Eee3420 lecture03 rev2011

1Week© Vocational Training Council, Hong Kong.
│ Lecture 3 │
Basic Programmable Logic Controller
(PLC) Programming
EEC3420 Industrial Control
Department of Electrical Engineering

2© Vocational Training Council, Hong Kong.
EEE3420 Industrial Control
Week
Learning Objectives

Know different types of programming languages used in
PLC

Understand the basic programming techniques of PLC

Week
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

Week
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)

Week
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

Week
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

Week
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

Week
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

Week
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

Week

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

Week

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

Week
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.

Week
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

Week
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

Week
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)

Week
A list of commonly used IL operators defined in IEC 61131-3

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

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

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

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

Week
example
demonstrating
the use of MCR
instruction in
Allen Bradley
PLC

Week
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”)

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

Week
Conversion of more complex logic gates

Week
Multiplexer

Week
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

Week

Week
a typical timing diagram, ignoring the brief pulse

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

Week
the SR Flip Flop may be implemented by using AND-OR
functions

Week
the SR Flip Flop may also be implemented by using SET-
RESET functions

Week
implement the SR Flip Flop in Omron PLC

Week
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

Week
Operation of a TON (non-retentive) Timer

Week
Operation of a RTO (retentive time on) timer

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

Week
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

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

Week
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

Week
Timers in Omron PLC

Week
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

Week
An up-down counter in Allen Bradley PLC

Week
Counter operation in Mitsubishi PLC

Week
Counter operation in Omron PLC

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

Week
in Mitsubishi FX series PLC, there are two edge
triggering instructions, namely, PLS (PuLSe) and PLF
(PuLse Falling)

Week
in Omron PLC, there are similar edge triggering
instructions, namely, the DIFU (DIFferentiate Up) and the
DIFD (DIFferentiate Down)

Week
Applications of Time Related Instructions
The timers and the counters find their
applications in many industrial process controls

Week
Pulse generator circuit

Week
Fixed width pulse generator circuit

Week
Delay 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

Week
Light flasher

Week
Variable delay on/variable delay off circuit with Mitsubishi
PLC

Week
Variable delay on/variable delay off circuit with Omron
PLC

Week
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.

Week
Basic PLC Programming
End of Lecture 3
 Revision
Mitsubishi PLC FX series hardware handbook.
Allen Bradley PLC handbook
Omron PLC manual

Eee3420 lecture03 rev2011

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