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ADVANCED PROGRAMMABLE LOGIC CONTROLLER
Reading and Interpreting Ladder Diagrams
Ladder Diagram #1:
Please review the following and answer the questions below.
I:001
00
I:001
B3
01
1
B3
1
B3
O:001
1
B3
00
O:001
1
I:001
02
L
01
O:001
U
01
Assumptions: I:001/01 is a normally open start button and I:001/00 is a normally closed stop button.
I:001/02 is a normally open limit switch.
1. Describe the operation once the start button is momentarily depressed.
2. When the stop button is pressed, do all outputs turn off? Why or Why not?
3. Give an example where a program like this can be used.
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Ladder Diagram #2:
Please review the following and answer the questions below.
SUB
Subtract
Source A
Source B
Dest
N7:0
10
N7:1
DIV
Divide
Source A
Source B
Dest
N7:1
2
N7:2
ADD
Add
Source A
Source B
Dest
N7:2
5
N7:3
A. If the value located at N7:0 is 12, what is the resultant value at N7:3?
B. If the value located at N7:3 is 80, what was the value at N7:0?
C. If the value located at N7:2 is 30, what was the values at N7:0 and N7:1, and what is the value at N7:3?
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Ladder Diagram #3:
Fig. 2
Refer to Fig. 2 to answer question 2a through 2e
2a) What is the starting sequence of the motor, M1, M2 and M3 ?
2b) From the time the START SWITCH is pressed, how long does it take before the last motor is ON ?
2c) How long is the delay between the starting of M1 and M2 ?
2d) How long is the delay between the starting of M1 and M3 ?
2e) How long is the delay between the starting of M2 and M3 ?
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Ladder Diagram #4
Please review the following program and answer the questions.
TON
Timer On Delay
Timer
Time Base
Preset
Accum
T4:0
DN
T4:0
1.0
5
0
CTU
T4:0
Count up
DN
Counter
C5:1
Preset
100
Accum
0
LES
Less Than
O:001
Source A
C5:1.ACC
Source B
3
01
LIM
Limit Test
Low Limit
3
Test
C5:1.ACC
High Limit
7
O:001
02
GRT
Less Than
O:001
Source A
C5:1.ACC
Source B
7
1. If C5:1.ACC is equal to 3, what happens?
2. How long is it before O:001/02 turns on and how long is it on?
3. When is the done bit set on counter C5:1?
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03
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Ladder Diagram #5:
In the following ladder program, what would be the content of B3:20 when the content of the Accumulated
Value Word of counter C5:0 is equal to the content of N7:10 for the above ladder program?
Given: Accumulated Value of the Counter C5:0 = 100 and N7:10 = 100
Given: B 3:15 = 000011112 and B3:16 = 001100112
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Ladder Diagram #6:
Refer to Fig. 3 to answer question 3a.
Fig. 3
3a) The OTE instruction (O:001/07) will be energized if the content of N7:1 is equal to what?
Refer to Fig. 4 to answer question 3b and 3c.
Fig. 4
3b) The OTE instruction (O:001/07)will be energized if the content of F8:0 is equal to what?
3c) If the content of F8:0 is 23.567, what will happen to the output O:001/07?
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HOW TO WRITE LADDER PROGRAM?
Problem #1:
Problem Description
Fig. 1 shows a main conveyor with a diverter gate for defective parts to be fed onto a rejection conveyor. A camera
vision sensor system is used to inspect the work-piece. If a work-piece fails inspection, the vision sensor will
energized the diverted gate and the parts is routed onto the Reject Conveyor. Proximity switch #2 is used to sense the
number of rejected work-piece. Another proximity switch, #1, is used to sense the total number of work-piece on the
main conveyor before it passing the vision sensor system. The system also have a normally open start switch and a
normally closed stop switch. Both the start and stop are instantaneous contact switches.
Write a ladder program to count the number of rejected work-piece, good work-piece, and total number of work-piece
inspected. The addresses of 120VAC input and output devices are listed as below. If you want to used more input and
output devices for the purpose of controlling the system, list out the device description and its address.
I:001/00
I:001/01
I:001/02
I:001/03
I:001/04
O:001/00
- Input from vision sensor
- Normally open start switch
- Normally closed stop switch
- Proximity switch #1
- Proximity switch #2
- Diverter gate
Fig. 1
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Solution:
Step #1: Read the problem twice. You must understand the problem thoroughly before start working on the
solution.
Step #2: Understand the logical relationship among inputs and outputs.
Step #3: Draw a block diagram showing all the inputs and outputs.
Proximity Sensor #1
Proximity Sensor #2
Conveyor
System
Controller
Diverter Gate
Vision Sensor
Step #4: Write down description of all inputs and outputs and their source.




Input 1: 120V AC normally open contact (NO) from Proximity Sensor 1
Input 2: 120V AC normally open contact (NO) from Proximity Sensor 2
Input 3: 120V AC normally open contact (NO) from Vision Sensor.
Output: 120V AC normally reenergized state, Diverter Gate activation solenoid.
Logical Relationship











Proximity Sensor 1 must trigger a retentive counter counting up (CTU), Counter 1
Proximity Sensor 2 must trigger a retentive counter counting up (CTU), Counter 2
Work-piece passed equal to the difference between the counter reading of Counter 1 and Counter 2.
Operation must be started and remained active for the proximity system, belt conveyors, and vision
system.
Conveyor motor should run continuously.
Start switch ON is the condition for starting conveyor belt, diverter gate.
Conveyor running & Proximity Sensor1activate – count-up for total parts
Conveyor running & Proximity Sensor2activate – count-up for total rejected parts
Start switch ON – compute for good parts
Activate Stop switch will stop the conveyor only without resetting the counter.
Locked reset switch will reset the counter.
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The Ladder Diagram
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Problem #2:
Problem Description
Figure 1 below shows a system which stacks metal plates in a group of tens. Conveyor#1 carries the metal
plates to be stacked onto conveyor#2. The lighter detector detects the metal plates as it falls from conveyor#1
to conveyor#2. Every time a metal plate from conveyor#1 breaks the detector’s light beam it produces a logic
1 (120V AC) Conveyor#2 moves for five seconds (one position) only after it stacks 10 plates. When
conveyor#2 moves conveyor#1 must not move. To initiate and stop the system there are two normally open
(NO) momentary contact switches.
Write a ladder program using PLC 5 instruction sets to operate the system. The addresses of 120 VAC input and
output devices are listed below. If you want to use more input and output devices for the purpose of controlling the
system, list the device and its address below with other device addresses.
I:001/00 - Light detector
I:001/01 - Normally open start switch
I:001/02 - Normally closed stop switch
O:001/00 - C1 relay to turn on motor 1 to run conveyor #1
O:001/01 - C1 relay to turn on motor 1 to run conveyor #2
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Solution:
Step #1: Read the problem twice. You must understand the problem thoroughly before start working on the
solution.
Step #2: Understand the logical relationship among inputs and outputs.
Step #3: Draw a block diagram showing all the inputs and outputs.
Lighter Detector
Stacking Conveyor
Controller
Counter
(Internal Input)
Conveyor #2 Motor
Conveyor #1 Motor
Step #4: Write down description of all inputs and outputs and their source.




Input 1: 120V AC normally open contact (NO) from Proximity Sensor 1
Input 2: 120V AC normally open contact (NO) from Proximity Sensor 2
Input 3: 120V AC normally open contact (NO) from Vision Sensor.
Output: 120V AC normally reenergized state, Diverter Gate activation solenoid.
Logical Relationship
 Light detector: Every time the light detector beam is interrupted a count-up(CTU) instruction is
going to count forward. The counter must be a retentive counter.
 Conveyor system#1 is running always except when conveyor system#2 is running for 5 second.
 Conveyor system#2 is running for 5 second and is adjustable depending on the dynamics of the
manufacturing process. The five second timer can be done by a TON (Time-On-Delay instruction.
 CTU retentive timer with light detector system counts up to ten and trigger the timing of the
conveyor#2 system.
 When the conveyor#2 completes its 5 second run the program goes back to Rung#1 and also RESETs
the CTU counter for next batch of 10.
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Ladder Diagram #7:
The following is segment of a ladder program that is transferring analog data from Analog Input Module to
Analog Output Module by using a BTR and BTW instruction. Answer the following questions by referring
the ladder program segment.
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5a) Which channel the analog input data is connected?
5b) Through which channel the analog output data is going output?
5c) What is the address of the Error Bit of the BTW instruction that is associated with the Analog output
module?
5d) Are these I/O modules are located in the same rack as the processor module?
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