0. Introduction With the development of technology, traditional DDC (Direct Digital Control) systems can no longer meet the control requirements of modern systems. Distributed Control System (DCS) is an effective method for solving the control of modern large-scale systems. Its essence is a new type of control technology that uses computer technology to centrally monitor, operate, manage, and distribute the production process. It features strong versatility, flexible system configuration, comprehensive control functions, convenient data processing, centralized display and operation, user-friendly human-machine interface, simple and standardized installation, convenient debugging, and safe and reliable operation. 1. Design of Distributed Control System As shown in Figure 1, a large-scale distributed control system is formed by using a computer as the central monitoring station, which oversees multiple programmable logic controllers (PLCs). Each PLC can simultaneously control multiple elevators. This system designs a MINI-type distributed control system, using two PLCs for control, each controlling one elevator. It can be easily expanded into a large-scale distributed control system. Figure 1 Distributed Control System 2. Elevator Model Design The structural diagram of the elevator is shown in Figure 2. The elevator's support frame is made of aluminum plate processed into a "几" (ji) shape. A DC motor and its drive circuit are mounted on the aluminum plate of the support base. A sensor for detecting the car's position is installed on the left side wall of the vertical aluminum plate for easy detection of the car's position signal. The elevator structure model uses a DC motor driving a pulley as the driving wheel. Another pulley is installed at the top of the elevator support frame as the driven wheel. A cotton thread is tied between the two pulleys to drive the car up and down. [align=center] Figure 2 Schematic diagram of the elevator structure[/align] 3. PLC control of the elevator This design utilizes a programmable logic controller (PLC) FX2N[1, 21]. Considering that only an external call signal is designed, the elevator's operating rules are as follows (taking a three-story building as an example): When the car is stopped at the first or second floor, pressing the third-floor call button will cause the car to rise to the third-floor limit switch and stop. When the car is stopped at the third or second floor, pressing the first-floor call button will cause the car to descend to the first-floor limit switch and stop. When the car is stopped at the first floor, pressing the second-floor call button will cause the car to rise to the second-floor limit switch and stop. When the car is stopped on the third floor, pressing the second-floor call button will cause the car to descend to the second-floor limit switch and stop. When the car is stopped on the first floor, and both the second and third-floor buttons are being used for calls, the car will rise to the second-floor limit switch, pause for 2 seconds, and then continue rising to the third-floor limit switch and stop. When the car is stopped on the third floor, and both the second and first-floor buttons are being used for calls, the car will descend to the second-floor limit switch, pause for 2 seconds, and then continue descending to the first-floor limit switch and stop. During the car's ascent, calls made in either direction are invalid. The reverse is also true. Taking the case where the car calls from the first floor, second floor, and third floor as an example, the analysis of part of the program is shown in Figure 3a): [align=center] Figure 3 Program Analysis Diagram[/align] 1) Setting the intermediate relay (for power outage retention) when ascending After the second floor call switch X004 is effective, the power outage retention relay M600 is set; After the car reaches the second floor, the second floor limit switch X001 is triggered, and M600 is reset; After the third floor call switch X006 is effective, the power outage retention relay M601 is set; After the car reaches the third floor, the third floor limit switch X002 is triggered, and M601 is reset. 2) The car calls from the first floor, second floor, and third floor (1) After both the power outage retention relays M600 and M601 are effective (there are calls from the second and third floors at the same time), the output MO is effective (drives the motor to rotate forward) and has self-holding. When the car reaches the second floor, the second-floor limit switch X001 is triggered, which causes MO to fail; (2) When the car reaches the second floor, the second-floor limit switch X001 triggers the time relay TO to start timing for 2 seconds; (3) After 2 seconds, the time relay switch TO closes, M1 is effective (driving the motor to rotate forward), and self-holds. When the car reaches the third floor, the third-floor limit switch X002 is triggered, which causes M1 to fail. Ladder diagrams can be drawn in Mitsubishi's programming software FXGPWIN, then converted into instructions, and the program can be solidified into the PLC using the programming line SC09. 4. Connection between PLC and elevator The floor limit switches x0, X1, X2 and the floor external call buttons x3, X4, X6 serve as input signals, and the output is to control the forward and reverse rotation of the DC motor. The actual wiring is shown in Figure 4. [align=center]Figure 4 Connection diagram of PLC and elevator model[/align] When connecting the limit switch (GK122), the 24V power supply of the PLC is used for power supply. Considering that about 20mA of current is required to light up the LED, a 1kΩ resistor is connected in series in the circuit. The forward and reverse rotation of the DC motor can be realized by the "H" type circuit. When Y0 and Y4 have output, the motor rotates forward; when Y10 and Y14 have output, the motor rotates in reverse. 5. MCGS Monitoring Interface Design MCGS (Monitor and Control Generated System) is a configuration software for quickly constructing and generating computer monitoring systems. It can run on various 32-bit Windows platforms based on Microsoft. Through the acquisition and processing of field data, it provides users with solutions to practical engineering problems in various ways such as animation display, alarm processing, process control and report output. The MCGS system consists of five major functional modules. The main functional modules are constructed in the form of components. Different components have different functions and are independent of each other. Three basic types of components (equipment components, animation components, and strategy components) complete all the work of the three main parts of the TMCGS system (installation of various drives, animation display, and process control). 6. Communication connection between PLC and MCGS The connection between the PLC and PC is shown in Figure 5, using RS485 communication. In the figure, one PC has two PLCs under it. Using the FX-485PC-IF adapter, the RS232C signal from the PC is converted into an RS485 signal, which is then connected to the PLC through the function expansion board FX2N 485 BD, and then connected to the elevator. The FXON-485ADP and FX2N-CNV-BD in the other parallel branch have the same function as the FX2N-485-BD. The system constructed in this way can have up to 16 substations. When using only the adapter, the distance can be extended by 500 m, and when using the function expansion board, it is 50 m. [align=center] Figure 5 Connection between PLC and PC[/align] The RS485 connection can be one or two pairs of wires. The wiring method is determined by the intended use; this design uses a two-pair wire connection. To establish a communication connection between the PLC and MCGS, the communication address and parameters can be set in the "PLC/Serial Port Settings" menu of the FKGPWIN PLC programming software, or directly through programming (MOV instructions) in the software. The specific settings according to RS485 specifications are: baud rate set to 9600 bit/s, data bits set to 7 bits, 1 start bit, 2 stop bits, even parity, and protocol 1. The programming software settings are shown in Figure 6: [align=center] Figure 6 Programming Software Settings[/align] The communication address is set in D8121, with the addresses of the two elevators set to 0 and 1 respectively. Communication parameters are set in D8120. The same settings must also be performed in the MCGS device window to establish a communication connection. Furthermore, it is necessary to establish channel connections between the PLC's input/output quantities and the corresponding variables in the MCGS database. This ensures synchronized movement between the physical elevator model and the elevator in the monitoring interface, as the data is collected synchronously. 7. Debugging After adjusting the DC motor's power supply voltage and the timer cycle time in MCGS, the elevator in the MCGS monitoring screen can now operate synchronously with the PLC-controlled elevator and be monitored. 8. Conclusion Although this design is only a MINI distributed control system, it can be easily expanded into a large-scale distributed control system, possessing strong versatility. With the expansion of control scale and the increase in control requirements, distributed control is gradually becoming a very important control method in the field of automatic control. Especially when combined with a PLC and using the powerful synchronous monitoring function of MCGS configuration software for real-time monitoring, it has broad application prospects.