- about PLC programming elements and basic instructions - programmable logic

In the traditional relay control system, for a goal to achieve a certain control in the practical work is connected with a low voltage apparatus according to certain logical relationship to achieve the control requirements. That is to say, we want to achieve certain control requirements, are in some low voltage apparatus of specific control circuit.

we are familiar with the total pressure of the motor start control, for example, is the use of the contactor, button switch, fuse and the thermal relay, according to the logic control requirements to connect them.

PLC is a software program to realize the control requirements. When programming is to use a variety of programming elements, these programming element is equivalent to all kinds of electrical appliances in the relay control circuit. Programming element refers to the input, output, registers, memory, timer, counter, general-purpose registers, data registers and special function of memory, etc.

PLC controller internal these memory function and relay control system used in the relay contact is very similar, there are also 'coil' and 'contact', and 'contact' to use is not restricted. But they are not 'hard' relay, but the PLC controller memory storage unit. When writing the logic state of the unit for a '1', represents the corresponding relay coil electricity, its dynamic contact closure, cut off contact. So, these internal relay calling it 'soft relay'. We put the PLC the storage unit is called 'soft relay' programming element.

1。 Contact coil order

1) Contact instruction format and function of

ladder diagram program contact instructions have often opened normally closed contact of two classes, similar to traditional control circuit of relay, contactor contact. Can use free series-parallel contact, with functions of and, or, not logic.

2) Coil output instruction format and function of

coil output commands and output terminals, contact circuit connected drive coil, the coil through the 'flow' ( Equivalent to current) , the corresponding terminal load circuit connected, load movement. Output instruction should be on the right side of the ladder diagram. In the same programs, the same number of coil can be used only once. If the same number of the coil in a program that USES two or more times, it is called a coil repeat output, easy cause misoperation, so should avoid such a design.

3) Contact coil

1, the network instruction application I0. 0 for normally open contacts, I0. 0 to 1 connect, Q0. 0 through, Q0. 0 to 1; I0. 0 disconnect is 0, the Q0. 0 disconnect, Q0. 0 0. 2,

network I0. 1 as the normally closed contact, I0 at this time. Switch on, Q0. Switch on, if I0. 1 the normally closed contact of disconnection, Q0. 1 disconnect.

4) Two application examples of

a. Two switch control, a lamp requires only two switches are connected, light is on. Try to design program. I0. 0,钱数。 1 state 1 are connected, Q0. Zero output is 1, the light; I0. 0,钱数。 Any one state of 0 or 1 both disconnected, Q0. Zero output is 0, the lights went out. This example and logic control.

b。 Is a lamp, two switch control requirements as long as there is a switch is closed, the light is bright, the two switches are disconnected lights went out. Try to design program. I0. 0,钱数。 1 the state any 1 processing, Q0. Zero output is 1, the light; I0. 0,钱数。 1 state are 0 is disconnected, Q0. Zero output is 0, the lights went out. This example for control or logic.

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