Bryan L. Programmable controllers. Theory and implementation
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As illustrated in Example 10-5, the programming of an input field device
depends on how it is wired to the input interface. A timing diagram can
provide tremendous help in determining the appropriate logic for a required
transition. Note that the same type of fail-safe circuit that is required in ladder
diagrams must also be incorporated when programming SFCs. A fail-safe
start/stop circuit can be implemented using ladder diagrams in an action, as
illustrated in Figure 10-46.
Figure 10-46. Fail-safe circuit implemented in an SFC using ladder diagrams.
DIVERGENCES AND CONVERGENCES
So far, we have only discussed sequential function charts that have one link
between their steps and transitions. However, SFCs can have multiple links
between these program elements (see Figure 10-47). These multiple links can
be one of two types:
• divergences
• convergences
Figure 10-47. An SFC with (a) one link between the steps and transitions and (b)
multiple links between steps and transitions.
10
11
12
10
11
12
10
11
13
10
11
21
20
21
13
One
link
between
each
step
Multiple
links
between
steps
10
10
9
(Motor M_1)
M1-1
Stop_1 Motor_1Start_1
(a)
(b)
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A divergence is when an SFC element has many links going out of it, while
a convergence is when an element has many links coming into it. Both
divergences and convergences can have either OR or AND configurations,
which relate to the Boolean logic operators of the same name.
OR Divergences and Convergences. Figure 10-48 shows an OR diver-
gence, or single divergence, which connects one step to many transitions. An
OR divergence allows an active step to pass its token to one of several steps
via connecting transitions; thus, it “diverts” one step to several transitions.
Although an OR divergence connects a step with several transitions, the step
can only activate one of these transitions at a time. In other words, like an
exclusive-OR (XOR) function, the transitions must be mutually exclusive,
triggering only one transition. Depending on the IEC 1131-3 system, an OR
divergence must have either mutually exclusive triggering signals (i.e., when
one transition is ON, the others are OFF) or programmed logic that creates a
mutually exclusive situation (i.e., only one divergence path can be triggered
at a time). Some systems avoid multiple divergence paths by selecting either
the left-most or right-most divergence if several triggering conditions occur
at once. This prioritizes divergence path selection.
Figure 10-48. OR divergence.
Figure 10-49 shows an SFC with an OR divergence after step 1. Once step 1
is activated, either step 10 or 20 can be activated if either transition 1 or 2 is
triggered. These two transitions have mutually exclusive triggering condi-
10
1
12
11
10
1
11
12
20
30
20
2
30
OR Divergence
(one step to several transitions)
OR Convergence
(several transitions to one step)
Figure 10-49. Example of an OR divergence and an OR convergence.
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tions, so that the token advances in only one branch of the divergence.
Therefore, if transition 1 is triggered, step 10 becomes active; if transition 2
is triggered, step 20 becomes active.
An OR convergence, also called a single convergence, is used to link several
transitions to the same step (see Figure 10-50). An OR convergence is the
opposite of an OR divergence; it “converges” several transitions to one step.
Referring to Figure 10-49, this SFC illustrates an OR convergence in
addition to an OR divergence. The OR convergence indicates that either of
two links, one containing transition 11 and the other containing transition 30,
can pass the control token to step 12. Because of the mutually exclusive
requirement of the transition triggers, OR convergences and divergences are
well suited for programming alarm circuit SFCs like the one shown in
Figure 10-51. In this program, if the circuit is working properly after
initialization, the program will begin the control sequence (transition 1 to step
20); whereas if an error occurs, the program will initiate an alarm action
(transition 2 to step 30), which will sound an alarm until the alarm acknowl-
edgment is triggered (transition 30). From step 1, the program can pass the
token through only one path (either transition 1 or 2), but not both, because
of the logical mutual exclusivity of the OR programming.
Figure 10-50. OR convergence.
Figure 10-51. An alarm circuit with an OR divergence and an OR convergence.
1
20
1
20
Run
AND
Not_Error
M1_Started
Start_Motor_M1
21
21 Time_Up
Start_Run
22
22 M1_Stopped
Stop_Motor_M1
30
2
30
Error
Acknowledge
Alarm
Initialize
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AND Divergences and Convergences. An AND divergence, also called a
double divergence, provides a link from one transition to many steps in
parallel form (see Figure 10-52). Unlike an OR divergence, an AND
divergence can pass the token through several branches at once. For example,
if transition 1 in Figure 10-53 is triggered, the program will pass control to
both step 40 and step 50 at the same time. The parallel lines that represent
an AND convergence indicate that it passes control to all the steps below
it in parallel.
Figure 10-52. AND divergence.
An AND convergence, also referred to as a double convergence, links
multiple steps to a single transition (see Figure 10-54). It is most commonly
used to group SFC branches that were separated by an AND divergence.
Referring to Figure 10-53, once steps 41 and 51 both have the token (i.e., their
actions are ON), the SFC program will wait for transition 2 to trigger and
then pass the control token to step 10. This is an AND convergence function
Figure 10-53. Example of an AND divergence and an AND convergence.
1
1
40
40
50
41 51
50
10
2
10
AND Divergence
(one transition to several steps)
AND Convergence
(several steps to one transition)
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Figure 10-54. AND convergence.
because both steps 41 AND 51 will be deactivated by the transition. If more
than two links converge at the transition, then all the steps immediately
preceding the convergence must be active before the transition can occur.
When it does occur, all the steps will converge to the step following the
transition. For example, if only step 51 is active and transition 2 occurs, the
SFC will not pass control to step 10. When both steps 41 and 51 are active and
transition 2 is TRUE, then control will pass to step 10.
AND divergences and convergences are ideal for running control programs
in a synchronized, parallel manner. For example, Figure 10-55 illustrates a
sequential function chart depicting two processes that occur in parallel (at
the same time). When transition 1 becomes active, it diverts activity to two
program sections, each controlling one of the processes. Each program
section, Process1 and Process2, must be completed (steps 21 and 31 active)
before transition 2 can occur, transferring control back to step 1. Note that in
an SFC transition like transition 2, which has the trigger variable True, the
transition is always triggered. When used in an application, this type of AND
convergence transition simply waits for both processes to finish before
transferring control to the next step.
Figure 10-55. An SFC using AND convergences and divergences to run two
processes in parallel.
1
1 Run
20
20 End_of_Process1 End_of_Process2
30
21 31
30
2 True
Initialize
Process1
Wait_for_Process2
Process2
Wait_for_Process1
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SUBPROGRAMS
Main Program Subprogram
Execution of parent (main) program
is suspended until subprogram ends.
IN
OUT
Main
Program
Chart 1
Main
Program
Chart 2
Main
Program
Chart 3
Main
Program
Chart 4
Figure 10-56. Process with several SFC programs.
As illustrated in Figure 10-56, a process may have several main program
charts executing different main control tasks within the PLC system.
Depending on the IEC 1131-3 software system, these main programs may
utilize one or more subprograms (smaller, independent programs) to imple-
ment specialized control sequences (see Figure 10-57). For example,
ISaGRAF, a software system manufacturer who produces an IEC 1131–
compatible program that runs in a “soft PLC” environment, provides the user
Figure 10-57. Execution of a subprogram within a main program—main program is
suspended until subprogram ends.
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with the ability to have a main program with one or more subprograms
organized in a “father-child” relationship (see Figure 10-58). A father
program can “call” (i.e., jump to) any of the child programs in a process, but
a child program can only have one father program.
Main
Program
(Father)
Subprogram 1
(Child)
Action that calls
subprogram 1
Action that calls
subprogram 2
Action that calls
subprogram 1 and
subprogram 2
Entry
Return
Subprogram 2
(Child)
Entry
Return
Father (main program) can call any child (subprogram)
Figure 10-58. Subprograms organized in a “father-child” relationship.
Subprograms are similar in operation to macrosteps, except that macrosteps
can actually be considered an SFC type of subroutine. They are also similar
to custom function blocks in the sense that they can be used over and over
where needed to implement a control function. Subprograms can be written
in any of the IEC 1131-3 languages and can be called directly from an SFC
action using any of the four languages. In contrast, a macrostep routine can
only be called from the macrostep action that contains it. Custom-built
function blocks, on the other hand, can be called from any main program’s
action once they are in the SFC program library. These function blocks,
however, cannot pass completed information to the main program like a
subprogram can.
Subprograms differ from custom blocks and macrosteps because they can
pass and receive variables and values in a manner similar to a computer
program. For example, the statement (in ST):
Actual_Weight:=SP_Weighing (Max_Wt, Tare_Wt)
states that the variable Actual_Weight will be equal to the value computed by
the subprogram SP_Weighing (SP denotes subprogram), which receives the
data values of the variables Max_Wt and Tare_Wt from the main program.
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When a program calls a subprogram, it asks for a value to be delivered when
execution returns from the subprogram (see Figure 10-59). This value may be
Boolean, real, or integer. In the previous Actual_Weight example, the main
program obtains or computes the variable values Max_Wt and Tare_Wt, which
are passed to the subprogram. The subprogram SP_Weighing uses these two
values to compute a value that is passed to the variable Actual_Weight. This
variable value can then be used in the main program. Because subprograms run
miniprograms within the larger control program, they can dramatically affect the
scan cycle time.
Actual_Weight:=SP_Weighing (Max_Wt and Tare_Wt)
Program computes Max_Wt and Tare_Wt
Subprogram is called
Variable Actual_Weight
is returned to the main
program.
Value of Actual_Weight
is computed and becomes
the variable Actual_Weight.
Main
Program
4
2
1
3
Subprogram
SP_Weighing
Instructions
Return
Figure 10-59. Interpretation of a subprogram call from a main program.
The syntax for calling subprograms may differ slightly from one software
system to another. Nevertheless, all subprograms execute a small routine and
then return a desired computed value to the main program. Figure 10-60
illustrates how an SFC program calls a subprogram from an instruction in
one of its actions. In this example, step 11’s action (Action_11) has several
10
11
12
(Action_11)
ST
Program
Instructions
End of ST
Program Instructions
Init_Value:=SP_Check_Start
SP_Check_Start
Return
Subprogram
Figure 10-60. A subprogram called by an action’s instruction.
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ST instructions with the instruction Init_Value:=SP_Check_Start initiating
a subprogram named SP_Check_Start. This subprogram calculates the value
of the variable Init_Value and sends this data back to the main program, so that
the main program can use the variable value in the control process.
Figure 10-61 illustrates several subprogram example calls using other lan-
guages. The SUBPROG_1 subprogram will be called and executed once it is
found directly in the program (IL and ST) or once the conditions are satisfied
(LD and FBD). Remember that the subprogram can be written in any of the
languages, regardless of the calling language. The PLC’s manufacturer can
provide IEC 1131-3 software system specifications for properly passing and
receiving subprogram parameters.
Figure 10-61. Subprogram calls in IL, ST, LD, and FBD languages. SUBPROG_1 is
defined as a subprogram during the program structure definition.
An SFC transition can also call a subprogram, as shown in Figure 10-62. In
this example, transitions 1 and 2 call for the subprograms ErrEval and
EvalCond, which are mutually exclusive. These subprograms determine
whether the process should be executed or whether an alarm condition should
be set. The two subprogram calls follow the syntax:
Subprogram_Name();
This syntax specifies the subprogram name and the return condition (), which
is a Boolean result that triggers the transition. The value returned by the
subprogram yields the following conditions:
Return value = 0 → FALSE condition
Return value <> 0 → TRUE condition
10
11
12
(Action_11)
ST Language
New_Value:=SUBPROG_1
IL Language
SUBPROG_1
LD Language
SUBPROG_1
BA
FBD Language
SUBPROG_1
A
B
&
OR
Sample
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An action can also call a subprogram directly using an instruction syntax
that is similar to a transition. For example, the following instruction:
Action:
Result_Variable:=Sub_Program();
End_Action;
may be used to call a subprogram that will give a Boolean TRUE/FALSE
value to the Result_Variable, which can be used to trigger a transition. Figure
10-63 illustrates a sample subprogram call from an SFC action. In this
example, when the action in step 1 is activated, it initiates a subprogram that
determines the value of the variable Init. The value of this variable (expressed
in Boolean) is then passed back to the main program, where it is used to either
trigger the start of a macrostep process program or sound an alarm. The
variable labels Error and OK must have been declared as Boolean variables
during programming (e.g., Error:=False, OK:=True) for the proper transition
to occur. The (P) in the action name in step 1 indicates a pulse-type action
(momentary), which we will discuss in the next section.
1
20
1
20
ErrEval( );
Acknowledge
(Alarm)
20
2
30
EvalCond( );
Process_End
(Process)
(Initialize)
Subprograms
Figure 10-62. Subprogram call from a transition.
1
20
Init = Error
Acknowledge
(Alarm)
30
Init = OK
True
(Process)
Action (P):
Init:=SP_Initialize( );
End_Action;
Figure 10-63. Subprogram call from an SFC action.