Chapter 19
Fundamental PLC Programming
1
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
2
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
3
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
4
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)5
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
6
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
7
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
8
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
9
Timer On-Delay Application
10
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
11
Retentive Timer-On Delay
• The accumulated
value of a retentive
time is held until a
reset command is
given, regardless of
rung conditions
12
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
13
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
14
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
15
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
16
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
17
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
18
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
19
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
20
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
21
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
Compare Not Equal
Compare Less Than
Compare Less Than or Equal
Compare Greater
Compare Greater Than or Equal
(EQU)
(NEQ)
LES)
(LEQ)
(GRT)
(GEQ)
22
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)
23
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
24