RS232 is an asynchronous transmission standard interface. Usually the RS-232 interface is available in 9-pin (DB-9) or 25-pin (DB-25) versions. The most common connection method for RS232 is three lines: one transmission line, one reception line and one ground line. Level signal: logic 1 (MARK) = -3V ~ -15V, logic 0 (SPACE) = +3 ~ +15V Transmission distance: RS-232-C standard stipulates that the drive is allowed to have a capacitive load of 2500pF. The communication distance will be limited by this capacitor. For example, when using a 150pF/m communication cable, the maximum communication distance is 15m; if the cable is charged per meter As the capacity is reduced, the communication distance can be increased. Another reason for the short transmission distance is that RS-232 is a single-ended signal transmission, there is common ground noise and can not suppress common mode interference (the signals on the two transmission lines become larger or smaller at the same time), so it is generally used within 20m. Communication. RS232 cannot implement multi-machine communication. Reason: RS232 is a full-duplex communication mode. The general connection method is TTL level "--level conversion chip--" RS232 level. When the line is idle, the TTL levels sent and received are high. The RS232 level obtained after the level shifting chip is always low. Now suppose that A is the master, B and C are the slaves, then A wants to send a signal to B when there is no problem, because the TXD line of A is connected to the RXD line of B and C, at this time, the RXD lines of B and C are both Normally waiting for receiving (idle) state, but if B wants to reply to A if it receives a signal, the TXD line of B and the TXD line of C are connected with the RXD line of A, and B is transmitting data at this time. The state (B wants to control the change between its own TXD line high and low level), however, C is in the idle state at this time (C wants to ensure that its TXD line is low level), this time the TXD line of B and C There is a communication conflict between them. Therefore, RS232 can implement point-to-point communication, but this method cannot realize the function of networking. Transmission rate: RS232 transmission rate is slower, and it can be less than 1Mbps. RS485 generally uses two-wire transmission: A and B transmission lines. Level signal: -2V to -6V means "0", +2V to +6V means "1", and the voltage is AB voltage. Transmission distance: Generally, there is no problem within 1Km. In theory, the maximum transmission distance of RS485 can reach 1200 meters when the communication rate is 100Kpbs or less, but the distance transmitted in practical applications is also different due to the transmission characteristics of chips and cables. In the transmission process, the signal can be amplified by adding a relay method, and up to eight relays can be added, that is to say, the maximum transmission distance of RS485 can reach 9.6 kilometers. If you really need long-distance transmission, you can use fiber as the propagation medium, and add a photoelectric converter to each end of the transceiver. The transmission distance of multimode fiber is 5~10km, and the propagation distance of single-mode fiber can reach 50km. RS485 can achieve multi-machine communication. Reason: RS485 is a half-duplex communication mode, that is, time-sharing is implemented. In the idle state of the bus, it is necessary to ensure that the state is logic 1, that is, the voltage of AB conforms to the level of logic 1. Suppose that 1 is the host at this time, 2 and 3 are slaves, the connection mode of the data lines is 1, 2, 3 A is connected together, and 1, 2, 3 B are also connected together, there is no problem of RS232 connection. . 　　 supplement: Synchronous transmission is a data transmission method in which data blocks are transmitted. In this mode, the time interval between data blocks and data blocks is fixed, and their time relationship must be strictly defined. A special character or bit sequence is added to the header and trailer of each data block to mark the beginning and end of a data block, and a check sequence is usually added to perform error control on the data block. Asynchronous Transmission: Asynchronous transmission divides bits into groups for transmission. The group can be 8 characters or 1 character longer. The sender can send these groups of bits at any time, and the receiver never knows when they will arrive. A common example is the communication between a computer keyboard and a host computer. The difference between synchronous and asynchronous is mainly because the clocks of the sending and receiving parties are synchronized. Baud rate calculation: If the baud rate is set to 115200, the data bit is 8bit, the start bit is 1bit, the end bit is 1bit, and the check bit is 1bit. The characters that can be transmitted continuously for 1 s (1 bit start bit + 8 bit data bit + 1 bit check bit + 1 bit end bit, total 11 bits) is 115200/11 = 10472; 10472/1024 result is about 10.227 so the rate is about 10kB /ps. Typical serial communication standards are RS232 and RS485. They define voltage, impedance, etc. However, the definition of the software protocol is not distinguished from RS232. The characteristics of RS485 include: 1. Electrical characteristics of RS-485: Logic "1" is represented by a voltage difference between two lines of +(2-6)V; logic "0" is represented by a voltage difference between two lines - (2-6)V . The interface signal level is lower than that of RS-232-C, and it is not easy to damage the chip of the interface circuit, and the level is compatible with the TTL level, which is convenient for connection with the TTL circuit. 2. RS-485 data maximum transmission rate is 10Mbps 3. RS-485 interface is a combination of balanced driver and differential receiver, which has enhanced anti-common mode dry capability, that is, good anti-noise interference. 4. The maximum transmission distance of the RS-485 interface is 4000 feet, which is actually up to 3000 meters. In addition, the RS-232-C interface allows only one transceiver to be connected to the bus, that is, the single station capability. The RS-485 interface allows up to 128 transceivers to be connected on the bus. That is, it has multi-station capability, so users can easily establish a device network with a single RS-485 interface. Because RS-485 interface has good anti-noise, long transmission distance and multi-station capability, it has become the preferred serial interface. Because the half-duplex network consisting of RS485 interfaces generally only needs two connections, the RS485 interfaces are transmitted by shielded twisted pair. RS485 interface connector adopts DB-9 9-pin connector, DB-9 (hole) is used for RS485 interface with smart terminal, and DB-9 (pin) is used for keyboard interface RS485. Car Phone Holder,Mobile Holder For Cars,Mobile Phone Holder For Dashboard,Mobile Phone Holder For Car Dashboard Ningbo Luke Automotive Supplies Ltd. , https://www.car-phone-holder.com
What is the difference between the RS232 protocol and the RS485 protocol?
RS232