Rockwell Automation 1785 PLC-5 User Manual download pdf (Page 28)

Publication 1785-UM012D-EN-P - July 2005

1-10 Understanding Your Programmable Controller

Programming Features

This table highlights the programming features of a PLC-5 programmable

controller.

This Capability Lets You

Ladder logic program using a language that is representative of relay logic.

Choose this language

• if you are more familiar with ladder logic than with programming languages such

as BASIC

Your plant personnel may be more familiar with ladder logic; consider their needs

as well.

• performing diagnostics

• programming discrete control

Subroutines store recurring sections of program logic that can be accessed from multiple program

files.

A subroutine saves memory because you program repetitive logic only once. The JSR

instruction directs the controller to go to a separate subroutine file within the logic

controller, scan that subroutine file once, and return to the point of departure.

Sequential Function Charts

(SFCs)

use sequence-control language to control and display the state of a sequential process.

Instead of using one long ladder program for your application, divide the logic into steps

and transitions. A step corresponds to a control task; a transition corresponds to a

condition that must occur before the programmable controller can perform the next

control task. The display of these steps and transitions lets you see what state the

machine process is in at a given time via a flowchart form.

SFCs offer constructs that enable execution of multiple paths of logic, or a single

selected path of logic, as well as the ability to jump forwards and backwards.

Troubleshooting can be reduced to a small routine of logic instead of an entire ladder

file.

SFCs are best for defining the order of events in a sequential process.

Structured text program using a language similar to BASIC.

Choose structured text if you are:

• more familiar with programming languages such as BASIC than with ladder logic

• using complex mathematical algorithms

• using program constructs that repeat or “loop”

• creating custom data-table monitoring screens

Main Control Programs

(MCPs)

separate sequential logic from ladder logic and structured text as a way of modularized

your process and making troubleshooting easier.

Use several main control programs (MCPs) to define one main control program for each

particular machine or function of your process. MCPs accommodate independent or

non-sequential activities.

A main control program can be an SFC file numbered 1-999 or a ladder-logic file or

structured-text program numbered 2-999.

One data table is used by all MCPs (i.e., you do not have a separate data table for each

MCP).

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Rockwell Automation 1785 PLC-5 User Manual Page 28

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