Switch and analog is most used in the early years of the PLC way - both input and output

Switch and analog is everybody to learn early PLC most used way of two kinds of input and output. What is the switch? What is analog? The problem must be clear.

figure 1 is a typical can switch output signal of the device.

high pressure when the two contact closure through to C and B, high output pressure signal, the pressure is low C and two contact closure through A low output pressure signal. With such signals will achieve the goal of in situ stress, far to the distance of the electrical control cabinet to participate in the automatic remote control, including C and B is A switch quantity, C and A, is also A switch quantity. So a switch contact is a switch, and the characteristics of it is the same time either on or off. Connect is 1, which represents have signal, disconnect is 0, representing no signal. This is the so-called on-off signal.

pressure gauge can though the pressure signals to the distance, but it is presence of pressure signal, cannot know how much pressure value in real time.

device called pressure transmitter in figure 2.

internal pressure transmitter is a piece of circuit board, circuit board connected to a pressure sensor F. F it is the working principle of pressure sensor to detect the pressure to C of circuit board, signal into the circuit board, by the transformation of the circuit board and calculation, the pressure signal is converted into A current signal output from the two points A and B. Schematic diagram on the right is the conversion process, it can put a 0 10 kpa pressure signal is transformed into a 4 - 20 ma current signal, the output from the two points A and B. When we say the two points A and B is an analog signal output. Analog signal is characterized by its value is within the scope of a numeric value is a continuous variable.

see below how analog signal for long distance transmission.

we pipe is installed on a range of 0 - 10 kpa pressure transmitter, positive pressure transmitter point B, the cathode series A multimeter to point A pressure transmitter, and the multimeter to current file. When the pressure transmitter C point pressure is 5 kpa, multimeter 12 is the current reading of the ma. Just 4 - 20 ma current signal of intermediate value, and 5 kpa also happens to be 0 10 kpa pressure value of the intermediate value. When the pressure is 10 kpa pressure transmitter point C, the current reading of the multimeter is 20 ma. The 0 - Corresponding to the four to 10 kpa pressure value 20 ma current signal value, as long as we in the distance through the 4 - a receiving device 20 ma current signal value extracted, and then through a certain calculation, can know how much is the pressure value in situ.

why converts the pressure signal into 4 - 20 ma current signal, instead of 0 Or 0 to 20 ma current signal 10 v voltage signal?

1。 0 - 10 v voltage signal is vulnerable to electromagnetic interference of the outside world, especially the long cable length is more obvious when the interference.

2。 Use a scale of 0 - 20 ma current signal, will not be able to determine when the current signal is 0 ma, what is the result of the cable bolt failure 0 ma, or is 0 kpa pressure and the output of the normal 0 ma.

figure 4 is the use of Siemens S7 - Read the pressure value of the pressure transmitter 200 PLC controller wiring diagram example, this is one of the most basic method of using switch quantity on the left, right is analog, different signal types to receive different input PLC.

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