GPRS-based wireless data acquisition and transmission terminal

Pick   Important: This article provides a wireless data acquisition and transmission terminal implementation scheme from the system, and discusses the technical difficulties in implementation in detail.

The application range of wireless data collection and transmission is very wide, involving industries such as electric power, water conservancy, public security, transportation, petroleum, security and finance. China Mobile officially launched the General Packet Radio Service GPRS network in May 2002 . The GPRS network supports the TCP / IP protocol and has a wide coverage. Compared with the use of short message and ultrashort wave wireless data transmission stations for wireless data transmission, GPRS has great advantages in terms of cost, reliability and implementability.

Terminal system composition

There are two schemes for the design of wireless data acquisition and transmission terminals: the first one is "Single Chip Computer + GPRS Modem". Although this scheme has low hardware cost, its functions are relatively limited, and there are certain difficulties in protocol development and support; The second set of solutions is "embedded CPU + GPRS module". Although this solution has a slightly higher hardware cost and requires an embedded CPU chip to support the embedded operating system, it can implement a rich protocol interface, which is convenient for transplantation and application upgrades to high-end systems. It is more convenient for the realization of data collection and transmission.

Figure 1 shows the principle reference diagram of a wireless data collection and transmission terminal, using Motorola Semiconductor ( Editor's Note, now renamed: Freescale) embedded CPU MPC8xx plus GPRS module CMS91 second set of implementation solutions.

The working principle of the terminal is to receive user data through the RS232/485 port, then type the data into IP packets, access the GPRS network through the GPRS module , and then send the data to the data processing center through various gateways and routes.

Figure 1  Reference diagram of terminal composition principle

The following is a detailed introduction to the composition principle in Figure 1 :

The embedded CPU chip is the core of the entire data acquisition terminal, which can well support the embedded operating system; considering the ease of transplantation and performance requirements of the embedded operating system, the current mature Motorola MPC8xx embedded CPU is adopted . Many operating system manufacturers have developed microcode and kits (BSP) for this type of CPU to facilitate user transplantation.

The GPRS module mainly completes the function of wireless Internet access. There are some mature products on the market, such as Sony/Ericsson 's GM47 ; Simens 's MC35, etc. Choose Cellon 's CMS91 here . It is a dual-band GSM/GPRS 10 -level module. The main advantages are: low power consumption, simple interface, complete AT command function, support for GPRS CLASS 10 , development of multimedia applications, lower price, etc. At the same time, it also provides SMS ( Short Message Service ) and voice functions. The GPRS module provides an RS232 interface, which can be used to complete the control of the module, such as dialing and switching modes. Once connected to the Internet through the module , the collected data can be sent to any host with a public IP address by TCP/IP transmission , so as to realize the wireless transmission of the collected data.

Data collection generally uses standard RS232 or RS485 interfaces to collect sensor data such as pressure and temperature. The CPU is responsible for calculating and processing the collected data, and then handing it over to the GPRS module to send it to the remote data center.

Watchdog is mainly used to prevent the terminal system from crashing. Write data to the WD hardware regularly through software . Once the system crashes, the software does not work normally, and the WD hardware generates a hardware interrupt because the data is not received, and the system automatically restarts.

ROM is mainly used to save embedded operating system, application programs and related configuration parameters, and directly communicate with the CPU through the internal bus . Here, Intel ’s 28F320-J3 , 32M bytes is selected. The file system can be completed on the ROM , but the file system processing module needs to be added to the operating system.

LCD is an optional module, used to display status information and so on. 10Base-T is also an optional module for easy debugging. You can also use NAT technology and 10Base-T port to use the terminal as a gateway to the Internet .

Technical difficulties in implementation

CPU communication port configuration

Choose MPC850 microprocessor in this design , It is a multi-purpose general-purpose chip, which integrates a microprocessor and common peripheral components, and can be used in various control fields. It is a low-cost implementation of MPC860 applied to communication systems, providing a higher cost performance, and enhancements in communication, such as the support of Universal Serial Bus (USB) . MPC850 integrates an embedded PowerPC core and a dedicated RISC communication processor module (CPM) for communication . The CPM of MPC850 supports 6 serial channels: a serial communication controller (SCC) , a USB , two serial management controllers (SMS) , an I2C interface, and a serial peripheral interface circuit SPI . Usually one SCC and two SMS can be configured as a universal serial port UART to control different modules and the rate is adjustable.

Implementation of File System on ROM

Ordinary ROM operations can only be operated with modules, which is very inconvenient to maintain, and because there is no buffer operation when reading and writing, data is easy to lose and make mistakes. File system implemented on ROM Each block can be allocated and used reasonably, reducing block migration and block overuse. In this design, Intel J3 series (28F320J3) flash memory is used as the storage device, Realized TrueFFS . First, configure the three-tier structure of TFFS in the system startup configuration file and modify the MTD . Initialize the file system and default path when the system starts. If successful, an operable file device symbol is generated. From then on, users can use the normal file operation methods ( generate, read, write, and delete ) . It should be noted that after reading and writing the file, the file should be displayed and closed to avoid data loss.

Implementation of Network Address Translation (NAT) when the device is used as a gateway

The use of NAT to access the Internet has changed the original way of using proxy software to access the Internet . Two NAT settings ( dynamic NAT and static NAT) can be used to achieve Internet access. Dynamic NAT enables all hosts to access the Internet . Because NAT has the function of hiding the internal topology of the network, external hosts cannot directly access internal websites or hosts. However, through the combined use of dynamic and static NAT , both internal and external visits can be realized, and the internal topology of the network can be hidden to ensure network security. In the specific implementation, first dial up the GPRS network through PPP to get the assigned address. If you get the internal IP address of the network communication allocated on the CPU communication port , then configure the NAT parameters, and configure two of the port parameters as the GPRS network address and the internal communication address. The last good NAT NAT initialized with configuration parameters.

Use of GPRS module working mode

There are three main methods of using CMS91 for data transmission :

1. SMS Messages -can be used to transmit character or binary data. Generally, the byte length of each SMS is 140 bytes. SMS is suitable for occasions where the amount of data is small and the real-time requirements are not high.

2. Data Calls -Transparent or non-transparent data transmission can be carried out after the data link is established. It is mainly suitable for occasions where the amount of data is relatively large and the number of transmissions is small.

3. GPRS data transmission-suitable for data transmission in all situations, it is the future development trend.

The use of CMS91 GPRS module for Internet- based data communication SMS and Data Call applications are very different. Both SMS and Data Call are supported by corresponding AT commands, and they are relatively simple to use and do not need to understand the actual operation process. , But for the GPRS data application, the knowledge of the network protocol involved is relatively more.

In this application, the CMS91 GPRS module is equivalent to an application system for wireless modem users. It needs to be connected to the operator’s Internet access server through PPP (LCP/PAP/IPCP) before using TCP/IP , UDP or more. The higher-level application layer programs http , FTP, etc. communicate. In the current GPRS application, if the application system is based on the operating system , the complete PPP protocol can be used due to the relatively strong system function , but if the application system uses the MCU , the simplified PPP protocol is generally used , which will cause some incompatibility. Rejected the information.

There are two structures for data transmission using GPRS , the main difference lies in the location of the server:

The server side adopts the host method on the ordinary Internet , or the server directly connects with the intermediate mobile network CMNET through DDN ( or other high-speed connection methods such as ASDL) .

According to the design and implementation technology introduced above, it is not difficult to write a complete data communication process based on the Internet using the GPRS module .

Concluding remarks

This article introduces a GPRS- based wireless data acquisition and transmission terminal implementation method from the system structure and implementation mode , especially focusing on how to use GPRS for data communication. Generally, wireless data acquisition terminals are used in dedicated systems, such as electricity, water conservancy, etc., which are particularly suitable for small data volume and low-speed data transmission requirements, and generally have higher requirements for working stability. Here are some ideas for cost and compatibility: 1. The embedded CPU chip is converted from MPC8xx to ARM chip. 2. The network supports upgrading from GPRS to CDMA , which can be realized by replacing the wireless communication module.

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