What is RS-485 interface? What are its characteristics compared with RS-232-C interface? Due to the early appearance of RS-232-C interface standard, there are inevitably some 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, and because it is incompatible with TTL level, it is necessary to use a level conversion circuit to connect with TTL circuit. (2) The transmission rate is low. In asynchronous transmission, the baud rate is 20Kbps. (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 noise interference resistance is weak. (4) The transmission distance is limited. The standard value of the maximum transmission distance is 50 feet, but it can only be used at about 50 meters. In view of the shortcomings of RS-232-C, some new interface standards have emerged, and RS-485 is one of them. It has the following characteristics: 1. The electrical characteristics of RS-485: the logic "1" is represented by the voltage difference between two lines as+(2-6) V; The logic "0" is represented by the voltage difference between the two lines being-(2-6) V. The interface signal level is lower than that of 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 can be easily connected with TTL circuit. 2. The highest data transmission rate of RS-485 is 10 MB PS3. The RS-485 interface is a combination of balanced driver and differential receiver, which enhances the anti-common mode interference ability, that is, the anti-noise interference ability is good. 4. The standard value of the maximum transmission distance of RS-485 interface is 4,000 feet, which can actually reach 3,000 meters. In addition, RS-232-C interface is only allowed to connect one 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 the ability of multi-station, so that users can easily establish the equipment network by using a single RS-485 interface. RS-485 interface is the preferred serial interface because of its good anti-noise interference, long transmission distance and multi-station capability. Because the half-duplex network composed of RS485 interfaces generally only needs two wires, RS485 interfaces all adopt shielded twisted pair transmission. The 9-core plug socket of DB-9 is used for RS485 interface connector, DB-9 (hole) is used for interface with intelligent terminal RS485, and DB-9 (pin) is used for keyboard interface RS485 connected with keyboard.