Implementation of embedded remote communication mode of SMTP protocol based on ARM processor

In this topic, a new embedded remote communication mode is provided through the SMTP protocol. That is, the SMTP protocol is implemented in the ARM processor and connected to the Internet through twisted pair cables. The remote control equipment or instruments developed on this platform realize remote data transmission through the Internet, and can be remotely monitored and controlled through the Internet on any computer connected to the Internet.

The project selected a 32-bit microprocessor based on ARM architecture and introduced the various components of the module, including the ARM-based hardware structure model, software structure, basic structure of SMTP and its operating mechanism. The specific implementation and debugging process of the embedded SMTP software module in the ARM system was analyzed in detail.

1. Overall system structure

In the system, the SMTP server is introduced into the ARM platform to realize the embedded SMTP server (Embedded SMTP Server). Based on this software and hardware system, control equipment and instruments are developed to transform traditional test and control equipment into Internet-based network test and control equipment with TCP/IP as the underlying communication protocol and SMTP technology as the core. Compared with the traditional SMTP Server, the embedded SMTP system simplifies the protocol structure and integrates information collection and information release into the on-site measurement and control equipment. Since SMTP is a cross-platform standard communication protocol in the Internet, the SMTP server embedded in the device can receive E-mail instructions and data sent by any standard mail sending tool. At the same time, due to the openness and independent platform characteristics of SMTP technology, the design and maintenance workload of the software system and communication system is greatly reduced, the cost of personnel training is saved, and the management level of on-site test and control equipment is improved.

1.1 System Hardware Structure

The core components of the system hardware are composed of an embedded microprocessor system running an embedded operating system, and the core software of the system runs on the embedded operating system. Due to the mature technology of ARM processors, high market share, and sharply reduced costs, from the perspective of industrial control, we chose MOTOROLA's MX9328MX1 based on the ARM core. This chip is based on ARM9T. On the one hand, it has the advantages of low power consumption and high performance of ARM processors; on the other hand, it has rich on-chip resources, which is very suitable for the development of embedded products.

The hardware system includes a microprocessor that implements the SMTP communication function. It can be directly integrated with the front-end application system or connected to the application system through the field bus. It includes: 8M FLASH memory for storing the system kernel of the real-time operating system, TCP/IP protocol stack, various control programs and other parallel modules; 32M SDRAM memory for use during system operation; Ethernet interface for connection with the Internet/Intranet; field bus expansion control module for distributed control of devices within a certain range on site; general IO port control module, etc. Its hardware structure is shown in Figure 1 [2].

Figure 1: Embedded SMTP remote control platform hardware structure

Considering the cost-effectiveness, the hardware structure of the system is adjusted according to different application environments, and the hardware resources are also limited. The system kernel, TCP/IP protocol stack, and various application programs of the real-time operating system must be written into FLASH and loaded into SDRAM at runtime, which places high demands on the software system.

1.2 System Software Structure

The software system of the whole system consists of five parts: ① embedded operating system; ② virtual file system; ③ SMTP engine; ④ configuration module; ⑤ security module; ⑥ control program interface module.

The embedded operating system uses Linux as the software foundation of the system. Its excellent cross-platform transplantation capability, open source code, and flexible configuration provide convenience for development. The built-in TCP/IP protocol stack allows users to quickly develop application layer protocol programs.

The virtual file system on Linux builds a disk-like usage environment based on FLASH and SDRAM. The virtual file system uses data structures to store information such as file size and modification time. The control programs and configuration texts required in the system are stored in the form of files on the system chip through the virtual file system.

The SMTP engine is responsible for responding to user requests, enabling users to remotely control the actions of the device through the application program interface, reporting critical value status, and regularly reporting working status to users.

The configuration module allows system administrators to set device parameters remotely using emails. The configuration environment variables defined during system startup include device critical values, status report cycles, network parameters such as Socket ports, device start and stop times, working cycles, etc.

The configuration and control information access of the remote network device is the focus of security protection. The security module protects sensitive information by defining security domains on the server and defining usernames/passwords for each security domain. It can also take encryption measures for request control actions and data to achieve security protection functions. The system mainly implements protection measures through ESMTP. ESMTP is an extended SMTP protocol, which is a security authentication service opened by the mail server system to restrict non-official users of the system from using the system to distribute spam or other improper behaviors.

The application program interface module realizes the control and data exchange with the embedded control system. In this system, the application program interface communicates with the embedded operating system and realizes the configuration, monitoring and control of the embedded system through the device driver. It is the core of the system control. The monitoring program module is implemented using CGI (Common Gateway InteRFace).