Low-cost remote monitoring system based on single-chip microcomputer

1 Introduction
With the continuous development of information technology, the application demand for remote monitoring of specific decentralized measurement and control objects is becoming increasingly widespread. The low-cost remote monitoring system designed with single-chip microcomputer as the core can be widely used in the command and control of field operations, enterprise production and military equipment, and can fully reflect the flexible networking, high reliability and maintainability, and satisfactory performance-price ratio.
2 System Overall Scheme Introduction
The system consists of a master station and several mobile slave stations. The master station is responsible for data acquisition, control commands and status information upload and download of the automated turntable and sensors. The slave station is located in a fixed machine room, unmanned room and field operation station, etc., and is responsible for receiving the indication data of the detection sensor, solving the production process equation after Karlman filtering, and related display and control processing. The master station and the slave station realize point-to-point code division multiple access communication, which can realize wired data communication (15km) and wireless time-division multiplexing digital/voice integrated communication (25km) supported by V/UHF radio.
The schematic diagram of the system working principle is shown in Figure 1. The circuit functions are as follows:
The master station and the slave station both use dual CPUs (80C196, 8031) to expand the STD bus structure, that is, in addition to the 56-core standard plug-in of STD, a 60-core dedicated system signal is customized. 0C000H-0C3FFH is allocated to the dual-port RAM (IDT7130) to facilitate transparent data exchange with the 8031 ​​of the data communication board. The non-full address decoding I/O space uses the online programmable chip ISP2031 for easy debugging.

·Payment station display and control circuit. MC6847 is used as the display and control main chip, and the display memory occupies 0A000-0C000H of the 80C196 address space. It is connected to an external industrial-grade CRT, and the display resolution is 256×192. According to the Chinese character fonts obtained on the microcomputer, a dedicated 12×12 non-standard Chinese character library is built. The main chip for keyboard control is 8279. With the support of application software, a table page or graphic monitoring effect similar to that
of an industrial computer can be obtained. ·Various I/O boards receive signals from the extended STD bus and complete the isolation drive of data, address and control signals. In order to prevent mutual interference between devices, relay isolation, photoelectric isolation, and transformer coil isolation supported by MC1413 are widely used on this type of circuit board to ensure the reliability of system operation. [page]
· Intelligent interface board, mainly with 3 MC6821 control chips as the core, forms 48 signals that can be defined by any bit (such as instantaneous IO direction, signal duration, etc.), and communicates data with the simulator. The circuit board can simulate the complex bus timing drive signal after widening.
· Turntable azimuth conversion circuit. The analog azimuth signal of the synchronous transmission machine is obtained by discrete component circuit, and the DC level is formed after the differentiation of coarse and fine signals. The multi-way switch is then controlled by the CPU, and the azimuth data is obtained by the built-in AD conversion circuit of 80C196. After identifying the synchronization pulse, the other sensor conversion circuits are directly counted by the high-performance counter and generate external interrupt signals for CPU processing.

3 Software and hardware design of wired/wireless data communication
The serial port of the microcontroller can only solve short-distance communication, but it is powerless for medium and long-distance communication. When the distance between each point exceeds 1 km or more, modulation and demodulation technology is needed to solve it, see Figure 2.
3.1 Wired data communication The
telephone line is used as the communication medium. The master and slave stations are both connected through the wired digital modem (Modem for short) to form FSK modulation signals.
In this solution, TCM3105 is used. It is a single-chip CMOS process Modem produced by TI. It complies with CCITT V. 23 recommendations and BELL202 standards. It contains basic modulation and demodulation, timing, carrier detection, and group delay equalization, etc. It can work in full-duplex or half-duplex mode. Its RD and TD pins can be directly connected to the serial port RXD and TXD of the microcontroller, and RDR1 and RDR2 are externally connected to the P1 port of 8031 ​​to set the baud rate (1200, 600bps). The RXB pin is used to adjust the bias voltage of the receiving signal and adjust the threshold level of the last comparator in the chip. The CDL pin is used to set the carrier detection level, so the potentiometers W1 and W2 should be adjusted accurately to ensure good transceiver performance. The CLK pin here is only used for observation and is generally not connected.
The modulation signal input RC and the output pin TC are isolated by two-stage op amps and 1:1 transformer coils to the external cable connector.
3.2 Wireless data and voice integrated communication
The transmission medium of wireless data transmission is electromagnetic waves, which is very suitable for applications such as complex terrain or mobile equipment, and can be time-division multiplexed with voice on a radio station. Its advantages are self-evident. Whether it is data or voice, V/UHF radio stations essentially transmit low-frequency baseband signals. MSM6927 is a single-chip integrated modem produced by OKI of Japan that uses FSK modulation. It complies with the ITU-TV. 23 communication protocol. Together with the 8031 ​​microprocessor and RF circuit, it can perform bidirectional reliable transmission of wireless link datagrams.
The sending RD and XD in Figure 2 are the data receiving and sending pins. In order to reuse the serial port of 8031 ​​with the wired modem, these two and the RD and TD pins of TCM3105 should be transferred to the RXD and TXD of 8031 ​​through the multi-way switch 54LS157. This is not drawn in the figure. The /CD2 pin indicates whether the carrier signal is received. The low level is valid and can be connected to the indicator light. The modulation signal input AIN and the output pin AO are fed to the radio after being processed by the op amp and low-pass filtered.
3.3 Communication software design
The communication adopts the method that the host first initiates a broadcast call, checks whether each slave is ready, and then deletes the absent one from the subsequent polling queue. Since the complex TCP protocol is not used, the rules for the main call and the response must be determined in advance, so as to avoid communication congestion caused by multiple field nodes calling at the same time. The data frame is defined as:

If there is no response from the calling party after the call, the call will be disconnected after the timeout mark is given. After receiving the data packet, if the extension finds that the address matches the local machine, it will accept it. If there is an error, it will be requested to resend or simply discarded depending on the importance of the information. During debugging, simulation communication with the microcomputer can be carried out to evaluate the control relationship and data response rate to reduce the workload of research and development.
4 Measures to improve reliability
· Appropriate redundancy design contributes to the reliability of military systems. For example, the wired/wireless communication, CRT and LET composite display, and backup channel control of this system have all played a good role.
· Lightning protection measures should be strengthened during field operations. This system was severely damaged by lightning, and then lightning protection devices were added, including lightning protection devices connected in series with wired data communication cables and power supply lightning protection devices. The system has withstood the test of severe weather.
· A special way to prevent keyboard deadlock. Keyboard deadlock is a common fault in industrial control systems and even home computers, but for military systems with relatively high real-time requirements, this phenomenon must be resolutely eliminated. This system has encountered a phenomenon that the 8279 was abnormally reset due to a small burr on the RST of the 8279, resulting in a keyboard deadlock. Therefore, the 8279 initialization code was added to the timer subroutine, that is, the 8279 was initialized once every 10ms, and the keyboard deadlock phenomenon did not occur again.
· Algorithm design optimization can also improve the usability of the system. For example, the "wild value" elimination processing after AD sampling, Karlman filtering, and error correction coding of data communication, this system uses the Hamming coding check and code sum combination method for error detection and correction.
5 Conclusion
The remote monitoring system designed with the above ideas has been mass-produced and equipped to the troops. It has been proved that it has fast startup, flexible networking, reliable operation, and has achieved good benefits.

References