Analog and digital signals are the two types of external inputs that PLCs can directly accept. Digital signals primarily refer to inputs from various switches and buttons, while analog signals come from various sensors such as temperature, pressure, and flow sensors.

Analog signals are divided into analog voltage and analog current. When the PLC’s hardware module receives external analog signals, it converts these signals into digital signals through hardware circuits. The converted digital signals are what we need to process. The conversion of analog signals to digital signals follows a linear pattern.

As shown in the figure, this is the wiring for the analog input module of a Siemens PLC. When the external analog input changes, the analog signal received by the PLC also changes, and correspondingly, the converted digital signal will also change. When programming, what we are most concerned about is this converted digital signal.

In the figure above, this is the conversion principle of analog signals. The red part on the far left represents the actual analog input, while the red circle on the right represents the converted digital signal. It is essentially a numerical value, and this value varies between different PLCs, but it can be set as needed, with the principle of convenience in calculation. The middle part represents the actual meaning of the analog signal. Through this table, we can clearly see the linear relationship of analog signal conversion. The conversion from analog to digital signals follows the same principle.

Regardless of the brand of PLC, the conversion of analog signals to digital signals follows the principle of linear conversion. Once you understand this principle, you can easily master PLC programming for analog signals. Mastering the principle is more valuable than mechanically learning thousands of instructions.