1. PLC programming for
applications and
maintenance
By
Dr. Palitha Dassanayake

2. Content
 Programming a simple application
 Faultfinding and maintenance
 Programming application for
sequential operations
 Another program application

3. Constructing a PLC Program
Methodology in programming a PLC are given as
follows.
2.Write down or draw the requirements, logics or/and sequences
in an understandable format using pseudocodes, flowcharts or
UML (in some special cases). This may be skipped for smaller tasks,
but essential when that the task becomes large or complicated.
However, it is good practice to conduct this step to avoid unnecessary
logical errors.
1. Identify the requirements or logical sequences that are
going to be handled by a PLC.

4. Draw the necessary tables or maps to illustrate the PLC
ogic that match with the requirement, but independent
om the language code (ladder, logo or statement list)
4. Connect the PLC to the system to be controlled and to the
computer that PLC is going to get programmed
Identify the input and output cables that are connected in
der to program.

5. 6. Write the PLC program according to step 3 using the identified
inputs/outputs in step 5.
7. See the possibilities of improving the program
8. Validate the program before the actual run. This may be achieved
through a simulation facilitated in the software or using a virtual
model using bulb and switches

6. Check the program. Remove the connection from the computer
necessary. It may be conducted part by part in a large application.
lso, when the program is run for the first time, required safety
ecautions must be made before hand in order to avoid any damages
ue to logical errors.
0.Remove the connection from the computer if that has not been
one and use the PLC for the application.

7. Example
A1
Is
C1
C2
Fig:Two views of the system
Is-Inductive
sensor
A1-Pneumatic Actuator
C1-Collector
C2-Collector
Gear
Box
motor-m
item
Os-Optical sensor
A0
A0

8. 1. When Os detects an item, it should be pushed by A0 and conveyor
should start moving
2. If the item is metal, it has to be identified by Is.
3. Metal items are to be pushed by A1 and collected in C1.
4. Non-metal items are to be collected at the end of the belt to C2
5. If an item is in process, even if Os detects an item, it should not be
pushed by A1 until the previous item is fully processed.

9. Start -No item Os=0
Os=1 A0 should be ejected provided no other item is in process
If the item is metal, it is sensed by Is.
If Is=1 item is metal, this should be remembered
If Is had been 1 it has to pushed by A1 after a time interval
If the item is not metal, it should not be pushed and collected at
the end. The time required to travel is also required.

10. Amplifier
Electro-pneumatic
valve
PLC
Operating
Voltage –24 v
I0.1
I0.0
Q0.0
Q0.1
I s-
C
C2
Gear
Box
m
Os
If output the sensor
is in mv
A/C motor
RelayA0
Q0.2
Electro-pneumatic
valve
A1

11. Os - I0.0
Is - I0.1
A1 - Q0.0
M - Q0.1
A0 - Q0.2
Input-output connections
M0.0 - An item is in process
M0.1- The item is Metallic
0.3 sec time is required for a pneumatic actuator to activate
Operation Time(s)
Activate A0 0
Near A1 10
End of the belt 15

12. Start -No item Os=0
Os=1 A0 should be ejected provided no other item is in process
Time=0.3 sec
T33 Q0.2
R
M0.0
S
Q0.2 T33
30+
IN
PT
TON
Q
ET
I0.0 M0.0 Q0.2
S

13. If the item is metal, it is sensed by Is.
If Is=1 item is metal, this should be remembered
I0.1 M0.1
S
If Is had been 1 it has to pushed by A1 after a time interval
T37 T34
30+
IN
PT
TOF
Q
ET
T34
T34 Q0.0M0.1
M0.0 T37
100+
IN
PT
TON
Q
ET
T37

14. If the item is not metal, it should not be pushed and collected at
the end. The time required to travel is also required.
You must reset already set items once the process is completed
M0.0 T38
150+
IN
PT
TON
Q
ET
T38
Q0.0
T38
M0.1
R
M0.0
R

15. Fault in a system
System is not working at all
1. Check whether PLC is in the run mode
Stop mode, change
Error Mode, Restart PLC again error go to Step 2
2. Check inputs
No input working check input power supply, connections etc
Some inputs not working, check the connected wires, sensors or switches
and interfaces
All inputs working go to Step 3
3. Check outputs
Check the outputs in the order it should work
Check the LED of the PLC, if it is working find the fault in the connection or device
No LED output is on, down load the program
Communication with PLC is not possible replace PLC
After downloading the programming Not working replace PLC

16. Faults and causes
Os Fault A0 –Not work and Motor not work
Is Fault A1-Not work
Motor Fault Motor not work
A0 Fault A0 Not Work
A1 Fault A1 Not Work
Sensor Faults Nothing Work
Pneumatic problem A0 and A1 Not work
PLC Fault Nothing Work

17. Fault Finding
A/C Motor is not working, but the pneumatic actuator A0 pushes
the item into conveyor
Check the output of the PLC connected to the relay of the PLC
That is Q 0.1 if it is on (LED on) fault is from that point
If it is off (Generally not possible)
check the program or check all sensors
Pneumatic Actuator A0 is not working, but the A/C motor is working
Check the output of the PLC connected to the electro-pneumatic valve
of the PLC That is Q 0.2 if it is on (LED on) fault is from that point
If it is off, Place a metal onto the conveyor and check A1 is working
A1 Not working Pneumatic problem
A1 is working (Generally not possible)
check the program or check all sensors

18. Fault Finding Contd.
A0 and A/C Motor is working, but the pneumatic actuator A1 does
not sort items
Check the output of the PLC connected to the electro-pneumatic valve
of the PLC That is Q 0.0 if it is on (LED on) fault is from that point
If it is off, check the sensor Is
Working generally not possible
check the program or check all sensors or sensor Is
System is not working at all
Check electrical connections, check sensor failure,
No program in PLC
Pneumatic failure+ Motor is not working
PLC is at fault

19. Setting and Resetting a Memory
I0.0 M0.0
S
I0.1 M0.0
R
Another method with one ladder
I0.0
M0.0
I0.1 M0.0

20. Cylinders are in the
original position
A
B
C
D
Cylinder 2
X2
Cylinder 1
X1

21. A
B
C
D
Cylinder 2
X2
Cylinder 1
X1

22. A
B
C
D
Cylinder 1
X1
Cylinder 2
X2

23. Cylinders are in the
original position
A
B
C
D
Cylinder 1
X1
Cylinder 2
X2

24. Cylinders are in the
original position
A
B
C
D
Cylinder 2
X2
Cylinder 1
X1
=1 =1

25. Cylinders are in the
original position
A
B
C
D
Cylinder 2
X2
Cylinder 1
X1
=0 =1

26. ABC
Pneumatic
Actuator
ProductProductProductProduct
Product
Product

27. ABC
Pneumatic
Actuator
Product
A=0 B=0 C=0
Product
A=1 B=0 C=0
Product
Time 1 sec after C=1
Product
A=1 B=1 C=0
If short when B=1 A=0
Product
A=0 B=1 C=1
If long when C=1 A=1
Product
Time 1.3 sec after C=1
If the product is long or short

28. A B M0.0
S
T34
M0.1
100+
IN
PT
TON
Q
ET
T33
30+
IN
PT
TOF
Q
ET
34T T33
M0.1
S
M0.0
A C
C
PLC programming using S7 Version

29. apT33 T34
M0.0
R
T33 T34 M0.1
R
C

30. END