Watlow LogicPro - Manual (Page 97)

Doc.# 28002-00 Rev 3.00 Watlow Anafaze 81
8
Ladder Diagrams
The Ladder diagram is an outgrowth of the ‘relay Ladder’
commonly used in the programming of PLCs. This offers you
a visual method of creating Boolean expressions. By
satisfying the elements of the expression power flows through
to a coil at the output of a rung.
Simply put, the Ladder diagram symbolizes a circuit with
energized left and right power rails. If conditions exist that
allow contacts, positioned along a rung of the Ladder, to close
then the circuit is complete and power flows to an output coil.
By placing several contacts in series you can create an AND
condition in Boolean Logic. Conversely, an OR condition
results from placing contacts in parallel with each other. The
Ladder logic executes from top to bottom and from left to
right. Although LogicPro does not draw the right power rail
on the screen, the circuit functions as if it were there.
This programming technique provides you with a variety of
contacts and coils, thus giving you considerable flexibility in
creating intricate program logic. This chapter explains the
various contacts and coils accessible to you in LogicPro along
with some insight on how to use them. If you have ever
programmed a ‘relay Ladder’ you are already familiar with
most of these concepts and symbols.
To select Ladder diagram as the language for a program
window, select
LD - Ladder from the Tools, Language
menu. However, if the program window has existing
Sequential Function Chart or Function Block Diagram
elements, a language change will not be allowed.