1. Chapter 19 Fundamental PLC Programming

2. PLC Program Execution
PLCs monitor input devices, execute instructions, and update output devices sequentially during the processor scan cycle
The steps of the scan cycle are:
Update the input image
Reads the CPU for instructions
Update the output terminals

3. Ladder Diagram Programming Language
The programming language most commonly used with programmable logic controllers is the ladder diagram
Ladder diagram programming is built into the software of most PLCs

4. Ladder Diagram Programming
Ladder logic language closely resembles hardwired relay circuits
The symbols represent an instruction set that perform various logic and on-off functions
There are five categories of instructions
Relay Logic
Timers
Counters
Data Manipulation
Arithmetic

5. Relay Logic Instructions
These are the most common instructions found in PLCs
The relay logic instructions are:
Examine On -| |-
Energize Output -( )-
Branching - used to implement parallel inputs
Examine Off -|/|-
Latch On Output -(L)-
Unlatch Output -(U)-

6. Timer Instructions Timers are internal instructions to the PLC
Timers are activated by changes in the logic continuity of the rung
Types of timers found are:
Timer-On Delay
Timer-Off Delay
Retentive Timer-On Delay

7. Timer Programming
A separate address file for timers is provided in PLCs
File addresses begin at T4:0
Once the address is entered, the following characteristics are entered:
Time base
Preset value
Accumulated value

8. Timer Words Each timer in a PLC uses three words to store data
The second and third words store the preset and accumulated values
The first word contains status bits related to time status

9. Timer On-Delay
The Timer On-delay begins timing when rung conditions go true
When the accumulated value equals the preset value, the timer stops timing and the output is energized as bit 13 is set

10. Timer On-Delay Application

11. Timer Off-Delay
When the condition of the rung goes false, the timer off-delay begins timing
When the accumulated value equals the preset value, the output is energized
When the rung goes true, the counter is reset to zero

12. Retentive Timer-On Delay
The accumulated value of a retentive time is held until a reset command is given, regardless of rung conditions

13. Cascading Timers
If the time valued needed exceeds the maximum value of a single timer, timers may be cascaded using the done bit of previous timers

14. Counter Instructions
Counters are output instructions internal to the PLC
Counters are incremented or decremented by changes in the logic continuity of the rung
Types of counters found are:
Up Counter
Down Counter

15. Counter Programming A separate file is used for counter functions
Counter instructions begin with the address C5:0
The following information must be entered
Counter and address
Preset value
Accumulated value

16. Counter Words Each counter in a PLC uses three words to store data
The second and third words store the preset and accumulated values
The first word contains status bits related to count status

17. Up Counter Application
The count-up instructions are useful for repeating processes
The drilling application at the right makes use of an up counter which then activates a robotic arm

18. The Down-Counter
The down counter decrements with each false-true transition of the ladder rung
The down counter is often used to end a cycle

19. Data Manipulation Instructions
Data manipulation instructions allow words to be moved within the PLC
Data manipulation instructions permit more complex operations than relay type instructions
These instructions are divided into three categories:
Data Transfer
Data Conversion
Data Compare

20. Data Transfer Instructions
Data Transfer Instructions are implemented by the move (MOV) instruction
Contents from one register are moved to another based on rung conditions

21. Data Conversion Data conversion is available as:
Convert to BCD (TOD)
Convert from BCD (FRD)
Both are output instructions and convert data from or to binary coded decimal
A typical application would be the implementation of BCD encoded thumbwheel switches to input data into a PLC

22. Data Compare Instructions
These commands instruct the PLC to compare the numerical contents of two registers and make decisions based upon their values and the results of the comparison
Compare instructions:
Compare Equal (EQU)
Compare Not Equal (NEQ)
Compare Less Than LES)
Compare Less Than or Equal (LEQ)
Compare Greater (GRT)
Compare Greater Than or Equal (GEQ)

23. Arithmetic Functions
Most PLCs have the capability to carry out arithmetic operations
The output of an arithmetic instruction is stored in a specified location
Arithmetic functions available are:
Addition (ADD)
Subtraction (SUB)
Multiplication (MUL)
Division (DIV)

24. Writing a Program
Use the following steps when developing a PLC program:
Choose the sequence you want the I/O devices to operate in
Write a description and make a drawing showing the sequence and conditions for each operation
Use the description to write the ladder diagram
Connect and label the I/O devices
Make a written record of each address used and what the address represents. Document all counters, timers, data instructions, etc.
Enter the program into the PLC