1. PROGRAMMABLE LOGIC CONTROLLER
BY: Salsabila Ahmad

2. CHAPTER 4 BASIC FUNCTION OF PLC AND APPLICATION
BY: Salsabila Ahmad

3. BASIC FUNCTION OF PLC AND ITS APPLICATION
Introduction to Pendant Programming
Input and Output
Internal Relay
‘AND’ and ‘OR’ circuit
Latching Relay
Program Editing
Program Observations
Q&A

4. PROGRAM SCAN

5. Program Scan During each operating cycle, the processor reads all
inputs, takes these values, and energizes or
de-energizes the outputs according to the user
program. This process is known as a scan.
I/O scan – records status data of input
devices. Energizes output devices that
have their associated status bits set to
ON (1)
Program scan – instructions are
executed sequentially
Because the inputs can change at any time, the PLC
must carry on this process continuously.

6. Scan Process
Scan time indicates how fast the controller can react to changes in inputs.
Scan times vary with computer model and program content, and length.
If a controller has to react to an input signal that changes states twice during the scan time, it is possible that the PLC will never be able to detect this change.
Scan time may be a concern in high speed operations

7. Scan Process The scan is a a continuous and sequential process
Read inputs
The scan is a
a continuous
and sequential
process
Adjusts
outputs
Run
program

8. Data Flow Overview Input image Output table file image table file
modules
data
Examine data
Check/compare/examine
specific conditions
Output
data
modules
Take some
action
Return results
Input
image
table file
Output
image
table file
Program

9. Scan Process Input Module Output Module Output device Input file
Program
When the input is
closed, the input
module senses a
voltage and an ON
condition (1) is
entered into the
input table bit I:3/6
The processor turns
light output O:4/7
ON during the next
I/O scan
I:3/6
O:4/7
During the program
scan the processor
sets instructions I:3/6
and O:4/7 to ON (1)

10. SCAN PATTERN
There are two basic scan patterns that different PLC manufacturers use to accomplish the scan function:
Horizontal scan
E.g. Allen-Bradley
Vertical scan
E.g. AEG Modicon

11. Horizontal Scanning Order
The processor examines
input and output
instructions from the
first command
top left in the program
horizontally
rung by rung
End of ladder
In addition to the program itself, the scan time is also
dependent on the clock frequency of the processor!

12. Vertical Scanning Order
The processor examines
input and output
instructions from the
first command,
vertically,
column by column
page by page.
Pages are executed in sequence.
End of ladder
Misunderstanding the way the PLC scans can cause
programming bugs!

13. PLC PROGRAMMING LANGUAGE
Methods use for user to communicates information with PLC
3 most common language structures
Ladder diagram language
Boolean language
Function chart

14. LADDER DIAGRAM LANGUAGE
The most commonly used for PLC.
Ladder diagram language

15. BOOLEAN LANGUAGE
Addressing format for input and output varies for different PLC models
Boolean language

16. COMPARING PROGRAMMING LANGUAGE
PB1
CR1
CR2
SOL
Relay Schematic
LS1
PB1
CR1
CR2
LS1
SOL
Equivalent ladder
diagram language
Equivalent Boolean language

17. FUNCTIONAL CHART Advantages:
Block of logic can be programmed as a module
Simpler programming
Faster scan time
Enhanced maintainability
Ease of future enhancements
Functional chart

18. SEQUENTIAL FUNCTION CHART
(SFC) PROGRAMMING
A functional chart program is a pictorial representation or a special type of flowchart of a sequential control process.
It allows the description of the process to become the actual program.
Functional chart programming uses function blocks
made up of steps and transition units.

19. Sequential Function Chart (SFC) Programming
Check for stop
Wait for
start
Check for
part
Lock the
clamp
Press the
part
Unlock
clamp
Move
conveyor
One
cycle
Continuous
Check mode

20. PROGRAM INSTRUCTIONS

21. Relay-Type Instructions
The ladder diagram language is basically a symbolic set of instructions used to create the controller program.
These ladder instructions symbols are arranged to obtain the desired control logic.

22. EXAMINE IF CLOSED (XIC)
INSTRUCTION

23. Similar to the normally open relay contact
Similar to the normally open relay contact. For this instruction we ask the processor to EXAMINE IF (the contact is) CLOSED (XIC)
Symbol
Typically represents any input. Can be a switch
or pushbutton, a contact from a connected output,
or a contact from an internal output.
Has a bit-level address which is examined for an ON condition.
The status bit will be either 1 (ON) or 0 (OFF).
The status bit is examined for an ON condition.

24. I:012 04 If the status bit is 0 (OFF), then the instruction
is FALSE.
If the status bit is 1 (ON), then the instruction
is TRUE.

25. EXAMINE IF OPEN (XIC)
INSTRUCTION

26. Similar to the normally closed relay contact
Similar to the normally closed relay contact. For this instruction we ask the processor to EXAMINE IF (the contact is) OPEN (XIO).
Symbol
Typically represents any input. Can be a switch
or pushbutton, a contact from a connected output, or a contact from an internal output.
Has a bit-level address which is examined for an
OFF condition.
The status bit will be either 1 (ON) or 0 (OFF).
The status bit is examined for an OFF condition.

27. I:012 I:012 04 If the status bit is 1 (ON), then the instruction
is FALSE.
If the status bit is 0 (OFF), then the instruction
is TRUE.
I:012 04

28. OUTPUT ENERGIZE (OTE)
INSTRUCTION

29. Similar to the relay coil
Similar to the relay coil. The processor makes this instruction true (equivalent to energizing a coil) when there is path of true XIC and XIO instructions in the rung.
Symbol
Typically represents any output that is controlled by
some combination of input logic.
Output can be a connected device or an internal output (internal relay).
If any left-to-right path of input conditions is TRUE,
the output is energized (turned ON).

30. OUTPUT ENERGIZE instruction - FALSE OUTPUT ENERGIZE instruction - TRUE01 11 15

31. EXAMPLE

32. Status Bit Examples Input module A Bit status Button not actuated
Button actuated
OFF ON
False
True
Button not actuated
True
False ON
OFF
Output A A
Output

33. Ladder Rung A B C D
Output
instruction
Input conditions
Output
instruction
A ladder rung consists of a set of input conditions,
represented by contact instructions, and an output
instruction at the end of the rung, represented by the
coil symbol.

34. For an output to be activated or energized, at least
one left-to-right path of contacts must be closed.
A complete path is referred to as having logic
continuity. When logic exists the rung condition is
said to be TRUE.
Each contact or coil symbol is referenced with an
address number that identifies what is being
evaluated and what is being controlled. The same
contact instruction can be used throughout the program
whenever that condition needs to be evaluated.

35. Rung Continuity 1 The Examine If Closed instruction is TRUE
Bit in memory
Bit in memory 1
The Examine If Closed instruction is TRUE
making the rung TRUE
The Examine If Closed instruction is FALSE
making the rung False
LS_1
SOL_5

36. INSTRUCTION ADDRESSING

37. Allen-Bradley SLC-500 Controller Addressing
Energized
output 6
O:0:4
Address
terminal
O0:4/6
Output image table
file 0
O:0:4/6
Bit address
Input image table
file 1
I:3/12
Bit address
I1:3 12
User-programmed rung
Address
input
terminal
I1:3/12
Closed input

38. Structure of A 16-Bit Word
OFF ON 1
Word
(16 bits)
I/O Connection Diagram L1 L2
PB1
LS1
I:4/5
I:4/6
O:2/3
O:3/6
SOL 1
PL 1 R

39. BRANCH INSTRUCTIONS

40. Parallel Input Branch Instructions
Branch instructions are used
to create parallel paths of
input condition instructions.
If at least one of these parallel
branches forms a true logic
path, the logic is enabled.

41. Parallel Output BranchingD E
On most PLC models, branches can be established at
both the input and output portion of the rung.
With output branching, you can program parallel
outputs on a rung to allow a true logic path to
control multiple outputs.

42. Nested Input and Output Branches
A nested branch starts or ends within another branch.
Input and output branches can be nested to avoid
redundant instructions and to speed up the processor
scan time.

43. Nested Contact ProgramB C D E Y
Nested
contact
On some PLC models,
the programming of a
nested branch circuit
cannot be done directly.
Contact
instruction
C repeated A B C D E Y
Reprogrammed to
obtain the required
logic.

44. PLC Matrix Limitation Diagram
Max series
contacts
Max parallel
lines
No. outputs
per rung and
location of the
output in the rung
There may be limitations to the number of series
contacts instructions, number of parallel lines, and
the number of outputs and their location on the rung.

45. Programming of Vertical ContactsB C D E Y
Original program
Y = (AD) + (BCD) + (BE) + (ACE) A D B C E Y
Reprogrammed to obtain the
required logic

46. Programming for Different Scan Patterns
Original program
Y = (ABC) + (ADE) + (FE) + (FDBC) A D B E F Y A B D E Y F C
Reprogrammed to obtain the
required logic

47. INTERNAL RELAY

48. Internal Control Relay
The internal output operates just as any other output
that is controlled by programmed logic; however, the
output is used strictly for internal purposes.
The internal output does not directly control an
output device.
The advantage of using internal outputs is that there
are many situations where an output instruction is
required in a program, but no physical connection to
a field device is needed. Their use in this type of
instance can minimize output card requirements.

49. Extending the Number of Series Contacts Using
an Internal Control Relay
Internal
relay coil
Rung 1
Rung 2
Internal
relay
contact
Discrete output (requires
one physical connection
on the output module)

50. AND and OR circuit

51. LATCH

52. LATCHING RELAY

53. LATCHING RELAY It is necessary to have a relay "latch" ON so
if the device that activated the relay is switched OFF
the relay remains ON
Useful for making a momentary pushbutton switch perform
like a maintained switch
Might as well we use maintained switch?

54. Consider, for example, the pushbuttons that switch a machine on and off
Refer to basic STOP/START circuit
Here we use the contact of the motor to hold the circuit
This can be done we include a relay in the circuit that is wired as a latch as shown in the ladder diagram

55. When power is applied to the rails
CRI is initially de-energized
N/O CRI contact is also open
Assuming S1 has not been pressed
there is no path for current to flow through the rung
it will be OFF.

56. Next, when S1(START) is press
it provides a path for current flow
through S1, S2, and coil of CR1
It energizes CRI
As soon as CR1 energizes
the N/O CRI contact in parallel with S1 closes (since the CRI contact is operated by the CRI coil)

57. When the relay contact closes
switch S1 is no longer needed to maintain a path for current flow through the rung
because it is provided by the N/O CR1 contact and N/C pushbutton S2
At this point, we can release S1 and the relay CRI will remain energized

58. The N/O CR1 contact "seals" or "latches" the circuit ON
This type of contact configuration has several commonly used names including a sealing contact, seal-in contact, or latching contact

59. The circuit is de-energized
by pressing the STOP switch S2
This interrupts the flow of current through the rung
de-energizes the CRI coil
opens the CRI contact in parallel with S1.

60. When S2 is released, there will still be no current flow through the rung because
both S1 and the CR1 N/O contact are open

61. WHY DON’T WE USE MAINTAINED SWITCH?
The latch circuit has one other feature that cannot be obtained by using a maintained switch.
Should power fail while the machine is on the latch rung will, of course, de-energize
However, when power is restored, the machine will not automatically restart
It must be manually restarted by pressing S1
This is a safety feature that is required on all heavy machines

62. PROGRAM ENTERING AND EDITING

63. Entering the Ladder Diagram
A personal computer is most often used to enter the
ladder diagram.
The computer is adapted
to the particular PLC
model using the relevant
programmable controller
software.

64. SETTING

65. Specify
File name
Type of device/ processor used
Network type

66. CX Programmer Main Screen
Different screens, toolbars and windows dialog boxes are used to navigate through the Windows environment

67. INSTRUCTION TOOL BAR

68. Tool Bar for Creating Ladder Diagram
Vertical line
Function
e.g
-Counter
-Timer
-End
NO contact
NC contact
NO branch
NC branch
Horizontal line
Output coil
Output NOT
Tool Bar for Creating Ladder Diagram

69. Branch contain in the same rung
Output must located at far right
Error
Input located at right
After complete program must have END instruction

70. To connect PC with PLC (online)
To transfer program from PC to PLC
To read program from PLC to PC

71. Run Mode
Work online
Transfer to PLC
Program
PLC monitoring
Monitor
Transfer from PLC
Debug

72. PROGRAM OBSERVATION

73. Monitoring a Ladder Logic Program
Operation of the logic is apparent from the highlighting
of rungs of the various instructions on screen, which
identifies the logic state in real time and has logic
continuity.
Highlighted rungs indicate the instruction is true

74. MODES OF OPERATION A processor has basically two modes of operation:
program mode
run mode
And some of its variation

75. PROGRAM MODE May be used to enter a new program
edit or update an existing program
upload files
download files
document programs
change software configurations
When the PLC is switched into the
program mode, all outputs from the PLC
are forced off regardless of their rung
logic status, and the ladder I/O scan
sequence is stopped.

76. RUN MODE and its variations
is used to execute the user program. Input devices are monitored and output devices are energized accordingly.
Test Mode
is used to operate, or monitor, the user program without energizing any outputs.
Remote Mode
allows the PLC to be remotely changed between program and run mode by a personnel computer connected to the PLC processor.