In data communication, computer network and distributed industrial control system, it is often necessary to use serial communication to realize data exchange. At present, there are rs-232,rs-485,rs-422 interface standards for serial communication. rs-232 is the earliest serial interface standard that can be used over short distances (< 15m), low baud rate serial communication has been widely used. Later, in view of the short communication distance and low baud rate of rs-232 interface standard, rs-422 interface standard and rs-485 interface standard are proposed on the basis of rs-232 interface standard to overcome these defects. rs-232,rs-422,rs-485 interface standards are described in detail below.
The rs-232 serial port standard is a single-ended standard for increasing communication distance in low-rate serial communications. rs-232 adopts unbalanced transmission mode, that is, single-ended communication. The data signal of the transceiver is relative to the ground signal. Therefore, its common mode rejection ability is poor, coupled with the distributed capacitance of the twisted pair, its transmission distance is about 15m, the highest rate is 20kbps, and it can only support point-to-point communication.
In view of the limitation of rs-232 serial port standard, rs-422 and rs-485 interface standards have been proposed. rs-485\/422 adopts balanced transmission and differential reception to achieve communication: the sending end converts the ttl level signal of the serial port into differential signal a and b two outputs, and the differential signal is restored to ttl level signal at the receiving end after transmission by cable. Because the transmission line usually uses twisted pair and differential transmission, it has a strong ability to resist common mode interference, and the bus transceiver is very sensitive and can detect voltage as low as 200mv. Therefore, the transmission signal can be recovered from kilometers away. The maximum communication distance of rs-485\/422 is about 1219m, the maximum transmission rate is 10mb\/s, the transmission rate is inversely proportional to the transmission distance, under the transmission rate of 100kb\/s, the maximum communication distance can be reached, if you need to transmit a longer distance, you need to add 485 repeater. rs-485 adopts half-duplex mode and supports multi-point data communication. rs-485 bus network topology generally adopts the terminal matching bus structure. That is, a bus is used to connect each node in series, and ring or star networks are not supported. If you need to use a star structure, you must use a 485 repeater or 485 hub. rs-485\/422 bus generally supports a maximum of 32 nodes, if the use of special 485 chip, can reach 128 or 256 nodes, the maximum can support up to 400 nodes.