Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (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).