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CHAPTER2 SEQUENCE PROGRAMS
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2.1 Sequence Program Overview
(2) Programming method
There are two programming modes for sequence programs.
• Ladder mode
• List mode
(a) Ladder mode
Ladder mode is a mode based on the concept of sequential control by relay circuits.
A program in ladder mode is similar to a schematic for a set of relay circuits. Programming in units of ladder
blocks is available.
A ladder block, which starts from the left rail and ends at the right rail, is the minimum unit for operating
sequence programs.
(b) List mode
List mode uses dedicated instructions for programming. The symbols, such as contacts and coils, used in
ladder mode are replaced with dedicated instructions.
The following instructions are used for normally open contacts, normally closed contacts, and coils.
• Normally open contact LD, AND, OR
• Normally closed contact LDI, ANI, ORI
•Coil OUT
There are two other types of programs that can be executed in the CPU module: SFC programs and ST programs.
For details, refer to the following.
MELSEC-Q/L/QnA Programming Manual (SFC)
MELSEC-Q/L Programming Manual (MELSAP-L)
MELSEC-Q/L Programming Manual (Structured Text)
Figure 2.2 Ladder mode
Y21
Y22
Y23
Y24
Y20
Y24
0
2
8
Left rail
N/O
contact
N/C
contact
Coil (output)
Right rail
Ladder block
Step number
* X0 to X5: Input, Y20 to Y24: Output
X0
X3
X4 X5
X1 X2
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