The basic elements of PLC ladder diagram: energy flow, bus, electric shock, coil and function frame. See Figure 1 for details

Figure 1: basic elements of PLC ladder diagram

Characteristics of ladder diagram:

a: The ladder diagram echoes with the relay schematic diagram, which is intuitive and easy to understand.

b: Ladder diagram can have multiple networks, each network only writes one language, and there can be one or more rungs in a network. See Figure 2

Figure 2: ladder diagram feature description

c: In each network, the ladder diagram starts from the left bus, passes through the contact, and ends at the soft relay coil or the right bus. See Figure 3

Figure 3: arrangement of contacts, coils and buses

d: The coil cannot be connected to the left bus. If the coil is executed unconditionally, the unused normally closed contact can be used. See Figure 4

Figure 4: treatment method of connecting the coil to the left bus

e: The output coil with the same number cannot be used twice in the same program, otherwise it is easy to cause misoperation. See Figure 5

Figure 5: double coil treatment method. Figure 6: coil parallel output

f: Different improved coils can be output in parallel. See Figure 6

g: The energy flow is not an actual current, but an imaginary current, and the energy flow direction is from left to right, and can not be reversed.

h: In the ladder diagram, each programming element shall be marked with letter and number strings according to certain rules, such as I0 0 and Q0 one

i: The contacts and coils in the ladder diagram are only the contacts and coils in the software, not the contacts and coils in the hardware. The actual contacts and coils need to be connected when controlling the equipment.

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