Nowadays, various embedded MODEMs that are commercially available can realize data transmission functions through the Public Telephone Switching Network (PSTN), including standard serial and parallel interfaces, and have synchronous and asynchronous communication data transmission formats, and support multiple MODEMs. Standard protocols - V.92, V.90, V.34, V.32bis, V.22bis, etc., support error correction protocols such as V.44, V.42, MNP, and data compression protocols such as V.42bis and MNP5. First, communication technology Communication refers to the transfer of data between a computer and a computer or a peripheral device. Therefore, here the "letter" is a kind of information, which is a data or a batch of data that consists of numbers "1" and "0" and has certain rules and reflects certain information. Data communication involves the problem of transmitting data between two devices. Commonly used data communication methods are parallel communication and serial communication. When the distance is short and the transmission rate is high, parallel communication is usually adopted. When the device is far away, the data is often transmitted in a serial manner. (I) Parallel Communication and Serial Communication Parallel communication is relatively simple and can be divided into parallel communication with different number of bits (width), such as 8-bit parallel communication, 16-bit parallel communication, and so on. In parallel data transmission, 8 bits of parallel communication have 8 data bits transmitted simultaneously from one device to another, and the sending device transmits 8 data bits to the receiving device through 8 data lines. After the receiving device receives the data, it can be used without any changes. The characteristic of parallel communication is that each bit of data is transmitted or received at the same time. The data transmission of serial communication is transmitted bit by bit, so under the same conditions, the transmission speed is slower than that of parallel communication, but serial data transmission is often chosen in practical applications. Because serial communication requires only two wires to send or receive data, one for transmission and the other for reception. According to the different working modes of serial communication, the transmission and reception lines can also be combined into a transmission/reception multiplexing line (such as half duplex). The hardware that implements serial data transmission is economical and practical. (II) Serial data transmission method In serial data transmission, only one bit of data is transmitted from the source to the destination at a time. Compared with parallel data transmission that transmits several bits of data at the same time, serial data transmission has a slower transmission speed than parallel transmission. In serial data transmission, each person sequentially sends from the source to the destination. This requires synchronization between the data source and the data destination to distinguish each bit, character, and message. The data link will control the synchronization between the two stations. It requires additional information when sending bits, characters or messages from one station to another. This information allows the hardware clocks in the receiving and sending stations to be Synchronization ensures that signals sent from the source are correctly identified by the destination. There are two data transmission methods for serial data communication, namely asynchronous serial data transmission and synchronous serial data transmission. Asynchronous communication: The data format (also called serial frame) transmitted by asynchronous communication consists of 1 start bit, 5, 6, 7 or 8 data bits, 1, 1.5, 2 stop bits and 1 check bit. composition. The start bit convention is 0, and the idle bit convention is 1. The essence of asynchronous communication is that nails and B communicate using separate clocks. Each data begins with a start bit and ends with a stop bit. The start bit triggers both A and B synchronized clocks. One bit in each asynchronous serial frame is strictly synchronized with each other and the bit period is the same. Asynchronous communication relies on the start bit and stop bit to keep the communication synchronized. It has lower hardware requirements and is relatively simple and flexible to implement. Synchronous communication: The data format (also called synchronous string frame) transmitted by synchronous communication is composed of a plurality of data. Each frame has two (or one) synchronization characters as a start bit to trigger the synchronous clock to start sending or receiving data. . Free bits need to send synchronization characters. Synchronous communication relies on synchronization characters to keep the communication synchronized. Data and data in the data group do not need to insert synchronization characters and no gaps. Therefore, the transmission speed is fast, but an accurate clock is required to achieve strict synchronization between the sending and receiving parties. The hardware has higher requirements and is suitable for transmitting batch data. Second, the working principle of embedded MODEM MODEM is composed of sending, receiving, controlling, interface, control panel and power supply. The data terminal equipment provides the transmitted data in the form of a binary serial signal, is converted into an internal logic level by the interface and sent to the sending part, and is modulated by the modulation circuit to send the signal required by the line to the line. The receiving part receives the signal from the line, after filtering, anti-modulation, and level conversion, it is converted into a digital signal and sent to the digital terminal equipment. The telephone line allows the two parties to communicate with each other at a distance of several thousand kilometers. This is because there are relay amplification devices at certain intervals to ensure clear voice. If MODEM is reconfigured on these devices, data can be transmitted wherever there is a telephone call. The voice bandwidth of a typical telephone line is in the range of 300-3400 Hz, and it is used to transmit a digital signal whose signal frequency must also be within this range. Commonly used modulation methods include frequency shift keying (FSK), phase shift keying (PSK, DPSK), amplitude modulation (PAM, QAM), pulse code modulation (PCM), and the like. MODEM usually has three working modes: on-hook mode, call mode, and online mode. If the telephone line is not connected, it is an on-hook mode; both parties call by telephone is a calling mode; MODEM has been connected, and the data transmission is online. After the MODEM is powered on, it usually enters the on-hook mode first, dials the line after dialing, and then enters the call mode. Finally, it enters the online mode through the MODEM handshake process. The connection between the MODEM and the computer is an interface problem between the data circuit terminating equipment (DCE) and the data terminal equipment (DTE) of the data terminal equipment. The interface between DCE and DTE is an important issue in the use of computer networks. (a) DTE and DCE DTE (Data Terminal Equipment) is a device that has certain data processing capabilities and the ability to send and receive data. The DTE can be a computer or terminal, or it can be various I/O devices. Most data processing terminal devices have limited data transmission capabilities. If two DTE devices that are located far apart are directly connected, communication is often not possible. A data circuit terminal device (DCE) must be added between the DTE and the transmission line. , Data Circuit-terminal Equipment). The role of the DCE is to provide signal conversion and coding between the DTE and the transmission line and to establish, maintain, and release the data link connection. A typical DCE is a MODEM that is connected to an analog telephone line. Digital devices communicate through MODEM access to the telephone network using analog signals to transmit digital data. (B) RS-232C serial port The embedded MODEM is usually connected to the computer via the RS-232C serial port signal line. RS-232 allows a sending device to connect to a receiving device to transmit data; its original specification has a maximum transmission speed of 20 Kbps, but in fact, applications are now well beyond this speed range. RS-232 can be said to be a relatively simple communication standard. If you do not use hardware flow control, you can use a maximum of three signal lines to achieve full duplex transmission. RS-232C serial port signals fall into three categories: transmit signals, contact signals, and ground lines. 1. Transmit signals: Refer to TXD (transmit data signal line) and RXD (receive data signal line). 2. Contact signal: refers to the six signals of RTS, CTS, DTR, DSR, DCD and RI. Their respective functions are: RTS (Request to Send) is the contact signal sent by the PC to the MODEM. CTS (clear send) is the contact signal sent by the MODEM to the PC. DTR (Data Terminal Ready) is the contact signal sent by the PC to the MODEM. DSR (data ready) is the contact signal sent by the MODEM to the PC. It indicates the working status of the local MODEM. DCD (transmission detection) is a status signal sent from the MODEM to the PC. RI (ringing indication) is the status signal sent from the MODEM to the PC. 3. The ground signal (GND) provides the same potential reference point for the connected host and MODEM. Third, the modulation and agreement standard In the field of communications, a protocol refers to a set of common technical rules or specifications that the two parties to the communication should follow. If this set of rules or specifications is accepted by more users, it can be called Standard. The most basic function of MODEM is modulation and demodulation. In recent years, a series of technical standards have been developed. In addition, today's MODEM products, in order to increase the transmission speed, mostly incorporate compression and error correction techniques. (a) Standard MODEM protocol The basic function of the MODEM is to convert between the binary digital signal provided by the computer and the analog signal supported by the telephone network, so that the computer can use the telephone network to carry out long-distance data communication. The core of modem technology is how to increase the transmission speed of digital information in a limited-bandwidth (≤ 4KHz) telephone channel. This speed is often at the bit rate, which is the number of bits per second (bits per second, bps for short). )to measure. The earliest MODEM Bell103 was introduced by AT&T in 1958. It uses a simple frequency modulation technique FSK (Frequency Shift Keying), which only provides 300bps transmission speed. CCITT has issued a similar technical standard V.21 according to Bell103. In the 1970s, AT&T Bell 212 used a 4-DPSK technique combining AM and AM to achieve a transmission speed of 1200 bps. A similar CCITT standard is called V.22. Bell103 (V.21) and Bell212 (V.22) are now rarely used, but in order to be compatible with earlier MODEM, many MODEM still integrate these two technologies in the product as an option. In the mid-1980s, the CCITT V.22 bis standard was adopted by most MODEM manufacturers, and its 16-QAM (12 phase angle and 4 amplitude modulated quadrature modulation) modulation techniques achieved a transmission speed of 2400 bps. Next, the CCITT issued a V.32 standard that uses 32-TCM (Grid Code Modulation) technology to achieve a speed of 9600 bps. CCITT issued the V.32bis standard in 1991. V.32bis adopts 128-TCM modulation technology, can achieve a maximum transmission speed of 14400bps, and according to changes in line quality, back to 12000 bps, 9600bps, 7200bps and 4800bps and other four speed files to work. In 1993, CCITT introduced the V.34 standard and achieved a maximum speed of 28,800 bps. The standard can also be used as 28.8K/26.4K/21.6K/19.2K/16.8K/14.4K/12K/9600/7200/4800bps, etc. Speed ​​gear to slow down work. V.90 is a 56 Kbps data transmission standard developed by ITU-T. V.90 enables MODEM to receive data at up to 56 Kbps on the PSTN. The V.90 connection technology uses a bidirectional channel: upstream and downstream channels. The downlink (receiving) channel of the V.90 client MODEM can achieve a higher transmission speed of 56K. The V.90 standard supports 33.6 Kbps upstream rate. (II) Protocol Standards for Compression and Error Correction Techniques In order to further improve the transmission speed of the MODEM data, in addition to the continuous improvement of the above-mentioned modulation and demodulation technology, data compression technology has also been introduced into the MODEM in recent years. Error correction technology was introduced with the adoption of compression technology. Microcom's error correction and compression protocols are often abbreviated as MNP (Microcom Network Protocol), which consists of a series of independent error correction and compression protocols. Among them, MNP1 to MNP4 and MNP10 are error correction protocols, and MNP5 and MNP7 are compression protocols. MNP has become the industry standard for compression error correction technology. In 1988, the CCITT issued the V.42 correction standard. V.42 has MNP4 as one of its options. If one of the two MODEMs supports V.42 and the other supports MNP4, the two can automatically negotiate and perform MNP4 error correction processing. In terms of data compression protocols, MNP5 and V.42bis are the most popular. The V.42bis issued by CCITT in 1989 is a more efficient compression protocol. Since V.42bis has an automatic test function, it can automatically switch between compressed mode and transparent mode (without compression processing) through on-line testing. Therefore, V.42bis is more adaptable than MNP5. Compression technology and error correction technology are closely related. If V.42bis compression protocol is selected, MODEM will automatically enable V.42 error correction protocol. If MNP5 compression is used, MNP error correction is naturally used. IV. Conclusion The embedded MODEM can have all the functions of the traditional MODEM, use the telephone line (PSTN) to solve the problem of data transmission, and its small size, high reliability, flexibility and convenience, is very suitable for the terminal equipment between the traffic is not too large Communication. It has broad application prospects in distribution automation, remote meter reading, tax-controlled POS machines, and silver tax POS machines, and has a large market space. Http://