6-3
Section
Basic Ladder Diagrams
335
Other terms used in describing instructions are introduced in
Section 7 Instruc-
tion Set
.
6-3
Basic Ladder Diagrams
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
Instruction
Instruction
00403
00001
HR 0109
LR 0003
25208
20001
00501
00502
00503
00504
20002
00100
00002
00010
00011
00003 HR 0050
00007 TIM 001 LR 0515
21001
21002
00405
21005
21007
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.
6-3-1
Basic Terms
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
Normally Open and
Normally Closed
Conditions
