7 C RLL S
RLL STAGE
PROGRAMMING
PLUS
CHAPTER
7
In This Chapter...
Introduction to Stage Programming . . . . . . . . . . . . . . . . . . . . . . . .7–2
Learning to Draw State Transition Diagrams . . . . . . . . . . . . . . . . . .7–3
Using the Stage Jump Instruction for State Transitions . . . . . . . . . . .7–7
Stage Program Example: Toggle On/Off Lamp Controller . . . . . . . .7–8
Four Steps to Writing a Stage Program . . . . . . . . . . . . . . . . . . . . . .7–9
Stage Program Example: A Garage Door Opener . . . . . . . . . . . . . .7–10
Stage Program Design Considerations . . . . . . . . . . . . . . . . . . . . . .7–15
Parallel Processing Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7–19
RLLPLUS (Stage) Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7–21
Questions and Answers about Stage Programming . . . . . . . . . . . .7–27
Chapter 7: RLLPLUS Stage Programming
1 Introduction to Stage Programming
Stage Programming provides a way to organize and program complex applications with
relative ease, when compared to purely relay ladder logic (RLL) solutions. Stage programming
2
does not replace or negate the use of traditional boolean ladder programming. This is why
You won’t have to discard any training or
Stage Programming is also called RLL
3
experience you already have. Stage programming simply allows you to divide and organize an
RLL program into groups of ladder instructions called stages. This allows quicker and more
intuitive
ladder program development than traditional RLL alone provides.
4
Overcoming “Stage Fright”
Many PLC programmers in the industry have
5
become comfortable using RLL for every PLC
program they write, but often remain skeptical or
6
even fearful of learning new techniques such as
stage programming. While RLL is great at solving
boolean logic relationships, it has disadvantages as
7
well:
• Large programs can become almost unmanageable,
8
because of a lack of structure.
• When a process gets stuck, it is difficult to find the
9
rung where the error occurred.
• Programs become difficult to modify later, because
they do not intuitively resemble the application
10
problem they are solving.
It’s easy to see that these inefficiencies consume a
11
lot of additional time, and time is money. Stage
programming overcomes these obstacles! We believe a few moments of studying the stage concept
12
is one of the greatest investments in programming speed and efficiency a PLC programmer
can make!
13
So, we encourage you to study stage programming and add it to your toolbox of
programming techniques. This chapter is designed as a self-paced tutorial on stage
programming. For best results:
14
• Start at the beginning and do not skip over any sections.
• Study each stage programming concept by working through each example. The examples build
A
progressively on each other.
• Read the Stage Questions and Answers at the end of the chapter for a quick review.
B
C
D
PLUS
X0
X4
C0
RST
C1
Y0
SET
STAGE!
X3
Y2
OUT
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DL06 Micro PLC User Manual, 3rd Edition, Rev. C
Chapter 7: RLLPLUS Stage Programming
Learning to Draw State Transition Diagrams
Introduction to Process States
Ladder
Outputs
Those familiar with ladder program execution know that Inputs
Program
the CPU must scan the ladder program repeatedly, over
and over. Its three basic steps are:
1. Read the inputs
2. Execute the ladder program
1) Read
Execute
Write
3. Write the outputs
Execute
2) Read
Write
The benefit is that a change at the inputs can affect the
3) Read
(Etc.....)
outputs in just a few milliseconds.
Most manufacturing processes consist of a series of activities or conditions, each lasting for
several seconds, minutes, or even hours. We might call these process states, which are either
active or inactive at any particular time. A challenge for RLL programs is that a particular
input event may last for just a brief instant. We typically create latching relays in RLL to
preserve the input event in order to maintain a process state for the required duration.
We can organize and divide ladder logic into sections called stages, representing process states.
But before we describe stages in detail, we will reveal the secret to understanding stage
programming: state transition diagrams.
The Need for State Diagrams
Sometimes we need to forget about the scan nature of PLCs, and focus our thinking toward
the states of the process we need to identify. Clear thinking and concise analysis of an
application gives us the best chance at writing efficient, bug-free programs. State diagrams are
just a tool to help us draw a picture of our process! You’ll discover that if we can get the picture
right, our program will also be right!
Inputs
Outputs
A 2–State Process
ON
X0
Ladder
Motor
Y0
Consider the simple process shown to the right, which controls OFF
Program
X1
an industrial motor. We will use a green momentary SPST
pushbutton to turn the motor on, and a red one to turn it off.
Transition condition
The machine operator will press the appropriate pushbutton for State
X0
just a second or so. The two states of our process are ON and
OFF
ON
OFF.
X1
The next step is to draw a state transition diagram, as shown to
Output equation: Y0 = On
the right. It shows the two states OFF and ON, with two
transition lines in-between. When the event X0 is true, we transition from OFF to ON.
When X1 is true, we transition from ON to OFF.
If you’re following along, you are very close to grasping the concept and the problem-solving
power of state transition diagrams. The output of our controller is Y0, which is true any time
we are in the ON state. In a boolean sense, Y0=ON state.
Next, we will implement the state diagram first as RLL, then as a stage program. This will
help you see the relationship between the two methods in problem solving.
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The state transition diagram to the right is a picture
of the solution we need to create. The beauty of it is
this: it expresses the problem independently of the
programming language we may use to realize it. In
other words, by drawing the diagram we have already
solved the control problem!
First, we’ll translate the state diagram to traditional
RLL. Then, we’ll show how easy it is to translate the
diagram into a stage programming solution.
RLL Equivalent
The RLL solution is shown to the right. Output
control relay, Y0, has a dual purpose. It turns the
motor on and off and acts as a latching relay. When
the On pushbutton (X0) is pressed, output coil Y0
turns on and the Y0 contact on the second row
latches itself on. So, X0 turns on the motor output
Y0 which now has power flow and sets the latch Y0.
It will remain on after the X0 contact opens.
When the Off pushbutton (X1) is pressed, it opens
the normally-closed X1 contact, which turns off
moter output Y0 and also resets the latch.
X0
OFF
ON
X1
Output equation Y0 = ON
Set
X0
Reset
X1
Output
Y0
OUT
Latch
Y0
Stage Equivalent
SG
The stage program solution is shown to the right.
OFF State
S0
The two inline stage boxes S0 and S1 correspond to
the two states OFF and ON. The ladder rung(s)
Transition
below each stage box belong to each respective stage.
S1
X0
This means that the PLC only has to scan those
JMP
rungs when the corresponding stage is active!
For now, let’s assume we begin in the OFF State, so
SG
ON State
S1
stage S0 is active. When the On pushbutton (X0) is
Output
pressed, a stage transition occurs. The JMP S1
SP1 Always On
Y0
instruction executes, which simply turns off the Stage
OUT
bit S0 and turns on Stage bit S1. So on the next PLC
scan, the CPU will not execute Stage S0, but will
Transition
execute stage S1!
S0
X1
In the On State (Stage S1), we want the motor to
JMP
always be on. The special relay contact SP1 is defined
as always on, so Y0 turns the motor on.
When the Off pushbutton (X1) is pressed, a transition back to the Off State occurs. The JMP
S0 instruction executes, which simply turns off the Stage bit S1 and turns on Stage bit S0. On
the next PLC scan, the CPU will not execute Stage S1, so the motor output Y0 will turn off.
The Off state (Stage 0) will be ready for the next cycle.
DL06 Micro PLC User Manual, 3rd Edition, Rev. C
Chapter 7: RLLPLUS Stage Programming
Let’s Compare
Right now, you may be thinking, “I don’t see the big advantage to Stage Programming ... in
fact, the stage program is longer than the plain RLL program.” Well, now is the time to
exercise a bit of faith. As control problems grow in complexity, stage programming quickly
out-performs RLL in simplicity, program size, etc.
For example, consider the diagram below. Notice how easy
SG
it is to correlate the OFF and ON states of the state
OFF State
S0
transition diagram below to the stage program at the right.
S1
X0
Now, we challenge anyone to
JMP
easily identify the same states
SG
in the RLL program on the
ON State
S1
previous page!
SP1
Initial Stages
Y0
OUT
X0
At powerup and Program-toOFF
ON
Run Mode transitions, the
X1
PLC always begins with all
normal stages (SG) off. So, the stage programs shown so far
have actually had no way to get started (because rungs are
not scanned unless their stage is active).
Assume that we want to always begin in the Off state
(motor off ), which is how the RLL program works. The
Initial Stage (ISG) is defined to be active at powerup. In
the modified program to the right, we have changed stage
S0 to the ISG type. This ensures the PLC will scan
contact X0 after powerup, because Stage S0 is active.
After powerup, an Initial Stage (ISG) works just like any
other stage!
We can change both programs so that the motor is ON at
powerup. In the RLL below, we must add a first scan relay
SP0, latching Y0 on. In the stage example to the right, we
simply make Stage S1 an initial stage (ISG) instead of S0.
S0
X1
JMP
Powerup in OFF State
ISG
S0
Initial Stage
S1
X0
JMP
SG
S1
SP1
Y0
OUT
S0
X1
JMP
Powerup in ON State
SG
S0
S1
X0
JMP
Powerup in ON State
X0
Y0
SP0
X1
Y0
OUT
ISG
S1
Initial Stage
SP1
Y0
OUT
First Scan
S0
X1
JMP
NOTE: If the ISG is within the retentive range for stages, the ISG will remain in the state it was in
before power down and will NOT turn itself on during the first scan.
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We can mark our desired powerup state as shown to
the right, which helps us remember to use the
appropriate Initial Stages when creating a stage
program. It is permissible to have as many initial
stages as the process requires.
Powerup
X0
OFF
ON
X1
What Stage Bits Do
You may recall that a stage is just a section of ladder program which is either active or inactive
at a given moment. All stage bits (S0 to 1777) reside in the PLC’s image register as individual
status bits. Each stage bit is either a boolean 0 or 1 at any time.
Program execution always reads ladder rungs from top to bottom, and from left to right. The
drawing below shows the effect of stage bit status. The ladder rungs below the stage
instruction continuing until the next stage instruction or the end of program belong to stage
0. Its equivalent operation is shown on the right. When S0 is true, the two rungs have power
flow.
• If Stage bit S0 = 0, its ladder rungs are not scanned (executed).
• If Stage bit S0 = 1, its ladder rungs are scanned (executed).
Actual Program Appearance
Functionally Equivalent Ladder
SG
S0
S0
(includes all rungs in stage)
Stage Instruction Characteristics
The inline stage boxes on the left power rail divide the ladder
program rungs into stages. Some stage rules are:
• Execution – Only logic in active stages are executed on any
scan.
• Transitions – Stage transition instructions take effect on the
next occurrence of the stages involved.
• Octal numbering – Stages are numbered in octal, like I/O
points, etc. So “S8” is not valid.
• Total Stages – The DL06 offers up to 1024 stages
(S0 to 1777 in octal).
SG
S0
SG
S1
SG
S2
• No duplicates –Each stage number is unique and can be used
just once.
• Any order – You can skip numbers and sequence the stage
numbers in any order.
• Last Stage – The last stage in the ladder program includes all
rungs from its stage box until the end coil.
DL06 Micro PLC User Manual, 3rd Edition, Rev. C
END
Chapter 7: RLLPLUS Stage Programming
Using the Stage Jump Instruction for State Transitions
Stage Jump, Set, and Reset Instructions
The Stage JMP instruction we have used deactivates the stage in which the instruction occurs,
while activating the stage in the JMP instruction. Refer to the state transition shown below.
When contact X0 energizes, the state transition from S0 to S1 occurs. The two stage examples
shown below are equivalent. So, the Stage Jump instruction is equal to a Stage Reset of the
current stage, plus a Stage Set instruction for the stage to which we want to transition.
X0
S0
SG
S0
X0
S1
S1
SG
S0
Equivalent
S0
X0
JMP
RST
S1
SET
Please Read Carefully – The jump instruction is easily misunderstood. The “jump” does not
occur immediately like a GOTO or GOSUB program control instruction when executed.
Here’s how it works:
• The jump instruction resets the stage bit of the stage in which it occurs. All rungs in the stage still
finish executing during the current scan, even if there are other rungs in the stage below the jump
instruction!
• The reset will be in effect on the following scan, so the stage that executed the jump instruction
previously will be inactive and bypassed.
• The stage bit of the stage named in the Jump instruction will be set immediately, so the stage will be
executed on its next occurrence. In the left program shown below, stage S1 executes during the same
scan as the JMP S1 occurs in S0. In the example on the right, Stage S1 executes on the next scan
after the JMP S1 executes, because stage S1 is located above stage S0.
SG
S0
Executes on next
scan after Jmp
SG
S1
X0
S1
S1
JMP
Executes on same
scan as Jmp
SG
S1
S1
Y0
Y0
OUT
SG
S0
X0
OUT
S1
JMP
Note: Assume we start with Stage 0 active and stage 1 inactive for both examples.
NOTE: Assume we start with Stage 0 active and Stage 1 inactive for both examples.
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Chapter 7: RLLPLUS Stage Programming
1 Stage Program Example: Toggle On/Off Lamp Controller
A 4–State Process
2
In the process shown to the right, we use an ordinary
momentary pushbutton to control a light bulb. The
3
ladder program will latch the switch input, so that we
will push and release to turn on the light, push and
release again to turn it off (sometimes called toggle
4
function). Sure, we could just buy a mechanical switch
with the alternate on/off action built in... However, this
5
example is educational and also fun! Next we draw the
state transition diagram.
6
A typical first approach is to use X0 for both transitions
(like the example shown to the right). However, this is incorrect (please keep reading).
Note that this example differs from the motor example, because now we have just one
7
pushbutton. When we press the pushbutton, both transition conditions are met. We would
just transition around the state diagram at top speed. If implemented in Stage, this solution
8
would flash the light on or off each scan (obviously undesirable)!
The solution is to make the push and the release of the pushbutton separate events. Refer to
9
the new state transition diagram below. At powerup we enter the OFF state. When switch X0
is pressed, we enter the Press-ON state. When it is released, we enter the ON state. Note that
X0
with the bar above it denotes X0 NOT.
10
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When in the ON state, another push and release cycle
similarly takes us back to the OFF state. Now we have
two unique states (OFF and ON) used when the
A
pushbutton is released, which is what was required to
solve the control problem.
B
The equivalent stage program is shown to the right. The
desired powerup state is OFF, so we make S0 an initial
C
stage (ISG). In the ON state, we add special relay
contact SP1, which is always on.
D
Note that even as our programs grow more complex, it
Inputs
Toggle
Outputs
X0
Powerup
OFF
Ladder Y0
Program
X0
ON
X0
Output equation: Y0 = ON
Powerup
X0
Push–ON
ISG
S0
X0
OFF State
S1
X0
OFF
ON
X0
Push–OFF
X0
JMP
SG
S1
Push–On State
S2
X0
JMP
SG
S2
ON State
Output
SP1
Y0
OUT
S3
X0
JMP
SG
S3
is still easy to correlate the state transition diagram with
the stage program!
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DL06 Micro PLC User Manual, 3rd Edition, Rev. C
Push–Off State
X0
S0
JMP
Chapter 7: RLLPLUS Stage Programming
Four Steps to Writing a Stage Program
By now, you’ve probably noticed that we follow the same steps to solve each example
problem. The steps will probably come to you automatically if you work through all the
examples in this chapter. It’s helpful to have a checklist to guide us through the problem
solving. The following steps summarize the stage program design procedure:
1. Write a Word Description of the application.
Describe all functions of the process in your own words. Start by listing what happens first,
then next, etc. If you find there are too many things happening at once, try dividing the
problem into more than one process. Remember, you can still have the processes
communicate with each other to coordinate their overall activity.
2. Draw the Block Diagram.
Inputs represent all the information the process needs for decisions, and outputs connect to
all devices controlled by the process.
• Make lists of inputs and outputs for the process.
• Assign I/O point numbers (X and Y) to physical inputs and outputs.
3. Draw the State Transition Diagram.
The state transition diagram describes the central function of the block diagram, reading
inputs and generating outputs.
• Identify and name the states of the process.
• Identify the event(s) required for each transition between states.
• Ensure the process has a way to re-start itself, or is cyclical.
• Choose the powerup state for your process.
• Write the output equations.
4. Write the Stage Program.
Translate the state transition diagram into a stage program.
• Make each state a stage. Remember to number stages in octal. Up to 1024 total stages are available
in the DL06, numbered 0 to 1777 in octal.
• Put transition logic inside the stage which originates each transition (the stage each arrow points
away from).
• Use an initial stage (ISG) for any states that must be active at powerup.
• Place the outputs or actions in the appropriate stages.
You’ll notice that Steps 1 through 3 just prepare us to write the stage program in Step 4.
However, the program virtually writes itself because of the preparation beforehand. Soon
you’ll be able to start with a word description of an application and create a stage program in
one easy session!
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Chapter 7: RLLPLUS Stage Programming
1 Stage Program Example: A Garage Door Opener
Garage Door Opener Example
2
In this next stage programming example, we’ll create
a garage door opener controller. Hopefully, most
3
readers are familiar with this application, and we can
have fun, too!
The first step we must take is to describe how the
4
door opener works. We will start by achieving the
basic operation, waiting to add extra features later.
5
Stage programs are very easy to modify.
Our garage door controller has a motor which raises
6
or lowers the door on command. The garage owner
pushes and releases a momentary pushbutton once to
raise the door. After the door is up, another push7
release cycle will lower the door.
In
order to identify the inputs and outputs of the
8
Up limit switch
system, it’s sometimes helpful to sketch its main
components, as shown in the door side view to the
9
right. The door has an up limit and a down limit
Raise
Motor
Lower
switch. Each limit switch closes only when the door
has
reach
the
end
of
travel
in
the
corresponding
10
direction. In the middle of travel, neither limit switch
is closed.
11
The motor has two command inputs: raise and lower.
Door
When neither input is active, the motor is stopped.
Command
12
The door command is just a simple pushbutton.
Whether wall-mounted as shown, or a radio-remote
control, all door control commands logical OR
13
Down limit switch
together as one pair of switch contacts.
Draw the Block Diagram
14
The block diagram of the controller is shown to the
right. Input X0 is from the pushbutton door control.
Inputs
Outputs
A
Input X1 energizes when the door reaches the full up
position. Input X2 energizes when the door reaches
B
the full down position. When the door is positioned
Ladder
between fully up or down, both limit switches are
Program
open.
C
The controller has two outputs to drive the motor.
Y1 is the up (raise the door) command, and Y2 is the
D
down (lower the door) command.
Toggle
Up limit
Down limit
7–10
DL06 Micro PLC User Manual, 3rd Edition, Rev. C
X0
To motor:
X1
Y1
X2
Y2
Raise
Lower
Chapter 7: RLLPLUS Stage Programming
Draw the State Diagram
Now we are ready to draw the state transition diagram. Like the previous light bulb controller
example, this application also has just one switch for the command input. Refer to the figure
below.
• When the door is down (DOWN state), nothing happens until X0 energizes. Its push and release
brings us to the RAISE state, where output Y1 turns on and causes the motor to raise the door.
• We transition to the UP state when the up limit switch (X1) energizes, and turns off the motor.
• Then nothing happens until another X0 press-release cycle occurs. That takes us to the LOWER
state, turning on output Y2 to command the motor to lower the door. We transition back to the
DOWN state when the down limit switch (X2) energizes.
Powerup
X0
Push–UP
X0
RAISE
X1
ISG
S0
DOWN
X2
LOWER
Push–DOWN
X0
S1
JMP
SG
S1
X0
Output equations: Y1 = Raise
DOWN State
X0
UP
Push–UP State
S2
X0
JMP
Y2 = Lower
The equivalent stage program is shown to the right. For now, we
will assume the door is down at powerup, so the desired powerup
state is DOWN. We make S0 an initial stage (ISG). Stage S0
remains active until the door control pushbutton activates. Then
we transition (JMP) to Push-UP stage, S1.
A push-release cycle of the pushbutton takes us through stage S1
to the RAISE stage, S2. We use the always-on contact SP1 to
energize the motor’s raise command, Y1. When the door reaches
the fully-raised position, the up limit switch X1 activates. This
takes us to the UP Stage S3, where we wait until another door
control command occurs.
In the UP Stage S3, a push-release cycle of the pushbutton will
take us to the LOWER Stage S5, where we activate Y2 to
command the motor to lower the door. This continues until the
door reaches the down limit switch, X2. When X2 closes, we
transition from Stage S5 to the DOWN stage S0, where we began.
SG
S2
RAISE State
SP1
Y1
OUT
S3
X1
JMP
SG
S3
UP State
S4
X0
JMP
SG
S4
Push–DOWN State
S5
X0
JMP
SG
S5
LOWER State
SP1
X2
NOTE: The only special thing about an initial stage (ISG) is that it is
automatically active at powerup. Afterwards, it is just like any other.
DL06 Micro PLC User Manual, 3rd Edition, Rev. C
Y2
OUT
S0
JMP
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Add Safety Light Feature
Next we will add a safety light feature to the door
opener system. It’s best to get the main function
working first as we have done, then adding the
secondary features.
The safety light is standard on many commerciallyavailable garage door openers. It is shown to the
right, mounted on the motor housing. The light
turns on upon any door activity, remaining on for
approximately 3 minutes afterwards.
This part of the exercise will demonstrate the use of
parallel states in our state diagram. Instead of using
the JMP instruction, we’ll use the set and reset
commands.
Modify the Block Diagram and State Diagram
Inputs
Outputs
To control the light bulb, we add an output to our
Toggle
controller block diagram, shown to the right, Y3 is the
X0
Y1
Raise
light control output.
Up limit
In the diagram below, we add an additional state
Y2
X1
Lower
called “LIGHT”. Whenever the garage owner presses
the door control switch and releases, the RAISE or
LOWER state is active and the LIGHT state is
Down limit
X2
Y3
simultaneously active. The line to the Light state is
Light
dashed, because it is not the primary path.
We can think of the Light state as a parallel process to the raise and lower state. The paths to
the Light state are not a transition (Stage JMP), but a State Set command. In the logic of the
Light stage, we will place a three-minute timer. When it expires, timer bit T0 turns on and
resets the Light stage. The path out of the Light stage goes nowhere, indicating the Light stage
just becomes inactive, and the light goes out!
Output equations: Y1 = RAISE
Y2 = LOWER
Y3 = LIGHT
RAISE
X1
X0
X0
Push–UP
X0
DOWN
LIGHT
UP
T0
X0
X2
LOWER
Push–DOWN
X0
7–12
Safety light
DL06 Micro PLC User Manual, 3rd Edition, Rev. C
X0
Chapter 7: RLLPLUS Stage Programming
Using a Timer Inside a Stage
The finished modified program is shown to the right. The
shaded areas indicate the program additions.
In the Push-UP stage S1, we add the Set Stage Bit S6
instruction. When contact X0 closes, we transition from S1
and go to two new active states: S2 and S6. In the PushDOWN state S4, we make the same additions. So, any time
someone presses the door control pushbutton, the light turns
on.
Most new stage programmers would be concerned about
where to place the Light Stage in the ladder, and how to
number it. The good news is that it doesn’t matter!
• Just choose an unused Stage number, and use it for the new
stage and as the reference from other stages.
ISG
S0
DOWN State
JMP
SG
S1
Push–UP State
S2
X0
JMP
S6
SET
SG
S2
RAISE State
• Placement in the program is not critical, so we place it at the
end.
SP1
You might think that each stage has to be directly under the
stage that transitions to it. While it is good practice, it is not
required (that’s good, because our two locations for the Set
S6 instruction make that impossible). Stage numbers and
how they are used determines the transition paths.
In stage S6, we turn on the safety light by energizing Y3.
Special relay contact SP1 is always on. Timer T0 times at 0.1
second per count. To achieve 3 minutes time period, we
calculate:
X1
K = 3 min. x 60 sec/min
0.1 sec/count
K = 1800 counts
S1
X0
Y1
OUT
S3
JMP
SG
S3
UP State
S4
X0
JMP
SG
S4
Push–DOWN State
S5
X0
JMP
S6
SET
SG
S5
LOWER State
SP1
The timer has power flow whenever stage S6 is active. The
corresponding timer bit T0 is set when the timer expires. So
three minutes later, T0=1 and the instruction Reset S6 causes
the stage to be inactive.
While Stage S6 is active and the light is on, stage transitions
in the primary path continue normally and independently of
Stage 6. That is, the door can go up, down, or whatever, but
the light will be on for precisely 3 minutes.
Y2
OUT
S0
X2
JMP
SG
S6
LIGHT State
SP1
Y3
OUT
TMR T0
K1800
T0
S6
RST
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Chapter 7: RLLPLUS Stage Programming
Add Emergency Stop Feature
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Some garage door openers today will detect an object under
the door. This halts further lowering of the door. Usually
implemented with a photocell (electric-eye), a door in the
process of being lowered will halt and begin raising. We will
define our safety feature to work in this way, adding the
input from the photocell to the block diagram as shown to
the right. X3 will be on if an object is in the path of the
door.
Next, we make a simple addition to the state transition
diagram, shown in shaded areas in the figure below. Note
the new transition path at the top of the LOWER state. If
we are lowering the door and detect an obstruction (X3),
we then jump to the Push-UP State. We do this instead of
jumping directly to the RAISE state, to give the Lower
output Y2 one scan to turn off, before the Raise output Y1
energizes.
X0
X0
RAISE
Push–UP
Inputs
Toggle
Up limit
Outputs
X0
Y1
X1
Y2
Down limit
X2
Ladder
Program
Y3
Raise
Lower
Light
Obstruction
X3
X1
X0
DOWN
X2
X3
X3
LIGHT
UP
T0
X0
LOWER
Push–DOWN
X0
X0
Exclusive Transitions
7–14
It is theoretically possible that the down limit (X2) and the obstruction input (X3) could
energize at the same moment. In that case, we would jump to the Push-UP and DOWN
states simultaneously, which does not make sense.
Instead, we give priority to the obstruction by
changing the transition condition to the DOWN
SG
LOWER State
S5
state to [X2 AND NOT X3]. This ensures the
obstruction event has the priority. The
SP1
Y2
modifications we must make to the LOWER Stage
OUT
(S5) logic are shown to the right. The first rung
to DOWN
S0
X2
X3
remains unchanged. The second and third rungs
JMP
implement the transitions we need. Note the
opposite relay contact usage for X3, which ensures
S2
to Push-UP
X3
the stage will execute only one of the JMP
JMP
instructions.
DL06 Micro PLC User Manual, 3rd Edition, Rev. C
Chapter 7: RLLPLUS Stage Programming
Stage Program Design Considerations
Stage Program Organization
The examples so far in this chapter used one self-contained state diagram to represent the
main process. However, we can have multiple processes implemented in stages, all in the same
ladder program. New stage programmers sometimes try to turn a stage on and off each scan,
based on the false assumption that only one stage can be on at a time. For ladder rungs that
you want to execute each scan, just put them in a stage that is always on.
The following figure shows a typical application. During operation, the primary
manufacturing activity Main Process, Powerup Initialization, E-Stop and Alarm Monitoring,
and Operator Interface are all running. At powerup, three initial stages shown begin
operation.
XXX
= ISG
Main Process
Powerup Initialization
Powerup
Idle
Agitate
Fill
E-Stop and Alarm Monitoring
Rinse
Spin
Operator Interface
Monitor
Recipe
Control
Status
In a typical application, the separate stage sequences above operate as follows:
• Powerup Initialization – This stage contains ladder rung tasks done just once at powerup. Its last
rung resets the stage, so this stage is only active for one scan (or only as many scans
that are required).
• Main Process –This stage sequence controls the heart of the process or machine. One pass through
the sequence represents one part cycle of the machine, or one batch in the process.
• E-Stop and Alarm Monitoring –This stage is always active because it is watching for errors that
could indicate an alarm condition or require an emergency stop. It is common for
this stage to reset stages in the main process or elsewhere, in order to initialize them
after an error condition.
• Operator Interface –This is another task that must always be active and ready to respond to an
operator. It allows an operator interface to change modes, etc., independently of the
current main process step.
Although we have separate processes, there can be
coordination among them. For example, in an error
condition, the Status Stage may want to
automatically switch the operator interface to the
status mode to show error information as shown to
the right. The monitor stage could set the stage bit
for Status and Reset the stages Control and Recipe.
Operator Interface
Recipe
Control
Monitor
Status
E-Stop and
Alarm Monitoring
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How Instructions Work Inside Stages
7–16
We can think of states or stages as simply dividing up our ladder program as depicted in the
figure below. Each stage contains only the ladder rungs which are needed for the
corresponding state of the process. The logic for transitioning out of a stage is contained
within that stage. It’s easy to choose which ladder rungs are active at powerup by using an
initial stage type (ISG).
Stage 0
Stage 1
Stage 2
Most all instructions work just like they do in standard RLL. You can think of a stage just like
a miniature RLL program which is either active or inactive.
Output Coils – As expected, output coils in active stages will turn on or off outputs
according to power flow into the coil. However, note the following:
• Outputs work as usual, provided each output reference, such as “Y3”, is used in only one stage.
• An output can be referenced from more than one stage, as long as only one of the stages is active at
a time.
• If an output coil is controlled by more than one stage simultaneously, the active stage nearest the
bottom of the program determines the final output status during each scan. Therefore, use the
OROUT instruction instead when you want multiple stages to have a logical OR control of an
output.
One-Shot or PD coils – Use care if you must use a Positive Differential coil in a stage.
Remember that the input to the coil must make a 0–1 transition. If the coil is already
energized on the first scan when the stage becomes active, the PD coil will not work. This is
because the 0–1 transition did not occur.
PD coil alternative: If there is a task which you want to do only once (on 1 scan), it can be
placed in a stage which transitions to the next stage on the same scan.
Counter – In using a counter inside a stage, the stage must be active for one scan before the
input to the counter makes a 0–1 transition. Otherwise, there is no real transition and the
counter will not count.
The ordinary Counter instruction does have a restriction inside stages: it may not be reset
from other stages using the RST instruction for the counter bit. However, the special Stage
counter provides a solution (see next paragraph).
Stage Counter – The Stage Counter has the benefit that its count may be globally reset from
other stages by using the RST instruction. It has a count input, but no reset input. This is the
only difference from a standard counter.
Drum – Realize that the drum sequencer is its own process, and is a different programming
method than stage programming. If you need to use a drum with stages, be sure to place the
drum instruction in an ISG stage that is always active.
DL06 Micro PLC User Manual, 3rd Edition, Rev. C
Chapter 7: RLLPLUS Stage Programming
Using a Stage as a Supervisory Process
You may recall the light bulb on-off controller
example from earlier in this chapter. For the purpose
of illustration, suppose we want to monitor the
productivity of the lamp process, by counting the
number of on-off cycles which occurs. This
application will require the addition of a simple
counter, but the key decision is in where to put the
counter.
Toggle
X0
Ladder Y0
Program
Powerup
Powerup
ISG
S0
Supervisor Process
Supervisor
OFF State
S1
X0
X0
Push–ON
X0
Main Process
OFF
JMP
SG
S1
ON
Push–On State
S2
X0
JMP
X0
Push–OFF
X0
New stage programming students will typically try to place
the counter inside one of the stages of the process they are
trying to monitor. The problem with this approach is that
the stage is active only part of the time. In order for the
counter to count, the count input must transition from off
to on at least one scan after its stage activates. Ensuring this
requires extra logic that can be tricky.
In this case, we only need to add another supervisory stage
as shown above, to watch the main process. The counter
inside the supervisor stage uses the stage bit S1 of the main
process as its count input. Stage bits used as a contact let us
monitor a process!
NOTE: Both the Supervisor stage and the OFF stage are initial
stages. The supervisor stage remains active indefinitely.
SG
S2
ON State
SP1
Y0
OUT
S3
X0
JMP
SG
S3
Push–Off State
S0
X0
JMP
ISG
S4
Supervisor State
S1
SGCNT
CT0
K5000
Stage Counter
The counter in the above example is a special Stage Counter. Note that it does not have a
reset input. The count is reset by executing a Reset instruction, naming the counter bit (CT0
in this case). The Stage Counter has the benefit that its count may be globally reset from
other stages. The standard Counter instruction does not have this global reset capability. You
may still use a regular Counter instruction inside a stage... however, the reset input to the
counter is the only way to reset it.
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Power Flow Transition Technique
Our discussion of state transitions has shown how the Stage JMP instruction makes the
current stage inactive and the next stage (named in the JMP) active. As an alternative way to
enter this in DirectSOFT, you may use the power flow method for stage transitions.
The main requirement is that the current stage be located directly above the next (jump-to)
stage in the ladder program. This arrangement is shown in the diagram below, by stages S0
and S1, respectively.
S0
X0
S1
SG
S0
SG
S0
S1
X0
Equivalent
All other rungs in stage...
JMP
X0
SG
S1
Power flow
transition
SG
S1
Remember that the Stage JMP instruction may occur anywhere in the current stage, and the
result is the same. However, power flow transitions, as shown above, must occur as the last
rung in a stage. All other rungs in the stage will precede it. The power flow transition method
is also achievable on the handheld programmer, by simply following the transition condition
with the Stage instruction for the next stage.
The power flow transition method does eliminate one Stage JMP instruction, its only
advantage. However, it is not as easy to make program changes as using the Stage JMP.
Therefore, we advise using Stage JMP transitions for most programmers.
Stage View in DirectSOFT
7–18
The Stage View option in DirectSOFT will let you view the ladder program as a flow chart.
The figure below shows the symbol convention used in the diagrams. You may find the stage
view useful as a tool to verify that your stage program has faithfully reproduced the logic of
the state transition diagram you intend to realize.
SG
Stage
Transition
Logic
Reference to
a Stage
J
Jump
S
Set Stage
R
Reset Stage
The following diagram is a typical stage view of a ladder program containing stages. Note the
left-to-right direction of the flow chart.
ISG
SO
J
SG
S1
J
SG
S2
S
SG
S4
J
SG
S3
J
SG
S5
DL06 Micro PLC User Manual, 3rd Edition, Rev. C
Chapter 7: RLLPLUS Stage Programming
Parallel Processing Concepts
Parallel Processes
Previously in this chapter we discussed how a state may transition to either one state or
another, called an exclusive transition. In other cases, we may need to branch simultaneously to
two or more parallel processes, as shown below. It is acceptable to use all JMP instructions as
shown, or we could use one JMP and a Set Stage bit instruction(s) (at least one must be a
JMP, in order to leave S1). Remember that all instructions in a stage execute, even when it
transitions (the JMP is not a GOTO).
Process A
S0
S1
S2
SG
S1
S3
X0
Process B
Push–On State
S2
X0
JMP
S4
S4
S5
JMP
Note that if we want Stages S2 and S4 to energize exactly on the same scan, both stages must
be located below or above Stage S1 in the ladder program (see the explanation at the bottom
of page 7–7). Overall, parallel branching is easy!
Converging Processes
Now, we consider the opposite case of parallel branching, which is converging processes. This
simply means we stop doing multiple things and continue doing one thing at a time. In the
figure below, processes A and B converge when stages S2 and S4 transition to S5 at some
point in time. So, S2 and S4 are Convergence Stages.
Process A
S1
S2
= Convergence Stage
S5
Process B
S3
S6
S4
Convergence Stages (CV)
While the converging principle is simple enough, it brings a new complication. As parallel
processing completes, the multiple processes almost never finish at the same time. In other
words, how can we know whether Stage S2 or S4 will finish last? This is an important point,
because we have to decide how to transition to Stage S5.
CV
Convergence
The solution is to coordinate the transition condition out of
S2
Stages
convergence stages. We accomplish this with a stage type designed
for this purpose: the Convergence Stage (type CV). In the example CV
S4
to the right, convergence stages S2 and S4 are required to be
grouped together as shown. No logic is permitted between CV
X3
S5
stages! The transition condition (X3 in this case) must be located
CVJMP
in the last convergence stage. The transition condition only has
SG
power flow when all convergence stages in the group are active.
S5
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Convergence Jump (CVJMP)
Remember, the last convergence stage only has
power flow when all CV stages in the group are
active. To complement the convergence stage, we
need a new jump instruction. The Convergence
Jump (CVJMP) shown to the right will transition to
Stage S5 when X3 is active (as one might expect),
but it also automatically resets all convergence stages in
the group. This makes the CVJMP jump a very
powerful instruction. Note that this instruction may
only be used with convergence stages.
CV
S2
Convergence
Jump
CV
S4
X3
S5
CVJMP
SG
S5
Convergence Stage Guidelines
7–20
The following summarizes the requirements in the use of convergence stages, including some
tips for their effective application:
• A convergence stage is to be used as the last stage of a process which is running in parallel to another
process or processes. A transition to the convergence stage means that a particular process is finished
and represents a waiting point until all other parallel processes also finish.
• The maximum number of convergence stages which make up one group is 16. In other words, a
maximum of 16 stages can converge into one stage.
• Convergence stages of the same group must be placed together in the program, connected on the
power rail without any other logic in between.
• Within a convergence group, the stages may occur in any order, top to bottom. It does not matter
which stage is last in the group, because all convergence stages have to be active before the last stage
has power flow.
• The last convergence stage of a group may have ladder logic within the stage. However, this logic
will not execute until all convergence stages of the group are active.
• The convergence jump (CVJMP) is the intended method to be used to transition from the
convergence group of stages to the next stage. The CVJMP resets all convergence stages of the
group, and energizes the stage named in the jump.
• The CVJMP instruction must only be used in a convergence stage, as it is invalid in regular or
initial stages.
• Convergence Stages or CVJMP instructions may not be used in subroutines or interrupt routines.
DL06 Micro PLC User Manual, 3rd Edition, Rev. C
Chapter 7: RLLPLUS Stage Programming
RLLPLUS (Stage) Instructions
Stage (SG)
The Stage instructions are used to create structured RLLPLUS
programs. Stages are program segments which can be activated by
transitional logic, a jump or a set stage that is executed from an
active stage. Stages are deactivated one scan after transitional logic, a
jump, or a reset stage instruction is executed.
SG
Operand Data Type
DL06 Range
Stage S
0–1777
S aaa
aaa
The following example is a simple RLLPLUS program. This program utilizes an initial stage,
stage, and jump instructions to create a structured program.
Direct SOFT
ISG
Handheld Programmer Keystrokes
U
S0
$
X0
Y0
OUT
X1
S2
SET
X5
S1
JMP
SG
S1
Y1
OUT
SG
S2
X6
Y2
OUT
X7
S1
STR
A
A
GX
OUT
A
$
B
STR
0
0
0
1
X
SET
SHFT
$
F
STR
K
JMP
B
2
B
$
X2
ISG
SG
STR
C
GX
OUT
B
2
C
$
SG
STR
G
GX
OUT
C
$
H
STR
5
1
1
2
1
2
6
2
7
V
AND
SHFT
K
JMP
A
0
ENT
ENT
ENT
ENT
S
RST
C
2
ENT
ENT
ENT
ENT
ENT
ENT
ENT
ENT
ENT
ENT
S
RST
B
1
ENT
ENT
S0
JMP
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Initial Stage (ISG)
The Initial Stage instruction is normally used as the first
segment of an RLLPLUS program. Multiple Initial Stages are
allowed in a program. They will be active when the CPU
enters the Run mode allowing for a starting point in the
program. Initial Stages are also activated by transitional logic,
a jump or a set stage executed from an active stage.
Operand Data Type
ISG
S aaa
DL06 Range
aaa
Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S
0–1777
NOTE: If the ISG is within the retentive range for stages, the ISG will remain in the state it was in
before power down and will NOT turn itself on during the first scan.
Jump (JMP)
The Jump instruction allows the program to transition from
an active stage containing the jump instruction to another
stage (specified in the instruction). The jump occurs when
the input logic is true. The active stage containing the Jump
will deactivate 1 scan later.
Operand Data Type
S aaa
JMP
DL06 Range
aaa
Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S
0–1777
Not Jump (NJMP)
7–22
The Not Jump instruction allows the program to transition
from an active stage which contains the jump instruction to
another which is specified in the instruction. The jump will
occur when the input logic is off. The active stage that
contains the Not Jump will be deactivated 1 scan after the
Not Jump instruction is executed.
Operand Data Type
S aaa
NJMP
DL06 Range
aaa
Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S
0–1777
In the following example, only stage ISG0 will be active when program execution begins.
When X1 is on, program execution will jump from Initial Stage 0 to Stage 1.
DL06 Micro PLC User Manual, 3rd Edition, Rev. C
Chapter 7: RLLPLUS Stage Programming
DirectSOFT
Direct SOFT32
ISG
Handheld Programmer Keystrokes
S0
X1
U
S1
JMP
SG
S1
$
X7
B
2
B
SG
C
Y5
STR
OUT
GX
OUT
F
$
H
S2
JMP
X7
B
STR
K
JMP
$
X2
A
ISG
S3
NJMP
STR
K
JMP
SHFT
C
N
TMR
0
1
1
1
2
5
7
2
SHFT
ENT
ENT
ENT
ENT
ENT
ENT
ENT
ENT
K
JMP
D
3
ENT
Converge Stage (CV) and Converge Jump (CVJMP)
The Converge Stage instruction is used to group certain stages together by defining them as
Converge Stages.
When all of the Converge Stages within a group become active,
CV
the CVJMP instruction (and any additional logic in the final CV
S aaa
stage) will be executed. All preceding CV stages must be active
before the final CV stage logic can be executed. All Converge
Stages are deactivated one scan after the CVJMP instruction is executed.
Additional logic instructions are only allowed following the
last Converge Stage instruction and before the CVJMP
instruction. Multiple CVJMP instructions are allowed.
S aaa
Converge Stages must be programmed in the main body of
CVJMP
the application program. This means they cannot be
programmed in Subroutines or Interrupt Routines.
Operand Data Type
DL06 Range
aaa
Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S
0–1777
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7–24
In the following example, when Converge Stages S10 and S11 are both active the CVJMP
instruction will be executed when X4 is on. The CVJMP will deactivate S10 and S11, and
activate S20. Then, if X5 is on, the program execution will jump back to the initial stage, S0.
DirectSOFT
Direct SOFT32
Handheld Programmer Keystrokes
U
$
ISG
S0
X0
X1
Y0
OUT
S1
JMP
S10
JMP
SG
S1
X2
CV
S10
CV
S11
S11
JMP
STR
A
$
B
STR
K
JMP
B
K
JMP
B
2
B
$
SG
X4
SG
C
STR
B
K
JMP
SHFT
C
SHFT
C
$
1
1
1
1
2
1
ENT
ENT
ENT
A
0
ENT
ENT
ENT
B
1
ENT
2
V
AND
B
D
$
E
STR
2
SG
STR
K
JMP
S20
CVJMP
S20
X5
0
ENT
2
D
$
0
ENT
B
STR
C
0
V
AND
GX
OUT
2
Y3
OUT
A
GX
OUT
SHFT
X3
A
ISG
S0
JMP
DL06 Micro PLC User Manual, 3rd Edition, Rev. C
3
3
4
A
5
0
0
1
ENT
ENT
ENT
C
A
B
ENT
SHFT
2
1
A
ENT
V
AND
F
1
0
ENT
ENT
K
JMP
ENT
C
2
A
0
ENT
Chapter 7: RLLPLUS Stage Programming
Block Call (BCALL)
The stage block instructions are used to activate a block of stages. The Block Call, Block, and
Block End instructions must be used together. The BCALL instruction is used to activate a
stage block. There are several things you need to know about the BCALL instruction.
• Uses CR Numbers — The BCALL appears as an output
C aaa
coil, but does not actually refer to a Stage number as you
BCALL
might think. Instead, the block is identified with a Control
Relay (Caaa). This control relay cannot be used as an
output anywhere else in the program.
• Must Remain Active — The BCALL instruction actually controls all the stages between the
BLK and the BEND instructions even after the stages inside the block have started
executing. The BCALL must remain active or all the stages in the block will automatically
be turned off. If either the BCALL instruction, or the stage that contains the BCALL
instruction goes off, then the stages in the defined block will be turned off automatically.
• Activates First Block Stage — When the BCALL is executed it automatically activates the
first stage following the BLK instructions.
Operand Data Type
DL06 Range
aaa
Control Relay . . . . . . . . . . . . . . . . . . . . . . . . . . . S
0–1777
Block (BLK)
The Block instruction is a label which marks the beginning of a
block of stages that can be activated as a group. A Stage instruction
must immediately follow the Start Block instruction. Initial Stage
instructions are not allowed in a block. The control relay (Caaa)
specified in Block instruction must not be used as an output any
where else in the program.
Operand Data Type
BLK
C aaa
DL06 Range
aaa
Control Relay . . . . . . . . . . . . . . . . . . . . . . . . . . . S
0–1777
Block End (BEND)
The Block End instruction is a label used with the Block
instruction. It marks the end of a block of stages. There is no
operand with this instruction. Only one Block End is
allowed per Block Call.
BEND
DL06 Micro PLC User Manual, 3rd Edition, Rev. C
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Chapter 7: RLLPLUS Stage Programming
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Direct SOFT32
DirectSOFT
In this example, the Block Call is executed when
stage 1 is active and X6 is on. The Block Call then
automatically activates stage S10, which
immediately follows the Block instruction.
This allows the stages between S10 and the Block
End instruction to operate as programmed. If the
BCALL instruction is turned off, or if the stage
containing the BCALL instruction is turned off,
then all stages between the BLK and BEND
instructions are automatically turned off.
If you examine S15, you will notice that X7 could
reset Stage S1, which would disable the BCALL,
thus resetting all stages within the block.
SG
BLK
SG
S1
X2
Y5
OUT
X6
C0
BCALL
C0
S10
Y6
X3
OUT
BEND
Handheld Programmer Keystrokes
SG
S(SG)
1
ENT
STR
X(IN)
2
ENT
OUT
Y(OUT)
5
ENT
STR
X(IN)
6
ENT
L
SG
B
C
A
SHFT
B
L
K
SG
S(SG)
1
0
STR
X(IN)
3
ENT
Y(OUT)
6
ENT
E
N
D
ENT
SG
S(SG)
1
5
ENT
STR
X(IN)
7
ENT
RST
S(SG)
1
ENT
OUT
B
S1
X7
SHFT
SHFT
S15
L
C(CR)
C(CR)
0
RST
0
ENT
ENT
ENT
Stage View in DirectSOFT
The Stage View option in DirectSOFT will let you view the ladder program as a flow chart.
The figure below shows the symbol convention used in the diagrams. You may find the stage
view useful as a tool to verify that your stage program has faithfully reproduced the logic of
the state transition diagram you intend to realize.
SG
Stage
Transition
Logic
Reference to
a Stage
J
Jump
Output
S
Set Stage
R
Reset Stage
The following diagram is a typical stage view of a ladder program containing stages. Note the
left-to-right direction of the flow chart.
ISG
S0
J
SG
S1
J
SG
S2
S
SG
S4
J
SG
S3
J
SG
S5
DL06 Micro PLC User Manual, 3rd Edition, Rev. C
Chapter 7: RLLPLUS Stage Programming
Questions and Answers about Stage Programming
We include the following commonly-asked questions about Stage Programming as an aid to
new students. All question topics are covered in more detail in this chapter.
Q. What does stage programming do that I can’t do with regular RLL programs?
A. Stages allow you to identify all the states of your process before you begin programming.
This approach is more organized, because you divide up a ladder program into sections. As
stages, these program sections are active only when they are actually needed by the process.
Most processes can be organized into a sequence of stages, connected by event-based
transitions.
Q. What are Stage Bits?
A. A stage bit is just a single bit in the CPU’s image register, representing the active/inactive
status of the stage in real time. For example, the bit for Stage 0 is referenced as “S0”. If S0
= 0, then the ladder rungs in Stage 0 are bypassed (not executed) on each CPU scan. If S0
= 1, then the ladder rungs in Stage 0 are executed on each CPU scan. Stage bits, when used
as contacts, allow one part of your program to monitor another part by detecting stage
active/inactive status.
Q. How does a stage become active?
A. There are three ways:
• If the Stage is an initial stage (ISG), it is automatically active at powerup.
• Another stage can execute a Stage JMP instruction naming this stage, which makes it active upon its
next occurrence in the program.
• A program rung can execute a Set Stage Bit instruction (such as Set S0).
Q. How does a stage become inactive?
A. There are three ways:
• Standard Stages (SG) are automatically inactive at powerup.
• A stage can execute a Stage JMP instruction, resetting its Stage Bit to 0.
• Any rung in the program can execute a Reset Stage Bit instruction (such as Reset S0).
Q. What about the power flow technique of stage transitions?
A. The power flow method of connecting adjacent stages (directly above or below in the
program) actually is the same as the Stage Jump instruction executed in the stage above,
naming the stage below. Power flow transitions are more difficult to edit in DirectSOFT,
we list them separately from two preceding questions.
Q. Can I have a stage which is active for only one scan?
A. Yes, but this is not the intended use for a stage. Instead, just make a ladder rung active for
1 scan by including a stage Jump instruction at the bottom of the rung. Then the ladder
will execute on the last scan before its stage jumps to a new one.
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Q. Isn’t a Stage JMP just like a regular GOTO instruction used in software?
A. No, it is very different. A GOTO instruction sends the program execution immediately to
the code location named by the GOTO. A Stage JMP simply resets the Stage Bit of the
current stage, while setting the Stage Bit of the stage named in the JMP instruction. Stage
bits are 0 or 1, determining the inactive/active status of the corresponding stages. A stage
JMP has the following results:
• When the JMP is executed, the remainder of the current stage’s rungs are executed, even if they
reside past(under) the JMP instruction. On the following scan, that stage is not executed, because it
is inactive.
• The Stage named in the Stage JMP instruction will be executed upon its next occurrence. If located
past (under) the current stage, it will be executed on the same scan. If located before (above) the
current stage, it will be executed on the following scan.
Q. How can I know when to use stage JMP, versus a Set Stage Bit or Reset Stage
Bit?
A. These instructions are used according to the state diagram topology you have derived:
• Use a Stage JMP instruction for a state transition ... moving from one state to another.
• Use a Set Stage Bit instruction when the current state is spawning a new parallel state or stage
sequence, or when a supervisory state is starting a state sequence under its command.
• Use a Reset Bit instruction when the current state is the last state in a sequence and its task is
complete, or when a supervisory state is ending a state sequence under its command.
Q. What is an initial stage, and when do I use it?
A. An initial stage (ISG) is automatically active at powerup. Afterwards, it works just like any
other stage. You can have multiple initial stages, if required. Use an initial stage for ladder
that must always be active, or as a starting point.
Q. Can I place program ladder rungs outside of the stages, so they are always on?
A. It is possible, but it’s not good software design practice. Place ladder that must always be
active in an initial stage, and do not reset that stage or use a Stage JMP instruction inside
it. It can start other stage sequences at the proper time by setting the appropriate Stage
Bit(s).
Q. Can I have more than one active stage at a time?
7–28
A. Yes, and this is a normal occurrence for many programs. However, it is important to
organize your application into separate processes, each made up of stages. And a good
process design will be mostly sequential, with only one stage on at a time. However, all the
processes in the program may be active simultaneously.
DL06 Micro PLC User Manual, 3rd Edition, Rev. C
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