1. LADDER DIAGRAM
A ladder diagram is a means of graphically representing the logic required in a relay logic system.
Rail
Rung

2. PLC WIRING DIAGRAM A B External switches Stored program Input PLC
Output A 01 01 02 20 11 C B 12 02 20 03 20 11
External
switches
Stored program

3. SCAN
A PLC resolves the logic of a ladder diagram (program) rung by rung, from the top to the bottom. Usually, all the outputs are updated based on the status of the internal registers. Then the input states are checked and the corresponding input registers are updated. Only after the I/Os have been resolved, is the program then executed. This process is run in a endless cycle. The time it takes to finish one cycle is called the scan time. In some controllers the idle state is eliminated. In this case, the scan time varies depends on the program length.

4. PLC Scan
begin
Input
Output
Resolve
logic
Idle
Scan cycle

5. PLC Ladder Diagram INSTRUCTIONS
1) Relay,
2) Timer and counter,
3) Program control,
4) Arithmetic,
5) Data manipulation,
6) Data transfer, and
7) Others, such as sequencers.

6. LOGIC STATES ON : TRUE, contact closure, energize, etc.
OFF: FALSE, contact open , de-energize, etc.
Do not confuse the internal relay and program with the external
switch and relay. Internal symbols are used for programming.
External devices provide actual interface.
(In the notes we use the symbol "~" to represent negation. AND and OR are logic operators. )

7. Power flows through these contacts when they are closed. The
PROGRAMMING
Normally Open
(NO)
Normally Closed
(NC)
Power flows through these contacts when they are closed. The
normally open (NO) is true when the input or output status bit
controlling the contact is 1. The normally closed (NC) is true
when the input or output status bit controlling the contact is 0. 7

8. Coils represent relays that are energized when power flows to
them. When a coil is energized it causes a corresponding
output to turn on by changing the state of the status bit controlling
the output to 1. That same output status bit maybe used to control
normally open or normally closed contact anywhere in the program. 8

9. Boxes represent various instructions or functions that are
Executed when power flows to the box. Some of these
Functions are timers, counters and math operations. 9

10. Each rung or network on a ladder program represents
AND OPERATION B C A
Rung
Each rung or network on a ladder program represents
a logic operation. In the rung above, both inputs A and B
must be true (1) in order for the output C to be true (1). 10

11. In the rung above, it can be seen that either input A or B
OR OPERATION C A
Rung B
In the rung above, it can be seen that either input A or B
is be true (1), or both are true, then the output C is true (1). 11

12. NOT OPERATION C A
Rung
In the rung above, it can be seen that if input A is be true (1),
then the output C is true (0) or when A is (0), output C is 1.

13. AND and OR LOGIC AND OR PB1 PB2 R1 R1 = PB1.AND.PB2 PB3 PB4 R2
R1 = PB1 .OR. PB2 OR
PB2

14. COMBINED AND & OR
R1 = PB1 .OR. (PB2 .AND. PB3)
PB1 R1
PB3
PB2

15. PROGRAMMING EXAMPLE 1

16. Operation id description state explanation
MSI microswitch 1 part arrive
R1 output to bar code reader 1 scan the part
C1 input from bar code reader 1 right part
R2 output robot 1 loading cycle
R3 output robot 1 unloading cycle
C2 input from robot 1 robot busy
R4 output to stopper 1 stopper up
C3 input from machine 1 machine busy
C4 input from machine 1 task complete

17. PLC WIRING DIAGRAM

18. Operation
Rung 1. If part arrives and no part is stopped, trigger the bar code reader.
Rung 2. If it is a right part, activate the stopper.
Rung 3. If the stopper is up, the machine is not busy and the robot is not busy, load the part onto the machine.
Rung 4. If the task is completed and the robot is not busy, unload the machine.

19. PLC Ladder

20. Sequential Function Chart
Action Qualifiers:
N non-stored, executes while the step is active
R resets a store action
S sets an action active
L time limited action, terminates after a given period
D time delayed action.
P a pulse action, executes once in a step
SD stored and time delayed
DS time delayed and stored
SL stored and time limited

21. Sequential Function Chart

22. A Detailed Design Process
1. Understand the process
2. Hardware/software selection
3. Develop ladder logic
4. Determine scan times and memory requirements

23. Specifications OUTPUT-PORT POWER RATINGS
Each output port should be capable of supplying sufficient voltage and current to drive the output peripheral connected to it.

24. SCAN TIME
This is the speed at which the controller executes the relay-ladder logic program. This variable is usually specified as the scan time per 1000 logic nodes and typically ranges from 1 to 200 milliseconds.

25. MEMORY CAPACITY
The amount of memory required for a particular application is related to the length of the program and the complexity of the control system. Simple applications having just a few relays do not require significant amount of memory. Program length tend to expand after the system have been used for a while. It is advantageous to a acquire a controller that has more memory than is presently needed.

26. PLC Status Indicators
Power On
Run Mode
Programming Mode
Fault

27. Troubleshooting
1. Look at the process 2. PLC status lights HALT - something has stopped the CPU RUN - the PLC thinks it is OK (and probably is) ERROR - a physical problem has occurred with the PLC 3. Indicator lights on I/O cards and sensors 4. Consult the manuals, or use software if available. 5. Use programming terminal / laptop.

28. List of items required when working with PLCs:
1. Programming Terminal - laptop or desktop PC.
2. PLC Software. PLC manufacturers have
their own specific software and license key.
3. Communication cable for connection from Laptop
to PLC.
4. Backup copy of the ladder program (on diskette, CDROM, hard disk, flash memory). If none, upload it from the PLC.
5. Documentation- (PLC manual, Software manual, drawings, ladder program printout, and Seq. of Operations manual.)

29. Examples of PLC Programming Software:
1. Allen-Bradley – Rockwell Software RSLogix500
2. Modicon - Modsoft
3. Omron - Syswin
4. GE-Fanuc Series 6 – LogicMaster6
5. Square D- PowerLogic
6. Texas Instruments – Simatic
7. Telemecanique – Modicon TSX Micro 8. Mitshibishi – MelSoft (GX Developer)

30. PLC
Outputs & Power Supply
Communication
Ports (RS-485)
Inputs

31. PLC Internal Architecture

32. PLC Input/Output

33. PLC Input Devices Push buttons
Switches (limit switches, level switches, etc.)
Sensors
...

34. PLC Output Devices Relay contacts Solenoid valves
Signal devices (such as lamps, alarms, etc.)
Motors
...

35. Programming terminal

36. Programming terminal Programming is done through programming terminal
Programming terminal translates engineering language (logic control) to machine language (binary code)

37. Programming through standard computer
Most PLC manufacturers offer software packages that allow a standard computer to be used as a programming terminal

38. Programming through standard computer

39. Relating the program to inputs and outputs

40. SWITCHES DPST SPDT N o n - l o c k i n g L o c k i n g N o r m a l l y1 P 2 M u l t i p l e T h r o w M u l t i p l e P o l e B r e a k - b e f o r e - m a k e M a k e - b e f o r e - b r e a k

41. TERMS Throw - number of states
Pole - number of connecting moving parts (number of individual circuits).
SPDT
A serial switch box (A-B box) has
two 25 pin serial ports to switch from. A B
Output
Input
DPST
Knob
How is this switch classified?

42. TYPES OF SWITCHES Selector switches Pushbutton switches
Photoelectric switches
Limit Switches
Proximity switches
Level switches
Thumbwheel switches
Slide switches
RATING:
24 Volts AC/DC
48 Volts AC/DC
120 Volts AC/DC
230 Volts AC/DC
TTL level
(Transistor-to-transistor
±5V)
Isolated Input

43. RELAYS
A switch whose operation is activated by an electromagnet is called a "relay"
contact
coil
input
Relay coil R1
Output contact R1

44. RELAY Ladder Logic
Example 1: For a process control, it is desired to have the process start (by turning on a motor) five seconds after a part touched a limit switch. The process is terminated automatically when the finished part touches a second limit switch. An emergency switch will stop the process any time when it is pushed.

46. Example 2: One motor with two pushbuttons: start and stop State variables: PB1(for start), PB2(for stop), M (for motor)

47. Logic
PB1 is on -> CR1 energized, normally open contact 1 is closed -> M=1
PB2 is on -> CR2 energized, normally close contact 2 is open -> M=0
Rung 1: CR1=(PB1+CR1) CR2
Rung 2: CR2=(PB2)
Rung 3: M=CR1 CR2

48. Cylinder Pneumatic
Also called Actuator

49. ElectroPneumatic Valve
Directional Control Valve which acts as a ‘switch’ to direct compressed air to each side of pneumatic actuator.

50. 5-Port 2 Way Valve
Also called Double Acting Pneumatic Actuator and 5/2 way solenoid operated directional control valve.
Two ports to allow air in, one for outstroke (extend) and one for in-stroke (retract). 

51. Cylinder & Valve Assembly

52. TIMER

53. TIMER
A timer consists of an internal clock, a count value register, and an accumulator. It is used for or some timing purpose.
Time 5 seconds.

54. ON-DELAY TIMER (TON)
For this example the timer has been set for 5 seconds. When S1 is closed, TR1 begins timing. When 5 seconds have elapsed, TR1 will close its associated normally open TR1 contacts, illuminating pilot light PL1. When S1 is open, de-energizing TR1,
the TR1 contacts open, immediately extinguishing PL1. This type of timer is referred to as ON delay..

55. TON Example
When the switch is closed input 4 becomes a logic 1, which is loaded into timer T37. T37 has a time base of 100 ms (.100 seconds). The preset time (PT) value has been set to 150. This is equivalent to 15 seconds (.100 x 150 ). The light will turn on 15 seconds after the input switch is closed.

56. Retentive On-Delay (TONR)
The Retentive On-Delay timer (TONR) functions in a similar manner to the On-Delay timer (TON). There is one difference. The Retentive On-Delay timer times as long as the enabling
input is on, but does not reset when the input goes off. The timer must be reset with a RESET (R) instruction.

57. TONR Example

58. TONR Example Cont.
The same example used with the On-Delay timer will be used with the Retentive On-Delay timer. When the switch is closed at input I0.3, timer T5 (Retentive timer) begins timing. If, for example, after 10 seconds input I0.3 is opened the timer
stops. When input I0.3 is closed the timer will begin timing at 10 seconds. A RESET (R) instruction can be added. Here a pushbutton is connected to input I0.2. If after 10 seconds input I0.3 were opened, T5 can be reset by momentarily closing input I0.2. T5 will be reset to 0 and begin timing from 0 when input I0.3 is closed again.

59. OFF-DELAY (TOFF)
The Off-Delay timer is used to delay an output off for a fixed period of time after the input turns off. When the enabling bit turns on the timer bit turns on immediately and the value is set to 0. When the input turns off, the timer counts until the preset time has elapsed before the timer bit turns off.

60. TIMER MAX VALUES

61. TIMER EXAMPLE
Start PB Pressed >> Pump1 ON (5 sec) >> Pump2 ON (3 sec) >> Mixer ON (60 sec) >> Drain Valve ON >> Pump 3 (8 sec)

62. Counter

63. COUNTER
Digital counters output in the form of a relay contact when a pre-assigned count value is reached. 5

64. CTU, CTD, and CTUD

65. UP COUNTER (CTU)

66. Down Counter (CTD)

67. UP/DOWN COUNTER (CTUD)

68. COUNTER EXAMPLE

69. Example

70. Example (cont.)