The difference and connection between the three serial communication serial physical interface standards of RS

1. RS-232-C is a serial physical interface standard developed by the Electronic Industry Association (EIA). RS is an abbreviation of the English "recommendation standard", 232 is the identification number, and C is the number of revisions. The RS-232-C bus standard has 25 signal lines, including one main channel and one auxiliary channel. In most cases, the main channel is mainly used. For general duplex communication, only a few signal lines are needed, such as a strip. Send line, one receive line, and one ground line. The data transmission rate specified by the RS-232-C standard is 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600, 19200 baud per second. According to the RS-232-C standard, the driver is allowed to have a capacitive load of 2500pF, and 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 capacitance per meter 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, which has common ground noise and cannot suppress common mode interference. Therefore, it is generally used for communication within 20m.

2. RS-485 bus, RS-485 serial bus standard is widely used when the communication distance is required to be several tens of meters to kilometer. RS-485 uses balanced transmit and differential receive, so it has the ability to reject common-mode interference. In addition to the high sensitivity of the bus transceiver, it can detect voltages as low as 200mV, so the transmitted signal can be recovered beyond the kilometer. RS-485 uses a half-duplex mode of operation, and only one point can be sent at any time. Therefore, the transmitting circuit must be controlled by an enable signal. RS-485 is very convenient for multi-point interconnection and can save many signal lines. Applications RS-485 can be networked to form a distributed system that allows up to 32 drives and 32 receivers to be connected in parallel.

3. RS422 bus, RS485 and RS422 circuit principle are basically the same, are sent and received in differential mode, do not need digital ground. Differential operation is the fundamental reason for the long transmission distance under the same rate condition. This is the fundamental difference between the two and RS232. Because RS232 is a single-ended input and output, at least three lines of digital ground transmission line and acceptance line are required for duplex operation. (Asynchronous transfer), you can also add other control lines to complete synchronization and other functions. RS422 can work and receive without full-duplex operation through two pairs of twisted pairs. RS485 can only work half-duplex, and transmission and reception cannot be performed at the same time, but it only needs one pair of twisted pairs. RS422 and RS485 can transmit 1200 meters at 19kpbs. A device can be connected to the line on the new transceiver.

In the past, PC and smart device communication mostly relied on RS232, RS485, Ethernet, etc., mainly depending on the interface specification of the device. However, RS232 and RS485 can only represent the physical media layer and link layer of communication. If you want to achieve bidirectional access to data, you must write your own communication application. However, most of these programs cannot conform to the ISO/OSI specification and can only be implemented. A single feature that works with a single device type and the program is not versatile. In the equipment network of RS232 or RS485 equipment, if the number of equipment exceeds 2, RS485 must be used as the communication medium. The communication between the RS485 network equipments can only be realized through the "master" equipment transfer. The master device is usually a PC, and only one master device is allowed in the device network, and the rest are all slave devices. The fieldbus technology is based on the ISO/OSI model and has a complete software support system that can solve problems such as bus control, collision detection, and link maintenance. Fieldbus devices are automatically networked, with no master/slave devices or multiple masters. Products from different manufacturers can be interchanged at the same level, and interoperability between devices.