IEEE802.11b wireless measurement and control system based on EPA protocol

　　introduction

　　The EPA (Ethernet for Plant Automation) standard is an industrial control network communication standard based on information network communication technologies such as Ethernet, wireless LAN and Bluetooth. It is suitable for data communication between industrial automation control system devices and instruments, and between industrial automation instruments.

　　802.11b is a flexible data transmission system that uses wireless radio frequency (RF) technology to send and receive data over the air, reducing the use of cable connections. Therefore, the 802.11b wireless network system can achieve the purpose of building a computer network system and allow devices to be freely arranged and moved at will. These characteristics of the 802.11b protocol give it a good application prospect in the field of industrial control. It can not only realize industrial remote control, remote adjustment, and remote measurement functions, but also realize real-time monitoring of remote images of industrial control sites, and can centrally manage the working status of various industrial control equipment to meet the requirements of the development of industrial control automation. This article mainly discusses the implementation and measurement and control of the EPA protocol in IEEE802.11b wireless field equipment.

　　1 Design of 802.11b wireless measurement and control system

　　As shown in Figure 1, the 802.11b wireless measurement and control system includes an EPA protocol communication module, an EPA host computer monitoring module, a wireless gateway module, and an LCD touch display module. In this system, the temperature transmitter collects the temperature change data of the thermocouple in real time through the serial port, and after the data is encapsulated by the EPA protocol stack, it is sent to the 802.11

b The wireless network sends the EPA message to the EPA wireless handheld device for display, and transmits it to the EPA host computer and other EPA devices in the EPA wired network through the wireless gateway to process the EPA message data.

　　

　　In addition to battery-powered handheld devices, the system also uses an Ethernet power supply module to transmit DC power through Ethernet, which simplifies the system design and improves the system reliability. Finally, the designed equipment (802.11b wireless gateway based on MPC852T, 802.11b wireless handheld device based on S3C2410 and 802.11b wireless temperature sensor based on S3C2410) constitutes a complete and applicable wireless measurement and control system.

　　2 Software Implementation of 802.1b Wireless Measurement and Control System

　　2.1 IEEE802.11b wireless gateway software design

　　The implementation of 802.11b communication protocol on embedded platform requires relatively high system performance. Therefore, this paper selects the hardware platform based on Motorola MPC852T processor (including Ethernet power supply module and 802.11b communication interface, etc.) to develop IEEE802.11b wireless gateway.

　　To realize the function of wireless gateway, the system software structure can be divided into the following modules from top to bottom: system configuration module, system monitoring module, bottom-level driver, data transceiver module, data forwarding module, command line module (Command line), IEEE802.1x authentication, authorization module (Authentication Authorization) and WEB management module. [page]

　　The wireless gateway software functional module flow is shown in Figure 2.

　　

　　The role of the wireless gateway in this system is to convert the data frame format between Ethernet and 802.11b wireless LAN, and it is required to be able to forward data between different network segments. This requires transplanting the Bridge module in the software module of the wireless gateway, compiling it in the embedded Linux system, and setting it after transplanting the Bridge module:

　　brctl addbr br0

　　brctl addif br0 eth1

　　brctl addif br0 wlan0

　　Connect the wireless and wired networks together, and then use the tool software ifconfig to perform corresponding network configuration on the wired network card and wireless network card, and the wireless gateway can operate normally.

　　2.2 IEEE802.1 Wireless Handheld Device Software Design

　　During the development process, wireless handheld devices are required to be small and flexible. The relatively small but powerful S3C2410 is used as the development platform. Based on this platform, field devices such as 802.11b temperature sensors and 802.11b wireless handheld devices are developed.

　　First, transplant the equally small and flexible USB wireless network card on the S3C2410 platform. The chip of this wireless network card is the Prism series chip. The wireless network card is the main device of the mobile terminal. Its software part is divided into three parts: interface driver module (Prism_usb.o), wireless network card driver (P802.o) and wireless network card configuration management software. Step 1: Download the source code of the wireless network card Linux-wlan-ng, which is the source code for PC. To make it run on ARM, cross-compile it. Step 2: When cross-compiling the USB wireless network card driver, you must first configure the network type, kernel path and cross-compilation tool accordingly. This is a kernel source code for 2.4.x. Errors will occur during the compilation process because the generated mkmetadef and mkmetastruct are for ARM arch and cannot be executed on i386. You must modify the automatically generated Makefile, or make a set of Makefiles suitable for i386. After the cross-compilation is passed, two modules, USB interface driver Prism_usb.o and wireless network card driver P802.o, are generated. Step 3: Load these two modules in the kernel of the ARM Linux system. There are two ways to load, dynamic loading and static compilation into the kernel. The author uses the dynamic loading method and uses the NFS Mount method to debug the modules to be loaded. Step 4: Copy the wireless network card configuration file rc.wlan and tool software wlanctl-ng, wland, nwepgen, wlancfg, etc. to the corresponding directory, and use them to perform system authentication, key authentication, WEP encryption and SSID settings on the 802.11b wireless network.

References:

[1]. MPC852T datasheet http://www.dzsc.com/datasheet/MPC852T_1032422.html.