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8 -1
Chapter 8 Step Instruction Description
Structured programming design is a major trend in software design. The benefits are high readability, easy
maintenance, convenient updating and high quality and reliability. For the control applications, consisted of many
sequential tasks, designed by conventional ladder program design methodology usually makes others hard to maintain.
Therefore, it is necessary to combine the current widely used ladder diagrams with the sequential controls made
especially for machine working flow. With help from step instructions, the design work will become more efficient, time
saving and controlled. This kind of design method that combines process control and ladder diagram together is called
the step ladder language.
The basic unit of step ladder diagram is a step. A step is equivalent to a movement (stop) in the machine operation
where each movement has an output. The complete machine or the overall sequential control process is the combination
of steps in serial or parallel. Its step-by-step sequential execution procedure allows others to be able to understand the
machine operations thoroughly, so that design, operation, and maintenance will become more effective and simpler.
8.1
The Operation Principle of Step Ladder Diagram
【
Example
】
【
Description
】
Y1
Y3
Y0
Y4
Y5
M1924
X1
X3
X4
X2
Y2
X5
X6
X10
STP S20
STP S22
STP S23
STP S21
STP S0
1.
STP Sxxx is the symbol representing a step
Sxxx that can be one of S0
~
S999. When
executing the step (status ON), the ladder diagram
on the right will be executed and the previous step
and output will become OFF.
2. M1924 is on for a scan time after program start.
Hence, as soon as ON, the stop of the initial step
S0 is entered (S0 ON) while the other steps are
kept inactive, i.e. Y1
~
Y5 are all OFF. This means
M1924 ON
Æ
S0 ON
Æ
Y0 ON and Y0 will remain
ON until one of the contacts X1 or X2 is ON.
3. Assume that X2 is ON first; the path to S21 will
then be executed.
X2 ON
⇒
⇒
Y2 will remain ON until X5 is ON.
4. Assume that X5 is ON, the process will move
forward to step S23.
i.e. X5 ON
⇒
⇒
Y4 and Y5 will remain ON until X6 is ON.
※
If X10 is ON, then Y5 will be ON.
5. Assume that X6 is ON, the process will move
forward to S0.
i.e. X6 ON
⇒
⇒
Then, a control process cycle is completed and the
next control process cycle is entered.
S23 ON
S21 OFF
Y4 ON
Y2 OFF
S21 ON
S0 OFF
Y2 ON
Y0 OFF
S0 ON
S23 OFF
Y0 ON
Y4
、
Y5 OFF