Basic Ladder Diagrams - Omron CPM1 - PROGRAMING MANUAL 02-2001 Programming Manual

Basic Ladder Diagrams

6-3 Basic Ladder Diagrams
6-3-1 Basic Terms
Normally Open and
Normally Closed
Conditions
Other terms used in describing instructions are introduced in Section 7 Instruc-
tion Set.
A ladder diagram consists of one line running down the left side with lines
branching off to the right. The line on the left is called the bus bar; the branching
lines, instruction lines or rungs. Along the instruction lines are placed conditions
that lead to other instructions on the right side. The logical combinations of these
conditions determine when and how the instructions at the right are executed. A
ladder diagram is shown below.
00000 20000
00001
00100 00002 00003 HR 0050
00010
00011
As shown in the diagram above, instruction lines can branch apart and they can
join back together. The vertical pairs of lines are called conditions. Conditions
without diagonal lines through them are called normally open conditions and
correspond to a LOAD, AND, or OR instruction. The conditions with diagonal
lines through them are called normally closed conditions and correspond to a
LOAD NOT, AND NOT, or OR NOT instruction. The number above each condi-
tion indicates the operand bit for the instruction. It is the status of the bit
associated with each condition that determines the execution condition for fol-
lowing instructions. The way the operation of each of the instructions corre-
sponds to a condition is described below. Before we consider these, however,
there are some basic terms that must be explained.
Note When displaying ladder diagrams with the SSS, a second bus bar will be shown
on the right side of the ladder diagram and will be connected to all instructions on
the right side. This does not change the ladder-diagram program in any function-
al sense. No conditions can be placed between the instructions on the right side
and the right bus bar, i.e., all instructions on the right must be connected directly
to the right bus bar. Refer to the SSS Operation Manual: C-series PCs for details.
Each condition in a ladder diagram is either ON or OFF depending on the status
of the operand bit that has been assigned to it. A normally open condition is ON if
the operand bit is ON; OFF if the operand bit is OFF. A normally closed condition
is ON if the operand bit is OFF; OFF if the operand bit is ON. Generally speaking,
you use a normally open condition when you want something to happen when a
25208 HR 0109 LR 0003 20001
00501 00502
00007 TIM 001 LR 0515
21001 21002
21005 21007
Section
20002
Instruction
00503 00504
00403 00405
Instruction
6-3
335