Network application equipment and data communication system are used to realize end-to-end data transmission, which is mainly used to express information in the form of traditional media, such as sound, image, animation, etc. These transmission endpoints can be called communication interfaces. Communication interface mainly refers to the interface between the central processing unit and the standard communication subsystem. Next, let's talk about the main communication interfaces of the industrial Android motherboard that jinhaichuang has been promoting, as well as their characteristics and functions.

     Universal asynchronous transceiver, usually called UART. UART is a kind of asynchronous serial communication protocol. Its working principle is to transmit each character of transmitted data bit by bit. It converts the data to be transmitted between serial communication and parallel communication. As a chip that converts parallel input signals into serial output signals, UART is usually integrated into the connection of other communication interfaces. UART is a universal serial data bus for asynchronous communication. The two-way communication of the bus can realize full duplex transmission and reception. In embedded design, UART is used for communication between host and auxiliary equipment.

     When sending data signals, a synchronous clock signal will be sent at the same time to synchronize the data sampling frequency of the sender and the receiver. During synchronous communication, the signal line 1 is a synchronous clock signal line, which switches levels at a fixed frequency with a frequency period of T. after the rising edge of each level, the data signal line 2 sent synchronously is sampled (high level represents 1, low level represents 0), and the output data information is obtained according to the level of the sampled data. If both parties do not synchronize the clock, the receiver will not know the sampling period and will not be able to obtain data information normally.

     In asynchronous communication technology, the data sender and the data receiver have no synchronous clock, only data signal lines, but the sender and receiver will sample data according to the negotiated protocol (fixed frequency). The data sender sends data at the speed of 57600bits per second, and the receiver also receives data at the speed of 57600bits, so as to ensure the effectiveness and correctness of the data. Baud rate is usually used in asynchronous communication to specify the transmission speed of both parties, and its unit is BPS (bits per second).

     Serial communication is like a column, with each data element arranged vertically in turn. During transmission, one bit is transmitted serially, and one bit is transmitted every clock cycle. This transmission method is relatively simple and slow, but the number of buses is small. Usually, one receiving line and one sending line can realize serial communication. Its disadvantage is to add additional data to control the beginning and end of a data frame.

     Parallel communication is like a row of lines, going hand in hand and transmitting at the same time. The amount of data transmitted in each clock cycle of this communication mode is proportional to its bus width, but the implementation is more complex. UART communication adopts serial mode.

   RS232 interface is one of the commonly used serial communication interface standards. Due to the early emergence of RS232 interface standard, it is inevitable to have shortcomings, mainly including the following four points:

1. The signal level of the interface is high, which is easy to damage the chip of the interface circuit. The voltage of any signal line of RS232 interface is a negative logic relationship. It is incompatible with TTL level, so it is necessary to use level conversion circuit to connect with TTL circuit.

2. The transmission rate is low. In asynchronous transmission, the bit rate is 20kbps; Therefore, in the 51cpld development board, the baud rate of the integrated program can only be 19200, which is also the reason.

3. The interface uses a signal line and a signal return line to form a common ground transmission form. This common ground transmission is prone to common mode interference, so the anti noise interference is weak.

4. The transmission distance is limited. The standard value of the maximum transmission distance is 50 feet. In fact, it can only be used at about 15 meters.

   In industrial control occasions, RS485 bus is widely used because of its simple interface, convenient networking, long transmission distance and other characteristics. There are four main points:

1. Electrical characteristics of RS485: differential signal positive logic is adopted, and logic '1' is represented by + (2~6) V voltage difference between two lines; Logic '0' is represented by the voltage difference between the two lines as - (2~6) v. the interface signal level is lower than RS-232-C, so it is not easy to damage the chip of the interface circuit, and this level is compatible with TTL level, which is convenient to connect with TTL circuit.

2. The maximum data transmission rate of RS485 is 10Mbps.

3. RS485 interface is a combination of balanced driver and differential receiver, which enhances the ability of anti common mode interference, that is, good anti noise interference.

4. The maximum communication distance of RS485 is about 1219m, and the maximum transmission rate is 10Mbps. The transmission rate is inversely proportional to the transmission distance. The maximum communication distance can be reached only at the transmission rate of 100kbps. If you need to transmit a longer distance, you need to add a 485 repeater. RS485 bus generally supports 32 nodes at most. If a special 485 chip is used, it can reach 128 or 256 nodes, and the maximum can support 400 nodes.

     RJ45 interface is usually used for data communication transmission, and the most common application is network card interface. J45 interface is a commonly used Ethernet interface, which supports 10MB and 100MB adaptive network connection speed. There are two types of common RJ45 interfaces: DTE type for Ethernet card, router Ethernet interface, etc., and DCE type for switch, etc. We can call DTE 'data terminal equipment' and DCE 'data communication equipment'. In a sense, DTE equipment is called 'active communication equipment', and DCE equipment is called 'passive communication equipment'. When two devices of the same type use RJ45 interface to connect communication, cross line connection must be used.