Bryan L. Programmable controllers. Theory and implementation
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line that defines the grouping of PLC ladder instructions, however, is usually
drawn between functional instruction categories. These instruction catego-
ries include:
• ladder relay
• timing
• counting
• program/flow control
• arithmetic
• data manipulation
• data transfer
• special function (sequencers)
• network communication
Although these categories are straightforward, the classification of them is
subjective. For example, some people believe that basic ladder instructions
include ladder relay, timing, counting, program/flow control, arithmetic, and
some data manipulation. Others believe that only ladder relay, timing, and
counting categories should be considered basic ladder instructions.
Regardless of classification, the effects of instruction categories are simple—
the more instruction categories a PLC has, the more powerful its control
capability becomes. Usually, small PLCs have only basic instructions with,
perhaps, some enhanced instructions. Larger PLCs usually have more
Table 9-1. PLC instruction set classifications.
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advanced instruction sets. However, recent advances in software develop-
ment and I/O hardware have increased the computational power of small
PLCs through advanced instructions. This new trend has made small PLCs
very desirable in single, as well as distributed control, applications.
BOOLEAN
Some PLC manufacturers use Boolean language, also called Boolean
mnemonics, to program a controller. The Boolean language uses Boolean
algebra syntax (see Chapter 3) to enter and explain the control logic. That is,
it uses the AND, OR, and NOT logic functions to implement the control
circuits in the control program. Figure 9-3 shows a basic Boolean program.
Figure 9-3. Hardwired logic circuit and its Boolean representation.
The Boolean language is primarily just a way of entering the control
program into a PLC, rather than an actual instruction-oriented language.
When displayed on the programming monitor, the Boolean language is
usually viewed as a ladder circuit instead of as the Boolean commands that
define the instruction. We will discuss Boolean programming, along with its
instruction set, at the end of this chapter.
L1 L1 L2L2
Boolean Program
LD 10
OR 12
AND 11
OUT 40
Displayed as ladder diagram
LS1
10
10
12
11 40
40 SOL1
L1 L2
LS1
LS2
11
PB1
PB1
12
LS2
SOL1
Hardwired Circuit
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GRAFCET
Grafcet (Graphe Fonctionnel de Commande Étape Transition) is a symbolic,
graphic language, which originated in France, that represents the control
program as steps or stages in the machine or process. In fact, the English
translation of Grafcet means “step transition function charts.” As we will
discuss in Chapter 10, Grafcet is the foundation for the IEC 1131 standard’s
sequential function charts (SFCs), which allow several PLC languages to be
used in one control program.
Figure 9-4 illustrates a simple circuit represented in Grafcet. Note that
Grafcet charts provide a flowchart-like representation of the events that take
place in each stage of the control program. These charts use three compo-
nents—steps, transitions, and actions—to represent events. The IEC 1131
standard’s SFCs also use these components; however, the instructions inside
the actions can be programmed using one or more possible languages,
including ladder diagrams.
Figure 9-4. Hardwired logic circuit and its Grafcet representation.
Few programmable controllers may be directly programmed using Grafcet.
However, several Grafcet software manufacturers provide off-line Grafcet
programming using a personal computer. Once programmed in the PC, the
Grafcet instructions can be transferred to a PLC via a translator or driver that
translates the Grafcet program into a ladder diagram or Boolean language
program. Using this method, a Grafcet software manufacturer can provide
different PLCs that use the same “language.” Figure 9-5 illustrates a typical
translation that occurs when using Grafcet. Chapter 10 provides more detail
about the versatility of this type of structural programming.
L1 L2
PB1
CR1
CR1 LS1 M1
CR2
CR1
Hardwired Circuit Grafcet
1
2
1
2
If PB1
LS2
M1 IF LS1
LS2 CR2
Step
Transition
Action
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Figure 9-5. Grafcet translation.
9-3 LADDER DIAGRAM FORMAT
The ladder diagram language is a symbolic instruction set that is used to
create PLC programs. The ladder instruction symbols can be formatted to
obtain the desired control logic, which is then entered into memory. Since this
type of instruction set consists of contact symbols, it is also referred to as
contact symbology.
A thorough understanding of ladder diagram programming, including func-
tional blocks, is extremely beneficial, even when using a PLC with IEC
1131 programming language capabilities. Because ladder diagrams are easy
to use and implement, they provide a powerful programming tool when used
in the IEC 1131 environment.
The main functions of a ladder diagram program are to control outputs and
perform functional operations based on input conditions. Ladder diagrams
use rungs to accomplish this control. Figure 9-6 shows the basic structure of
a ladder rung. In general, a rung consists of a set of input conditions
(represented by contact instructions) and an output instruction at the end of
the rung (represented by a coil symbol). The contact instructions for a rung
may be referred to as input conditions, rung conditions, or the control logic.
Figure 9-6. Ladder rung structure.
Output
Instructions
Input
Conditions
L1 L2
A continuous path is required for logic continuity
Software
Translator
Grafcet PLC Ladder Language
PE3
IF LS1
AND
LS2
LS4
PB2
LS1 LS2 CR1
PE1
PE2
M2
PE3 LS4
TMR1
M2
CR1 SOL1
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A ladder rung is TRUE (i.e., energizing an output or functional instruction
block) when it has logic continuity. Logic continuity exists when power
flows through the rung from left to right. The execution of logic events that
enable the output provide this continuity. In a ladder rung, the left-most side
(left power line) simulates the L1 line of a relay ladder diagram, while the
right-most side (right power line) simulates the L2 line of the electromechani-
cal representation. Continuity occurs when a path between these two lines
contains contact elements in a closed condition, allowing power to flow from
left to right. These contact elements either close or remain closed according
to the status of their reference inputs. Figure 9-7 illustrates several continuous
paths that provide continuity and energize the output of the rung. Power
continuity is normally represented on a PLC’s monitoring device (e.g., a PC)
by bold or emphasized lines, as shown in Figure 9-8a. Figure 9-8b illustrates
power continuity through only one energized contact element; note that the
output is not ON. We will explain how these contact symbols are interpreted
to be ON or OFF in the next section.
Figure 9-7. Illustration of several different continuity paths in a ladder rung.
Power (continuity)
Power
Power
Power
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When a ladder diagram contains a functional block, contact instructions are
used to represent the input conditions that drive (or enable) the block’s logic.
A functional block can have one or more enable inputs that control its
operation. In addition, it can have one or more output coils, which signify the
status of the function being performed. For example, the block shown in
Figure 9-9a has an enable block line, which when energized (i.e., continuity
exists), will activate the block to perform the instruction. Thus, this instruc-
tion says: IF the enable is ON because the desired logic has continuity, THEN
execute the block instruction. Depending on the instruction, other enable lines
(see Figure 9-9b) may drive the block using reset or other control functions.
Figure 9-8. Monitoring device showing (a) power continuity through the rung—inputs 11 and
12 are ON, turning output 40 ON—and (b) power continuity through only input
12, thus output 40 is not ON.
Figure 9-9. Functional block instructions with (a) one enable line and one output and
(b) one enable line, a start timing command, and two outputs.
(a)
(b)
10 12 40
11
(b)
10 12 40
11
(a)
Output
Enable
Reset
Time Enable
Time = Preset
Enable
Reset
Time
Input Conditions Functional Blocks and Outputs
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The ladder rung matrix determines the maximum number of ladder
contact elements that can be used to program a rung (see Figure 9-11). The
size of this matrix differs among both PLC manufacturers and the program-
ming devices used (CRT screens versus miniprogrammers). For functional
block operations, a ladder matrix may have less available ladder contact
elements because the functional block instruction display takes up room in
Input Conditions Output
Figure 9-11. Ladder rung matrix.
Figure 9-10. A functional block instruction that is always enabled.
To make a block active at all times without any driving logic, the user can omit
all contact logic and place a continuity line in the block during programming
(see Figure 9-10).
Enable
Line
Output
Functional
Block
Intruction
L1 L2
Left
Power
Line
Right
Power
Line
Power Flow
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the matrix (see Figure 9-12a). In PLCs with enhanced ladder format
functional instructions instead of block-type instructions, the ladder matrix
may use one or more contact symbol spaces to represent the instruction in the
programming device (see Figure 9-12b).
Figure 9-12. Ladder matrix with (a) functional block instructions and (b) enhanced
ladder format functional instructions.
A ladder matrix represents all the possible locations where a contact symbol
instruction can be placed. The programming device usually displays all of
these possible locations on the screen, allowing the user to place contact
symbols in the desired locations. However, according to the maker of the
PLC, certain rules apply to contact placement. One rule, which is present in
almost all PLCs, prevents reverse (i.e., right-to-left) power flow in a ladder
rung (see Figure 9-13). PLC logic does not allow reverse power to avoid
sneak paths. Sneak paths occur when power flows in a reverse direction
through an undesired field device, thus completing a continuity path. If a
PLC’s logic requires reverse power flow, the user must reprogram the rung
with forward power flow to all contact elements. The next example illus-
trates the solution to the reverse power flow rung in Figure 9-13.
Output
Output
Input Conditions
Input Conditions
Enhanced
Functional Instruction
Block Instruction
Enable
Reset
(a)
(b)
Move R10 to R20
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Figure 9-13. Reverse power flow at contact D.
EXAMPLE 9-1
Solve the logic rung shown in Figure 9-13 so that no reverse power
flow condition exists. The reverse condition is not part of the required
logic for the output to be energized.
S
OLUTION
The forward power flow of the logic determines output
Y
. Let’s
implement it using logic concepts. The output
Y
is defined, using
forward paths only, as:
Y ABC ADE FE
=••+••+•()()()
1st line 2nd line 3rdline
6748467484678
which can be minimized, using Boolean algebra’s distributed rule, to
(see Chapter 3):
YABCDE FE
=••+•+•()()
Figure 9-14 shows the implementation of this logic gate, while Figure
9-15 gives the ladder-equivalent solution.
Figure 9-14. Logic solution for Example 9-1.
A
B
C
D
E
F
E
(B • C + D • E)
D • E
F • E
B • C
A • (B • C + D • E)
Y
AYBC
F
DE
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EXAMPLE 9-2
Solve the ladder logic shown in Figure 9-13 so that no reverse power
flow exists. Assume that the reverse path logic through contact D and
then forward through contacts B and C is required in the PLC logic
solution to energize the output.
S
OLUTION
Following the same procedure as in Example 9-1, we can obtain the
desired logic for output
Y
using Boolean logic expressions. Therefore,
output
Y
, including the reverse power flow logic, is represented by:
Y ABC ADE FE FDBC
ABCDE FEDBC
=••+••+•+•••
=••+• + +••
()()()( )
()()
1st line 2nd line 3rd line
Reverse path
67484674846786744844
The term
F
•
D
•
B
•
C
implements the reverse power flow sequence
that output
Y
requires. Figure 9-16 shows the ladder diagram of this
solution.
A B C
F E
Y
D E
Figure 9-15. Ladder diagram implementation for Example 9-1.
A B C
D B C
F E
Y
D E
A
Figure 9-16. Ladder diagram implementation for Example 9-2.