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Chapter 1 PLC Ladder Diagram and the Coding Rules of Mnemonic
In this chapter, we would like to introduce you the basic principles of ladder diagram, in addition, the coding rules of
mnemonic will be introduced as well, it's essential for the user who use FP-07C as a programming tool. If you are familiar
with PLC Ladder Diagram and mnemonic coding rules, you may skip this chapter.
1.1 The Operation Principle of Ladder Diagram
Ladder Diagram is a type of graphic language for automatic control systems it had been used for a long period since World
War II. Until today, it is the oldest and most popular language for automatic control systems. Originally there are only few
basic elements available such as A-contact (Normally ON), B contact (Normally OFF), output Coil, Timers and Counters.
Not until the appearance of microprocessor based PLC, more elements for Ladder Diagram, such as differential contact,
retentive coil (refer to page 1-6) and other instructions that a conventional system cannot provide, became available.
The basic operation principle for both conventional and PLC Ladder Diagram is the same. The main difference between the
two systems is that the appearance of the symbols for conventional Ladder Diagram are more closer to the real devices,
while for PLC system, symbols are simplified for computer display. There are two types of logic system available for Ladder
Diagram logic, namely combination logic and sequential logic. Detailed explanations for these two logics are discussed
below.
1.1.1 Combination Logic
Combination logic of the Ladder Diagram is a circuit that combines one or more input elements in series or parallel and then
send the results to the output elements, such as Coils, Timers/Counters, and other application instructions.
【Instruction】
Circuit 1
NO(A)
NC(A)
Circuit 2
NC(B)
NC(B)
Circuit 3
NC
NC
NO
NO
Actual wiring diagram
AC110V
AC110V
X0
X0
X1
X1
X4
X2
X2
X4
NO
NO
X3
X3
1 - 1
Y0
Y0
Y1
Y1
Y2
Y2
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