The relay control system is a kind of “hardware logic system”, as shown in Figure 1 (a). Its three branches work in parallel. When the button SB1 is pressed, the intermediate relay K is powered on, the two contacts of K are closed, and the contactors KML and km2 are powered on and act at the same time. So the relay control system adopts the parallel working mode.
Programmable controller is a kind of computer control system, its working principle is based on the working principle of the computer, that is, it is realized by executing the user program reflecting the control requirements, as shown in Figure 1 (b). Because the CPU is to execute the program to deal with various tasks, so in each moment can only do one thing, belongs to the serial working mode. Through the execution of the program, the corresponding actions are completed in sequence.
Figure 1 Schematic diagram of PLC control logic