(Issue 1, 2004)
on the "stage" wagon
by Richard Palmer
The Technical Support Team at AutomationDirect
receives a large number of calls on RLL Plus (Relay Ladder
Logic Plus) programming, also known as "Stage" programming.
Most of the callers want to understand how and when it should
be used. The short answer to the "when" question
is that it depends on the process that is being controlled.
The "how" question will be answered later.
is an advanced style of programming that is suitable for a
moderately experienced programmer. A thorough knowledge of
how the PLC scans the program is necessary to avoid complications.
However, it is not a "different" language than RLL.
It uses the same instruction set and is scanned in much the
same way with a few exceptions.
RLL Plus or "Stage" programming is well suited to a process that has
clearly defined "steps" or stages that will occur multiple times during
the process. It is not suited for processes that are linear, and have poorly
defined "steps". For example, making a batch of cookie dough might
be a good use for Stage programming, while baking the cookies would not be, since
baking only has one step (i.e., cooking the dough.)
A good example to illustrate the use of Stage programming is a machine that mixes
bread dough. A bread dough machine has several unique "steps" to create
the final product. Some of those steps will be done more than one time throughout
the process. As you can see in Figure 1, small portions of the standard RLL program
are needed multiple times. For example, the mixer will be turned on and off many
times throughout the program. It will always need the same alarms and safeties
each time it operates. While you could minimize the amount of ladder by paralleling
contacts, it does make a program harder to read. By placing the startup, safety,
alarm, and shutdown for the mixer all in one stage, you can enable the same code
many times in the program and cut down on your programming time (Figure 2).
Flowchart example using standard RLL programming
Flowchart example using stage programming
are not like subroutines. They are scanned just as if they
are regular ladder logic. The closest thing to a stage is
a Master Control Relay. If the MCR is on, then the power
rail is processed. If the MCR is off, then the power rail
is skipped. Stages operate in much the same fashion. If the
stage is enabled, the power rail has power and the logic
is executed. If it is disabled, then all logic within that
stage is skipped (not processed) because the power rail has
There are three main behaviors to Stage programming:
1) When the stage bit is on, the
corresponding stage is scanned and logic within that stage is evaluated and processed.
2) When the stage bit is transitioning from on to off, the corresponding stage
will be scanned one more time to turn off all outputs that are currently on.
3) When the stage bit is off, the
corresponding stage is not scanned and therefore no logic in that stage is
evaluated or processed.
There are some caveats to be aware of with stage programming that concern turning
stages on and off. These are detailed in AutomationDirect PLC user manuals, which
can be purchased separately or downloaded from www.automationdirect.com.
Stage programming can save PLC memory and improve scan time by reducing the number
of repeated sequences and by scanning only the stages that are active. These
are two more good reasons to think about using stage programming, but they are
not good reasons to use
stage if the process is not suited for it.
Stage programming and standard RLL logic style programming can be intermixed
by placing standard RLL at the beginning of the program, but usually there is
no need to do this. A proper stage program will start with one or more initial
stages and have numerous stages that will be turned on and off throughout the
There are two ways to activate a stage: with a "Jump" instruction or
by setting the stage bit. The Jump instruction deactivates the current stage
while activating the specified stage. It does not jump over other code.
Setting the stage bit will activate the specified stage while leaving the current
stage running as well. By using Set instructions you can have many stages active
at one time.
The Reset instruction works in much the same manner as the Set instruction. You
can reset one stage or a whole group of stages at one time using the Reset instruction.
When exiting a stage, whether by jumping out of it or by resetting the stage
bit, the stage will always finish scanning before turning the power rail off.
When jumping out of the current stage, the current stage will be scanned one
more time after the current scan is completed. This behavior can cause problems
with some processes if it is not accounted for.
A stage is turned off by jumping out of it or by resetting it. All outputs, timers
and counters, unless set, will be turned off. The simple explanation is that
turning off a stage turns off the power rail. If there is no power, then the
instructions are turned off. Timers and Out instructions, for example, rely on
the power rail to maintain their state. Once power is removed, they turn off.
This happens with other instructions as well. The following examples show Stage
programming and how Stage would look if using
programming can certainly help organize a program, but it
isn't always useful for every program. The
to help determine
if Stage programming
should be considered is, "Can this program be broken down into logical
steps?" If you can answer "Yes" to that question, then learning
Stage programming might help save you some time, both in program development
and in execution speed.
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