RS232 interface is a serial communication standard developed by the American Electronic Industry Association (EIA) in 1970 in conjunction with Bell Systems, modem manufacturers and computer terminal manufacturers. Its full name is \"Technical Standard for Serial Binary Data Exchange Interface between Data Terminal Equipment (DTE) and Data Communication Equipment (DCE).\" The standard specifies the use of a 25-pin DB25 connector, the signal content of each pin of the connector is specified, and the level of various signals is specified. The serial port of DB25 generally only uses pins 2(RXD), 3(TXD) and 7(GND). With the continuous improvement of the equipment, the DB25 pin is rarely seen now. Instead, it is replaced by the interface of DB9. The pins used by DB9 are different from those used by DB25, namely 2(RXD), 3(TXD) and 5(GND). Therefore, the RS232 interface is now called DB9. In the market, the male connector is called DRXX, and the female connector is called DBXX, such as the serial port on our computer, which is called DR9 in the market, not DB9, many people mistakenly call DB9, in fact, DB9 is the interface that connects two DR9 to each other. In general, in textbooks or articles, we often call all serial device interfaces uniformly RS232 interfaces. Since the RS232 interface standard appeared earlier, it is inevitable that there are shortcomings, mainly the following four points: (1) the signal level value of the interface is high, easy to damage the chip of the interface circuit, and because it is incompatible with the TTL level, it is necessary to use the level conversion circuit to connect with the TTL circuit. (2) The transmission rate is low, in the asynchronous transmission, the baud rate is 20Kbps. (3) The interface uses a signal line and a signal return line to form a common transmission form, which is easy to produce common mode interference, so the anti-noise interference is weak. (4) The transmission distance is limited, the maximum transmission distance standard value is 50 feet, in fact, it can only be used in about 50 meters. In view of the shortcomings of RS232 interface, some new interface standards continue to appear, RS-485 is one of them, it has the following characteristics: (1) the electrical characteristics of RS-485: logic \"1\" to the voltage difference between the two lines is (2-6) V; The logical \"0\" is represented by the voltage difference between the two lines as -(2-6)V. The interface signal level is lower than RS-232, it is not easy to damage the chip of the interface circuit, and the level is compatible with the TTL level, and it can be easily connected with the TTL circuit. (2) The maximum data transmission rate of RS-485 is 10Mbps. (3) RS-485 interface is a combination of balanced driver and differential receiver, anti-common mode dry ability is enhanced, that is, anti-noise interference is good. (4) The standard value of the maximum transmission distance of the RS-485 interface is 4000 feet, in fact, it can reach 3000 meters, and the RS-232 interface only allows the connection of 1 transceiver on the bus, that is, single station capability. The RS-485 interface allows the connection of up to 128 transceivers on the bus. That is, it has multi-station capability, so that users can easily establish equipment networks with a single RS-485 interface. Because the RS485 interface consists of a half-duplex network, generally only two wires (we are generally called AB line), so the RS485 interface is shielded twisted pair transmission. Because some devices are 232 interface, some 485 interface, if there is a 232 interface device and a 485 interface device communication, it needs an RS232\/RS485 converter, the 232 interface of the device into a 485 signal, and then communicate with the 485 interface device. If it is two 232 interface equipment to communicate over a long distance, it is only necessary to add two RS232\/RS485 conversion circuits. The so-called \"passive RS232\/RS485 converter\" on the market uses the technology of stealing power from the computer serial port, while the \"active RS232\/RS485 converter\" is similar to the former in the circuit schematic diagram, but the power part has been changed. RS485 interface EMC Design Reference RS232 interface EMC Design Reference Differences between RS485 and RS232 protocols: 1. Electrical characteristics of RS-485: Logic \"1\" is represented by the voltage difference between the two lines as (2-6) V; The logical \"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, it is not easy to damage the chip of the interface circuit, and the level is compatible with the TTL level, which can be easily connected with the TTL circuit. 2. The maximum data transmission rate of RS-485 is 10Mbps3. The RS-485 interface is a combination of balanced driver and differential receiver, which enhances the ability to resist common mode dry, that is, good anti-noise interference. 4. The maximum transmission distance of the RS-485 interface is 4000 feet, which can actually reach 3000 meters, and the RS-232-C interface only allows the connection of 1 transceiver on the bus, that is, single station capability. The RS-485 interface allows the connection of up to 128 transceivers on the bus. That is, it has multi-station capability, so that users can easily establish equipment networks with a single RS-485 interface. RS-485 interface has the advantages of good anti-noise interference, long transmission distance and multi-station capability, which makes it the preferred serial interface. Because the RS485 interface consists of a half-duplex network, generally only two wires are needed, so the RS485 interface adopts shielded twisted pair transmission. The RS485 port connector uses the 9-pin connector of the DB-9, the DB-9 (hole) port of the RS485 port connected to the smart terminal, and the DB-9 (pin) port of the RS485 port connected to the keyboard.