1. Introduction In conventional automatic control systems, sensors and actuators are independently wired, and systems consisting of multiple sensors and actuators require a large number of wires. Applying communication buses to measurement and control systems not only saves a significant amount of wiring but also improves system reliability. Widely used industrial buses generally fall into two categories. One is a master-slave structure, such as RS-485 communication. This communication bus is widely used in industrial control, and its communication method is command-response. The master controller periodically sends query signals to each sub-controller, which then reports its status. This communication method is relatively easy to develop, but it consumes a considerable portion of the master controller's resources. Therefore, this method does not fully utilize the powerful computing capabilities of the master controller. The other type is an autonomous communication method where each node communicates independently, such as the CAN bus from Omron and Mitsubishi, and the LONWORKS bus from NEWLIFT. This type of bus offers significantly improved reliability and communication speed compared to the former, but it is relatively more expensive. 2. Siemens AS-Interface Actuator-Sensor Interface Bus: Addressing the advantages and disadvantages of the two currently popular serial bus control methods, Siemens has incorporated the strengths of both and launched the AS-Interface (Remote I/O) bus technology. AS-Interface is an actuator-sensor interface bus system that collects dispersed I/O signals through slave stations and transmits them to the AS-I master station using only two signal lines. The AS-I master station calls sequentially, with a maximum cycle time of 5ms. In case of an error, an AS-I slave node has an automatic bus shutdown function, severing its connection to the bus and preventing other slave stations from being affected. Faults are promptly reflected on the AS-I master station. Each frame of AS-I information includes CRC checksum and other error detection measures, ensuring the high reliability of the AS-I bus. The maximum direct communication distance of the AS-I bus is 100m, and the maximum distance extended through relay stations is 300m. The AS-I bus can accommodate up to 248 sensors and actuators. The connection between the Siemens PLC master and the actuator-sensor-interface slave station is through the AS-I master station, requiring no additional editing of the communication program. For engineers, the remote I/O corresponds to the corresponding bit in the image area, which conforms to their programming habits and is very convenient. Due to the application of two-wire communication, the system connection cable adopts a wire-piercing structure, greatly reducing the amount of wiring. The unique AS-I ladder cable eliminates the possibility of wiring errors, saving a significant amount of cable compared to previous PLC control systems, and greatly reducing installation workload. 3 Elevator Control System The elevator control system has undergone a considerable technological leap from relay control to PLC plus speed controller control. Existing products are well-established and have quite stable performance. The basic structure of the existing elevator control system is shown in Figure 1. The control center is located in the rooftop machine room. All signals in the hoistway and car are transmitted to the control center in a point-to-point manner through a large number of cables. [align=center] Figure 1 Traditional Elevator Control System[/align] Traditional elevator control systems suffer from drawbacks such as excessive wiring, complex installation, difficulty in modification and expansion, resulting in poor maintenance and low efficiency. Elevator users' demands have expanded beyond basic functions like safety and reliability. They now require greater comfort, efficiency, self-diagnostic capabilities, remote monitoring, and ease of commissioning and maintenance. Therefore, a high-efficiency, high-reliability fieldbus technology is urgently needed to meet these requirements, and AS-Interface bus technology is the optimal choice. The AS-I bus uses a two-wire communication system with a wire-piercing cable design, allowing AS-I slaves to easily connect to the bus. Furthermore, the unique AS-I ladder cable eliminates the possibility of wiring errors. The following discussion explores the implementation of elevator control using an AS-Interface bus system with a Siemens S7-200CN PLC. 4. Application of AS-Interface Bus in Elevator Control System 4.1 Hardware Implementation The Siemens S7-200CN PLC, equipped with AS-Interface bus functionality, boasts superior performance and powerful features, supporting trigonometric functions, square root operations, logarithmic calculations, etc.; it allows online editing and monitoring; remote monitoring via modem; fault diagnosis; single-scan execution; forced output; and the ability to edit variable status tables, displaying signal status and status tables simultaneously using multiple open windows. Therefore, the elevator control system based on the S7-200CN PLC is a networked, intelligent, and cost-effective control system. Regarding the system's hardware implementation, careful investigation and analysis revealed that the elevator control system's sensors and actuators are primarily concentrated in the hoistway and car, with only one actuator (the speed controller) in the machine room, lacking sensors. Therefore, using the machine room as the control center is not entirely reasonable. To optimize the system's hardware layout, the project made the following adjustments to the traditional elevator control system: the elevator's control and drive systems were physically separated, changing the traditional inseparable state of control and drive in elevator systems. The advantage of this approach is that it truly separates strong and weak current signals, greatly improving the system's anti-interference capability and further ensuring the safety and reliability of the elevator system. Since most elevator signals reside in the car and hoistway, placing the control center in the machine room, even with AS-Interface bus technology, would require a considerable number of AS-I slave stations (for example, a 10-story, 10-stop elevator has approximately 100 signals in the car and hoistway; with a maximum of 8 I/Os per AS-I slave station, 13 slave stations would be needed to meet the requirements). Such a system, while advanced, lacks economic viability and is unlikely to be accepted by engineering projects. The project's approach is to move the control center to the top of the car. This move significantly reduces the cost of the control system without compromising its advanced features (again, for a 10-story, 10-stop elevator, there are approximately 48 signals in the hoistway and machine room, requiring only 6 slave stations). Figure 2 shows the elevator control system based on the above ideas and using AS-I bus technology. The control center is located on the car top and consists of three parts: CPU226CN (PLC), EM223 (PLC expansion), and AS-I master station. The signals on the car are directly connected to the I/O of the PLC, and the signals in the hoistway and machine room are transmitted to the AS-I master station through the AS-I slave station. The on-site installation is very simple. [align=center] Figure 2 AS-I elevator control system[/align] 4.2 Introduction to related Siemens control components The following is a brief introduction to the performance and function of CPU226CN, AS-I master station CP243-2, expansion EM223 and AS-I slave station. (1) S7-200CN master controller (CPU226cn) ● It integrates 14 inputs/10 outputs, a total of 24 digital I/O points. It can connect 7 expansion modules, and can be expanded to a maximum of 168 digital I/O points or 35 analog I/O points. 13K bytes of program and data storage space. Six independent 30kHz high-speed counters, two independent 20kHz high-speed pulse outputs, PID controller, and one RS-485 communication/programming port. It is a controller with strong control capabilities. As shown in Figure 3: [align=center] Figure 3 CPU226CN[/align] ● The device is located on the top of the car and is responsible for controlling the car position, driving the car door, receiving various electrical signals from the car, and processing various signals such as communication with the AS-I master station and modem. (2) AS-I Master Station ● Performance The AS-I cycle time is no more than 5ms, the maximum allowable current of the AS-I connection cable is 3A, and it can be directly connected to an external 24V power supply. Its address range is: one 8DI/8DO digital module and one 8AI/8AO analog module. It can be seen that the AS-I master station has an exceptional response time and load-carrying capacity. As shown in Figure 4: [align=center] Figure 4 CP243-2[/align] ● The device is located on the top of the car and is responsible for communicating with the master controller and controlling the AS-I slave station. (3) Expansion ● Performance The EM223 expansion unit has 16 digital signal input/output ports (8I/8O), with opto-isolation and low power consumption. ● Function One important reason for placing the control center on the car top is that most of the elevator signals are concentrated on the car. These signals can be sent directly to the control center in parallel, which is a very economical and feasible method. Therefore, when the number of CPU224 local I/O points cannot meet the requirements, it is necessary to make up for the insufficient number of points through expansion (EM223). (4) AS-I slave The signals from the hoistway and machine room are connected to the elevator control system through the AS-I slave. Therefore, the AS-I slaves are distributed in the hoistway and machine room, responsible for processing the signals in the call box and controlling the speed controller. 4.3 Software implementation The Siemens S7-200CN series PLC automatically maps the AS-I slaves to 8 analog input words (AIW0～AIW7) and 8 analog output words (AQW0～AQW7). For engineers, programming the AS-I slave station is no different from programming ordinary I/O. Only a small piece of code needs to be added to map the slave station I/O to the PLC. The AS-I startup and mapping conversion program list is as follows: LD SM0.1 SI Q3.7, 1 RI Q3.0, 4 LD SM0.7 BMW AIW0, VW1000, 8 BMW VW2000, AQW0, 8 4.4 Workflow The core of elevator control is the process of analyzing various signals and controlling the speed controller, door operator, etc., to move the car. In the Siemens S7-200CNPLC serial system, various control and data signals from the hoistway and machine room are transmitted to the AS-I master station via the AS-I slave station, and then to the CPU226CN via the AS-I master station. Similarly, the CPU226CN needs the AS-I master station to issue commands to a slave station. All signals on the car are directly sent to the CPU226CN through parallel I/O points. The following is a brief overview of the workflow, using the process of handling a call signal in an elevator as an example. When the system powers on, the CPU226CN performs a power-on self-test. This includes I/O checks, communication checks with the master station, and the correctness of the elevator's current status parameters (door status, automatic, maintenance, or operator, elevator position, etc.). If an error is detected, the system enters a fault state, blocking the express train until all faults are cleared, at which point it returns to normal operation. Once the slave station detects a call signal, it immediately transmits it to the AS-I master station via the AS-I signal cable. The master station then sends an interrupt signal to the CPU226CN, ultimately transmitting the call signal to the CPU226CN for processing. The transmission time for a single signal is less than 5ms. After receiving the signal, the CPU226CN determines the elevator's direction of travel and stopping position based on its current status, and sends commands to the slave station via the AS-I master station to control the speed controller and traction machine. 5. Elevator Remote Monitoring System Based on Siemens PLC 5.1 Communication Port Introduction The internally integrated PPI interface provides powerful communication capabilities for S7-200CN users. The physical characteristics of the PPI interface are RS-485, and it can work in three modes: (1) PPI mode The PPI communication protocol is a communication protocol developed by Siemens specifically for the S7-200CN series PLC. It can be networked through ordinary two-core shielded twisted pair cable. The baud rate is 9.6kbps, 19.2kbps and 187.5kbps. The programming port integrated on the S7-200CN series CPU is also the PPI communication protocol. Communication is very simple and convenient. Only two statements, NETR and NETW, are needed to transmit data signals. No additional modules or software are required. The PPI communication network is a token passing network. Without the addition of repeaters, up to 31 S7-200CN series PLCs, TD200, OP/TP panels or host computers (with MPI cards) can be used as stations to form a PPI network. (2) MPI mode The S7-200CN can be connected to the MPI network through the built-in interface. The baud rate is 19.2/187.5kbps. It can communicate with S7-300/S7-400 CPU. (3) Free port mode The free port mode is a very distinctive feature of S7-200CNPLC. It enables S7-200CNPLC to communicate with any other device or controller with an open communication protocol. 5.2 Hardware implementation We use the free port mode to connect the 485 port of CPU226CN to the modem via cable and connect it to the telephone line. In the monitoring room, the modem is connected to the computer. After the connection is completed, the elevator on site can be monitored by dialing up the Internet. Among them, the modem is selected as Realtek Internet Star 5600db+. The hardware block diagram is shown in Figure 5: [align=center] Figure 5 Remote monitoring hardware block diagram[/align] 5.3 Software settings Since the Siemens STEP-7MicroWIN programming software itself has the relevant settings for remote monitoring, the engineers do not need to redevelop the communication, saving a lot of money. After entering the interface of STEP-7MicroWIN programming software, remote monitoring can be realized with simple settings. In the Communication section, the Local Modem and Remote Modem should be set to the same model (otherwise, the Local Modem cannot be programmed). If the selected hardware Modem cannot be found in the options, it must be customized using the Custom Modem Configure method, as shown in Table 1. Table 1: Custom Modem Configure 6. Comprehensive Index Analysis The application of bus technology in elevators (also known as serial communication elevators) has been adopted in some elevators in China. Large elevator manufacturers such as Shanghai Mitsubishi, Guangzhou Hitachi, and Tianjin Otis have begun to adopt this technology extensively. However, for the vast majority of small and medium-sized elevator companies in China, introducing and developing this system will undoubtedly consume a lot of manpower and resources. In 2000, Sichuan Jianning Elevator Factory imported a complete set of the Taiwan TS868 elevator serial communication system. Compared with its current independently developed serial communication system based on Siemens AS-I bus technology, a comparison table is shown in Table 2. Table 2 Comparison of TS868 and SIEMENS (S7-200CN) The comparison table clearly shows that the development of a serial communication system based on Siemens AS-I bus technology excels in both cost and technology, making it particularly suitable for product upgrades in small and medium-sized elevator companies. 7 Conclusion Serial communication systems based on Siemens AS-I bus technology are highly suitable for independent development by small and medium-sized enterprises. Without requiring engineers to abandon their familiar PLC control systems, and without significant investment, elevator products can be upgraded to a higher level, keeping pace with international trends and enabling companies to secure a favorable market position in fierce competition. This technology was successfully implemented in two elevators in the main teaching building of Leshan Normal University in Sichuan Province in October 2001.