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Product Family:
DirectLogic PLC
Number:
AN-MISC-022
Date Issued: 06/16/04
Revision: Original
Jumping on the "stage" wagon
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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.
Stage 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).
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