Remote Communication System Designed Based on Single Chip Microcomputer 8251 and MODEM

1 Introduction

Most of my country's medium and large petrochemical enterprises use a small current grounding system for power supply, and the power system is relatively large. This type of system generally has several or even more than a dozen 35kV level total step-down stations, dozens of 6-10kV level high-voltage distribution rooms, and a wide distribution range. Some substations are more than ten kilometers away from the main dispatch. Therefore, the accurate and reliable transmission of the power parameters (voltage, current, phase, power factor, etc.) of each substation is particularly important for the main dispatch to make timely decisions, improve the quality of power supply, and ensure the safety of power operation. Here, we use a single-chip microcomputer to control the modem for remote real-time data transmission, and achieved satisfactory results.

2 Hardware system design
The system network diagram is shown in Figure 1. The host computer of this system adopts PII microcomputer, which is connected to the single-chip microcomputer system as the lower computer through MODEM←→ switching network←→MODEM. The data signal collected by the power sensor is input into the single-chip microcomputer system. After being processed by the software program, the asynchronous transceiver 8251 controls the MODEM to dial automatically. After the host computer responds, the data is sent out. After the host computer receives the data through the MODEM, the data table or graphic processing can be performed, which completes the whole process of remote data collection. The host computer can perform patrol control and detection on multiple lower single-chip microcomputer systems through the MODEM in time-sharing.

The hardware circuit of the single-chip microcomputer system is shown in Figure 2. It is mainly composed of a single-chip microcomputer 89C51 and an asynchronous/synchronous transceiver controller chip. Here, 8251 is used as an asynchronous transceiver. Among them, COM8046 is a programmable dedicated clock generator that can provide clock signals for 89C51 at the same time. The signals of the receiving clock RXC and the transmitting clock TXC required by 8251 are also provided by it.

Since the input and output of 8251 are TTL level, a serial transceiver driver is required for level conversion and driving. This system uses the more commonly used MC1488 and MC1489 chips. RS-232 is a standard 9-pin interface that can be connected to any standard external MODEM interface.
The performance requirements for MODEM are: telephone number parameter storage; automatic dialing function, automatic answering function; speed buffering and flow control; MNP5/V. 42bis data compression method; V4.2 error control protocol/MNP 2-4 error correction protocol; rate greater than 14.4kbps; select standard products compatible with the American Hayes series.
Since the telephone line as a switching network medium will inevitably have line interference, the MODEM as the two end points must adopt corresponding anti-interference and error control technology, and the line transmission rate should not be too high. This system uses a baud rate of 1200dps.
3 Software Design
The software design of the single-chip microcomputer system mainly consists of two parts: 8251 initialization and data transmission. The program flow charts are shown in Figures 3 and 4 respectively.

The initialization settings of 8251 are: working in asynchronous working mode; baud rate is 1200dps; data length is 8 bits, one stop bit; even parity is used; DTR and RTS signals are valid.
The control of MODEM is determined by DSR. When DSR is valid, the MODEM is reset first, and then a command is sent to make the MODEM dial automatically. When the DSR signal remains valid, data is transmitted with the host computer.
In the data transmission program, a group of storage units stores the data to be sent (that is, the data converted by the sensor input, or the manually input data of the extended keyboard system). When the data is sent, the checksum data is also sent. After sending the data, check whether the sending register is zero. If it is zero, it means that the data has been sent. After each data is sent, wait for the host computer to return the signal. If it is incorrect, it needs to be resent; if it is correct, send the next data again until the data is sent.
4 Conclusion
Using a single-chip microcomputer to control MODEM for remote data transmission has achieved good results and solved the problem of data exchange at remote locations that cannot be reached by the LAN dedicated line. It is particularly suitable for remote areas, such as docks, warehouses, and high tower inspections at remote locations of enterprises.
The system can be expanded on the basis of this hardware principle, that is, to add display, key input and single-chip output control, alarm and other functions, and can realize remote key input data and parameter setting through key function conversion to meet some special needs.
If different sensing methods are changed, it can be widely used in industry, agriculture and other social occasions. Therefore, this system has a broad market prospect.