Dual-machine hot standby control function has a wide range of applications in industry. In addition to important process automation (PA) control objects in chemical, metallurgical and power industries, some important machinery and equipment also require 24-hour uninterrupted operation, high reliability and online maintenance. Air traffic control radar (ATM) is such a typical electromechanical device. Its reliability is directly related to the safety of aircraft take-off and landing in the airspace. In view of the application characteristics and requirements of air traffic control radar, we have taken advantage of the speed and flexibility of PCC in computing control and network communication, and adopted two sets of mutually coupled PCC controllers. Using a set of driver software we developed, we have successfully built a small, inexpensive, efficient and practical dual-machine hot standby air traffic control radar servo control system. The entire servo system has a symmetrical control function structure as shown in the figure below: The main functions of the above system are as follows: 1) Control the start, stop, forward, reverse and speed adjustment of the 7.5kw azimuth motor driving the radar antenna through the CAN bus, and at the same time monitor the speed, current, temperature rise and fault alarm information of the azimuth motor in real time. 2) Control the start, stop, and speed adjustment of the 0.75kW azimuth lubrication pump motor via the CAN bus, while simultaneously monitoring the pump motor's speed, current, temperature rise, and fault alarm information in real time. 3) Control the movement of the polarization motor to achieve circular and linear polarization of the antenna feed. 4) Monitor the speed, oil pressure, oil temperature, and other mechanical information and status of the radar servo system , and implement control. For redundancy and hot standby design considerations, all the above electromechanical equipment consists of two sets, arranged in a roughly symmetrical layout. These two parts, serving as mutually redundant hot standby components, constitute an inseparable whole structure of the entire air traffic control radar servo system. During actual operation, the system software employs a certain strategy, selecting one half as the main control unit and the other half automatically becoming the backup unit for the main control unit. Unlike a typical backup, the backup part is called "hot standby," meaning it is powered on during operation and functionally ready. When the main control system fails and control is interrupted, the hot standby system automatically switches over and takes over the control role of the original main control system. In the aforementioned control system, in addition to two symmetrically arranged PCCs, a PowerPanel touchscreen is also provided for on-site monitoring. This touchscreen monitors all system operating information in real time and enables the switching of primary and standby control between the two PCCs. Simultaneously, through the system's backbone network—Ethernet—another management computer on the network can also monitor the system's operating status and remotely switch between primary and standby units. 1. Hardware Coupling Implementation The two symmetrically arranged PCCs are the key and core of the entire system. The implementation of their software and hardware coupling is the core technology ensuring the dual-machine hot standby control function, as shown in the following diagram: As shown in the diagram, the two coupled PCCs have the following functional elements: Primary/Standby Selection Switch: As a hardware means of selecting primary/standby function, this switch has the highest selection setting authority. When the system powers on, the dual-machine hot standby software sets the corresponding PCC as the primary control unit according to the selection status of this switch, and the other PCC enters the "hot standby" standby working state. Program Synchronization Button: This button controls the synchronous loading between the primary and backup controllers when loading dual-machine hot standby software. When this button is pressed, the control software within the current controller can be automatically transmitted to the other controller unit via a synchronous data network coupling. This is a crucial function for system software maintenance. LifeGuard Signal: This signal is cross-connected between the input/output channels of the primary and backup control systems. Through this signal, the backup control system can monitor the operating status of the primary PCC at any time. Accordingly, when the primary PCC fails, the backup PCC will automatically and seamlessly switch to become the primary control unit, taking over the software operation of the original primary PCC. HeartBeat Signal: This signal is cross-connected between the input/output channels of the primary and backup control systems. Accordingly, during its program execution, the primary PCC directs the software operation rhythm of the backup PCC according to a fixed heartbeat rhythm to ensure the consistency of the operating status of the primary and backup system software. The transmission of this signal is a necessary condition to ensure a smooth transition between the two PCCs during the primary/backup switch. SyncData Network Coupling: This signal connects the two PCCs via industrial Ethernet or a high-speed serial port, enabling the master PCC to synchronously and hot-back up its intermediate-level output data to the standby PCC. This signal transmission is another essential condition for ensuring uninterrupted operation when the two PCCs switch between master and standby roles. Upper-Level Management Network Link: This connection is implemented using industrial Ethernet and serves as the network connecting the two control systems to the upper-level monitoring touchscreen or management computer. Input/Output Signals: This part, like conventional non-dual-machine hot standby control systems, consists of various sensors and actuators on the air traffic control radar equipment. It can also be accessed via a CAN fieldbus network. Unlike conventional systems, considering the functional architecture of a dual-machine hot standby system, all these signals are designed with two redundant sets, connected in parallel to the two PCC control systems that serve as both master and standby. 2. Implementation of Dual-Machine Hot Standby Control Software The core function of this system software is to achieve coordinated management and control of two mutually hot-standby control systems. This ensures that the system's external control functions have online hot standby capabilities. In summary, the software design needs to meet the following functional characteristics: 1) High Speed: The system software must operate at high speed, achieving accurate data and status acquisition and control externally. After the main control unit fails, it must automatically switch between the primary and backup PCCs in a short time. 2) Synchronization: The two mutually hot-standby PCC control systems must maintain synchronization. During the switch between primary and backup roles, the output control of the hardware devices must be smooth and undisturbed. This cannot be guaranteed by a purely symmetrical hardware structure design; targeted software task scheduling design and data synchronization processing are necessary. This is the core of the entire system software. 3) Symmetry: This system design not only needs to achieve hardware symmetry but also pure software symmetry and uniformity. That is, the internal software of the two PCCs must be completely identical. This is a customer requirement that allows users to perform normal daily hardware and software maintenance after the system is put into operation. 4) Openness: This system needs to connect to the control computer touchscreen via a standard industrial Ethernet interface to enable real-time monitoring of the servo control system by the air traffic control radar's upper-level control computer. Based on the above principles, the following is a schematic diagram of the data synchronization and task scheduling process of our designed dual-machine hot standby software: According to the above process design, the precise timing of data synchronization and task scheduling of this dual-machine hot standby software can be described as shown in the following diagram: 3. Postscript The above dual-machine hot standby application system using two PCCs breaks away from the conventional "ultra-luxurious" hardware and software design architecture of dual-machine hot standby in the industrial field. It achieves small-scale, practical application requirements with a low-cost and economical solution. The system has been successfully deployed in the air traffic control radar networks of multiple civilian and military airports in Northeast and North China over several years. It has partially replaced the imported air traffic control radar products from developed countries in Europe and America that currently dominate the market. With its excellent performance in reliability, stability, and ease of maintenance, it has won unanimous praise from customers and demonstrated a promising market prospect. It is currently in the domestic promotion stage. References: 1. *Ruggedized and Enhanced Controller Product Selection Manual*, Shanghai Yingshuo Automation Technology Co., Ltd. 2. *PCC Dual-Machine Hot Standby Software Manual*, Shanghai Yingshuo Automation Technology Co., Ltd. 3. *Programmable Computer Controller Technology*, Qi Rong, Xiao Weirong 4. *Introduction to ALENIA Air Traffic Control Radar Maintenance Experience*, Civil Aviation Administration of Southwest China, Sichuan Provincial Administration