Digital and analog inputs are the two most commonly used input/output methods when first learning PLC . What are digital inputs? What are analog inputs? This question must be clarified.
Figure 1 shows a typical device that outputs a switching signal. When the pressure is high, contacts C and B close, connecting the circuit and outputting a high pressure signal. When the pressure is low, contacts C and A close, connecting the circuit and outputting a low pressure signal. This signal allows the local pressure signal to be transmitted to a remote electrical control cabinet for automatic remote control. Here, C and B represent a switching quantity, and C and A also represent a switching quantity. Therefore, each switch contact represents a switching quantity, characterized by being either closed or open at any given time. Closed is represented by a 1, indicating the presence of a signal; open is represented by a 0, indicating the absence of a signal. This is what is known as a switching signal.
Although a pressure gauge can transmit pressure signals over long distances, it only transmits a signal indicating whether there is pressure; it cannot determine the actual real-time pressure value.
The device in Figure 2 is called a pressure transmitter. Internally, a pressure transmitter consists of a circuit board connected to a pressure sensor F. Its working principle is that the pressure sensor F transmits the detected pressure to point C on the circuit board. After the detected signal enters the circuit board, it is converted into a current signal by the circuit board's conversion and calculation, and output from points A and B. The right side of the figure shows a schematic diagram of the conversion process. It can convert a 0-10 kPa pressure signal into a 4-20 mA current signal, output from points A and B. We then say that the outputs from points A and B are analog signals. The characteristic of analog signals is that their values are continuously variable within a certain numerical range.
Let's take a look at how analog signals are transmitted over long distances.
We installed a pressure transmitter with a range of 0-10 kPa on our pipeline. The positive terminal of the power supply is connected to point B of the pressure transmitter, and the negative terminal is connected in series with a multimeter to point A of the pressure transmitter, with the multimeter set to the current range. When the pressure at point C of the pressure transmitter is 5 kPa, the multimeter current reading is 12 mA, which is exactly the midpoint of the 4-20 mA current signal range. 5 kPa is also exactly the midpoint of the 0-10 kPa pressure range. When the pressure at point C of the pressure transmitter is 10 kPa, the multimeter current reading is exactly 20 mA. Thus, the 0-10 kPa pressure range corresponds to a 4-20 mA current signal value. By extracting this 4-20 mA current signal value remotely through a receiving device and performing certain calculations, we can determine the local pressure value.
Why convert the pressure signal into a 4-20mA current signal instead of a 0-20mA current signal or a 0-10V voltage signal?
Voltage signals of 1.0-10V are susceptible to external electromagnetic interference, especially when the cable is very long.
2. If a 0-20mA current signal is used, it is impossible to determine whether the 0mA current signal is due to a cable breakage or a normal 0mA output due to a pressure of 0kPa.
Figure 4 shows a wiring diagram for reading the pressure value of a pressure transmitter using a Siemens S7-200 PLC. This is a basic usage method. The left side is for digital signals, and the right side is for analog signals. Different signal types should be connected to different input terminals of the PLC.
