Based on ZigBee and ModBus distributed electronic police system (1)

0 introduction

With the development of a modern city, both in developing or developed countries, heavy traffic, traffic accidents, traffic issues such as the deterioration of the environment is becoming increasingly serious.

Intelligent transportation system (Intelligent Trans-portation System, ITS) has become increasing transport efficiency of important technical means. Electronic police system is intelligent traffic enforcement system components, as evidence of violation of the traffic management system, the actors have played an active role in the specification.

Based on ZigBee and ModBus distributed electronic police system, combined with a variety of innovative technology that is relevant to today's and future direction of its next-generation electronic police system.

1 ZigBee network programme

1.1 ZigBee intro

ZigBee is a group of approved based on IEEE 802.15.4 wireless standards adopted by the development of the networking, security and application software related technical standards.

Relative to existing wireless communication technology, ZigBee is the lowest power consumption and cost of technology. Suitable for short distances, a small area of the control based on wireless communication fields, has a broad application prospects.

1.2 system of ZigBee network project

IEEE 802.15.4 and ZigBee specification explicitly defines three topologies: star structure, tree structure and reticular formation.

Star network consists of a Coordinator (full function device FFD) and multiple terminal device node (simplified functional device RFD), Coordinator of the network, responsible for handling the entire Centre network, the terminal equipment is located in the coverage of the different locations. Star network control and synchronization of relatively simple, applicable to the device number is relatively small. Tree-structured network is an extension of the star network, is a multi-level structure of the star. Mesh network is a special way by relay transmission point-to-point network structure, the routing can automatically build and maintain. Tree and mesh network to be able to combine the two to form a mixed network, apply to the network are extremely complex.

System of ZigBee network communication section consists of one master controller, some from the controller.

System main, relatively independent from the controller, the master, slave controllers through ZigBee wireless mesh communications, host controllers to communicate with remote wireless communication with control centre. Network topology diagram shown in Figure 1.

The system uses ZigBee star network topology.

Star network to the network coordinator, all devices can be used only with network coordinator for communication, so the star formation process of the network, the first step is to establish a network coordinator. Main controller as a network coordinator is first activated, then the surrounding nodes send broadcast from the controller, and to network selection a unique identifier; from controller node receives the master controller to broadcast, to the access application and waits for the master node response; the main controller node receives a network application, will be made according to the request information is allowed to join the network of the judgment, if allowed to, main controller node will issue the request response, told from the controller node. From controller node after the receipt of a request-response, you get a master node is assigned to one of its network address as the only within the network, thus successful identity to join the network, the network is complete.

2 ModBus bus control system

2.1 system overview

ModBus Protocol is applied to the electronic controller on the World Wide Web.

Through this agreement, controller, controller with each other through the network (e.g., Ethernet) and other devices can communicate. The Protocol defines a controller can understand the use of message structure that describes a controller requests access to other devices, developed a message domain structure and content of the common format. ModBus has become a common industry standards, is an effective support for distributed control and real-time control of serial communication network. Electronic police system of vehicle detection unit, light detector, HD cameras, embedded computer, cell occupies the same ModBus fieldbus. The system diagram as shown in Figure 2.

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