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The internal sequential operation can be regarded as the program of
the programmable controller. The program is saved in the program
memory as similar to the instruction list
Step number
Instruction
word
Device
0 LD X6
1 OR Y74
2 ANI T1
3 OUT Y74
4 LD Y74
5 ANI X6
6 OUT T1 K30
0
4
X6
Y74
Y74
T1
X6
Y74
K30
T1
10
END
10 END
Repeat
operation
(a) Ladder diagram (b) Instruction list (program list)
Figure 1.2 Program
• A program consists of a large number of instruction words and
devices.
• The instructions contain instruction words and devices. In addition,
the instructions are numbered to represent the order of operations.
The numbers are called step numbers.
(Instruction words are also called instructions.)
• The number of steps varies depending on the types of instructions
or the setting method for the values to be used for the I/O numbers
and operations. (The more steps are needed for the operation with
complicated operation.)
• The instructions repeat from the step number 0 to the END
instruction. (This is called "repeat operation", "cyclic operation" or
"scanning".)
Amount of time necessary for one cycle is called operation cycle
(scan time).
• The number of steps from the step number 0 to the END instruction
is the length or size of the program.
• The program is stored in the program memory inside the CPU. The
operation is executed in a ladder block unit.
One ladder block ranges from the operation start instruction (LD,
LDI) to the OUT instruction (including the data instruction).