SM933, SM934, and HollySys modules. Shanghai Sijian Intelligent Technology Co., Ltd. is engaged in technology development, technical consulting, technology transfer, and sales of electrical equipment, automation equipment, electronic products, communication products, and electromechanical products in the fields of intelligent technology, information technology, and automation technology. Industrial automation products: PLC, DCS, sensors, frequency converters, encoders, relays, servo drives, soft starters, touch screens. Brands involved in the products include: Eurotherm, Schneider, Siemens, ABB, SICK, P+F, E+H, Yokogawa, Yamakawa, Mitsubishi, Emerson, Honeywell, FANUC, GE, Eaton Electrical, CT.
SM933, SM934, and HollySys modules. The SM934 module works in conjunction with AC contactors (relays) to achieve automatic switching functionality for dual 220VAC power sources. One circuit serves as the main power source, and the other as the backup power source. When the main power source drops below the normal operating voltage (<190V), and if the backup power source is normal (>210V), the output switch operates (disconnects), automatically switching to the backup power source. If the backup power source is also below the normal operating voltage (<190V) at this time, the output switch does not operate, and no switching occurs. When the main power voltage returns to the normal operating voltage (>210V), the switch operates (closes), switching back to the main power source.
The switching action is completed by the switching control switch in the SM934. The output delay time of the switching control switch is <300ms. This 300ms switching time is very fast and insufficient to cause a power loss, ensuring continuous power supply during the switching process. If the power were to disappear, the function of this module would be meaningless. The input device uses the normally closed contact of button SB0. The coil status of input relay X0 depends on the state of SB0. When the button is not pressed, the coil status of input relay X0 is "1" (energized), and all X0 contacts in the program operate accordingly—normally open contacts close, and normally closed contacts open. If the button is pressed, the coil status of input relay X0 becomes "0" (de-energized), and all X0 contacts in the program return to their normal state. If the input device connected to the input relay is the normally open contact of button SB0, the situation is exactly the opposite: when the button is not pressed, the coil status of input relay X0 is "0" (de-energized), and all X0 contacts in the program do not operate; when the button is pressed, the coil status of input relay X0 becomes "1" (energized), and all X0 contacts in the program operate.
The stop button uses a normally closed type.
Since the PLC determines the on/off status of contacts during program execution based solely on the state of the input relay coils in its memory and does not directly recognize external devices, the selection of external devices is closely related to the type of contacts used in the program during programming. This is an issue that easily arises when programming a PLC based on an electrical control schematic diagram. *A typical example is the stop control in the basic control—"start-maintain-stop control".