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Chapter Two Sequence Programming
5
Volume I
Programming
CHAPTER TWO SEQUENCE PROGRAMMING
The operation principle is different with the common relay circuit, because the PLC sequence
control is carried out by compiling the on-line diagram; and therefore, it is better to thoroughly
comprehend the sequence control principle when designing the PLC sequence programming.
2.1 Performance Process of Sequence Programming
In the general relay controlling circuits, each of them can be simultaneously operated. When the
relay A is operated in the following figure, the replay D and E can be operated (when the contactor A
and B are entirely closed) at the same. Each replay in the PLC sequence control is operated in turn.
The relay D is operated before relay A, and then the relay E operates (refer to the following figure).
Namely, each relay is operated based upon the sequence of the ladder diagram (compilation
sequence).
A
A
B
C
D
E
Fig. 2.1 (a) Circuit illustration
The differences between the relay circuit and PLC programming operation are shown below in
the Fig. 2.1 (b) and Fig. 2.1 (c).
A
A
C
B
C
Fig. 2.1 (b)
