Automation Direct DirectLogic Series Nota de aplicación

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Product Family:

Subject:

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).

APPLICATION NOTE

THIS INFORMATION PROVIDED BY AUTOMATIONDIRECT.COM TECHNICAL

SUPPORT IS PROVIDED "AS IS" WITHOUT A GUARANTEE OF ANY KIND.

These documents are provided by our technical support department to assist others. We do

not guarantee that the data is suitable for your particular application, nor do we assume any

responsibility for them in your application.

DirectLogic PLC

Number:

AN-MISC-022

Date Issued: 06/16/04

Revision: Original