be combined using AND and OR operations best suited to compute Boolean expressions IEC 61131-3 defines a complete LD set of instructions,

consisting of different types of contacts and coils



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Ladder diagram (LD) contacts lead (according to their type) the power

from the left to the right coils store the incoming value both, contacts and coils, are assigned to Boolean

variables an LD network can be supplemented by jumps,

returns, labels and comments



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Sequential Function Charts (SFC) SFC is the IEC 61131 -3 method to program

sequential controls actions may contain Boolean variables or entire

networks, networks are programmed in Ladder Diagram, true action associations from several steps, several concurrent SFC networks in one diagram



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Function Block Diagram (FBD) FBD defined in IEC 61131-3 consists of different

functions and function blocks that are either linked or connected to variables

in FBD networks, jumps, labels and returns can be used similar to the creation of functional diagrams or block

diagrams, with the control logic being created by simply connecting blocks and elements

frequently used in the process industry due to the flow-oriented properties of function blocks



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Structured Text language (ST) similar to the high-level language PASCAL or BASIC uses statements like IF...THEN or FOR…NEXT concerned compiler is responsible for syntactically

correct insertion of the language constructions text editor simplifies the programming work through

syntax highlighting syntax and the set of instructions of ST are optimally

suited for mathematical calculations and data manipulations



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Instruction List (IL) an assembler type textual programming language large (and mostly incompatible) sets of instructions

have been created by many vendors the IEC 61131 Instruction list reduced this set to a

minimum and favours the usage of functions and function blocks for more complex tasks

code in IL consists of a sequence of instructions separated by lines, which consist of one operator ("command"), one operand (variable, constant or instance name) and one optional modifier respectively



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Instruction List (IL) jumps and labels can be used concerned assembler is responsible for syntactically

correct insertion of the language constructions text editor simplifies the programming work through

syntax highlighting (colour marking of language elements)

mainly used if the processing speed of the PLC program has high priority



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Instruction List (IL) Basic instructions used in Allen Bradley PLC

Mnemonic Function Mnemonic Function

LD Load ANDN And Not

LDN Load Not ORN Or Not

ST Store MPS Memory Push

STN Store Not MPP Memory Pop

AND And MRD Memory Read

OR Or MCR Master Control Relay

ANB And Block

ORB Or Block

Note: The instruction mnemonic may vary across different series of the Allen Bradley PLCs.



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Instruction List (IL) Basic instructions used in Omron PLC


LOAD Load NOP No Operation

LOAD NOT Load Not TR Temp. Register

OUT Out IL InterLock (eq. to MC)

AND And ILC InterLockClear(eq. to MCR)

AND NOT And Inverted

AND LD And Block

OR Or

OR NOT Or Not

OR LD Or Block



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Instruction List (IL) Basic instructions used in Mitsubishi PLC FX series


LD Load NOP No Operation

LDI Load Inverted MPS Memory Push

OUT Out MPP Memory Pop

AND And MRD Memory Read

ANI And Inverted MC Master Control

ANB And Block MCR Master Control Reset

OR Or

ORI Or Inverted

ORB Or Block



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IL, LD and its associated logic equation

operation labeled with circle 1 is performed first as it is the deepest nested term, and operations proceed with circle 2,circle 3 and circle 4 consecutively

(Mitsubishi PLC codes)



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Instruction List (IL) A list of commonly used IL operators defined in IEC 61131-3



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Instruction List (IL) example of IL using the syntax defined in IEC 61131-3



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Instruction List (IL)

example of IL using the syntax defined in IEC 61131-3



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example of IL program using the instructions MPS, MPP & MRD to implement multiple outputs using the syntax defined in IEC 61131-3



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example demonstrating the use of TR instruction (similar to MPS, MPP & MRD instructions defined in IEC 6113-3) in Omron PLC



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example demonstrating the use of MCR instruction in Allen Bradley PLC



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example demonstrating the use of MC & MCR instructions in Mitsubishi PLC

(N0 is the nested number, maximum 7,i.e. a total of 8 nested levels; SP means “space”)



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example demonstrating the use of IL & ILC instructions in Omron PLC



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Conversion of more complex logic gates



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Multiplexer



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Time Related Instruction – the Latch

• a latch in ladder logic uses one instruction to latch, and a second instruction to unlatch

• the output with an L inside will turn the output D on when the input A becomes true

• D will stay on even if A turns off• D will turn off if input B

becomes true



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a typical timing diagram, ignoring the brief pulse



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The SR (Set-Reset) Flip FlopLatches are not used universally by all PLC vendors, others such as Siemens use flip-flops



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The SR (Set-Reset) Flip Flopthe SR Flip Flop may be implemented by using AND-OR functions



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The SR (Set-Reset) Flip Flopthe SR Flip Flop may also be implemented by using SET-RESET functions



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The SR (Set-Reset) Flip Flop implement the SR Flip Flop in Omron PLC



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The Timers• four fundamental types

• on-delay timer will wait for a set time after a line of ladder logic has been true before turning on, but it will turn off immediately

• off-delay timer will turn on immediately when a line of ladder logic is true, but it will delay before turning off

• retentive timer will sum all of the on or off time for a timer, even if the timer never finished

• non-retentive timer will start timing the delay from zero each time



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Operation of a TON (non-retentive) Timer



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Operation of a RTO (retentive time on) timer



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Operation of a TOF (non-retentive t-off) timer



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Timers in Mitsubishi PLCs • in FX1N and FX2N series, there are four types of

timers • T0 ~ T199 are non-retentive time on timer with

timing interval of 100 ms• T200 ~ T245 are non-retentive time on timer with

timing interval of 10 ms• T246 ~ T249 are retentive time on timer with timing

interval of 1 ms• T250 ~ T255 are retentive time on timer with timing

interval of 100 m



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Using non-retentive time on timer in Mitsubishi FX series PLC



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Timers in Omron PLC • in CP series, there are 48 timers/counters with

timing range running • from 0.1s to 999.9s if it is set as common timer • or 0.01s to 99.99s if it is set as high speed timer



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Timers in Omron PLC



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The Counters• two basic counter types:

count-up and count-down. • when the input to a count-up counter goes true the accumulator

value will increase by 1, if the accumulator value reaches the preset value the counter DN bit will be set

• a count-down counter will decrease the accumulator value until the preset value is reached



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An up-down counter in Allen Bradley PLC



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Counter operation in Mitsubishi PLC



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Counter operation in Omron PLC



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The Edge Triggering Instructionsin Mitsubishi FX series PLC, there are two edge triggering instructions, namely, PLS (PuLSe) and PLF (PuLse Falling)



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The Edge Triggering Instructionsin Mitsubishi FX series PLC, there are two edge triggering instructions, namely, PLS (PuLSe) and PLF (PuLse Falling)



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The Edge Triggering Instructionsin Omron PLC, there are similar edge triggering instructions, namely, the DIFU (DIFferentiate Up) and the DIFD (DIFferentiate Down)



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The Edge Triggering Instructionsin Omron PLC, there are similar edge triggering instructions, namely, the DIFU (DIFferentiate Up) and the DIFD (DIFferentiate Down)



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Applications of Time Related Instructions

The timers and the counters find their applications in many industrial process controls



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Pulse generator circuit



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Fixed width pulse generator circuit



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Applications of Time Related InstructionsDelay off timer - Many PLCs such as Mitsubishi do not provide delay off timer, but this function can easily be built by the delay on timer



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Light flasher



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Applications of Time Related InstructionsVariable delay on/variable delay off circuit with Mitsubishi PLC



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Applications of Time Related InstructionsVariable delay on/variable delay off circuit with Omron PLC



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Summary of Basic PLC Programming ․ The Instruction List programming language uses stack

operation to perform the operations specified.

․ Logic can be represented with basic Boolean operations AND, OR, NOT.

․ Boolean equations can be converted to and from ladder logic or instruction list.

․ The MPS, MPP and MRD enable intermediate results to be pushed, pop and read to facilitate multiple outputs.

․ Use of the MC/MCR instructions simplifies the organization of the ladder logic.

․ The time related instructions realize the sequential functions.



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Basic PLC Programming

End of Lecture 3

RevisionMitsubishi PLC FX series hardware handbook.Allen Bradley PLC handbookOmron PLC manual