Serial port is a kind of interface standard, which stipulates the electrical standard of the interface. Simply speaking, it is only a standard of the physical layer. The interface plug-in cable and the protocol used are not specified, so as long as the interface plug-in cable we use conforms to the serial port standard, it can be used flexibly in practice, and various protocols are used for communication and equipment control on the serial port interface standard.Therefore, RS485 has a wide range of applications and strong versatility. Typical serial communication standards are RS232 and RS485.
RS232 (also known as EIA RS232) is one of the commonly used serial communication interface standards, which was jointly formulated by the American Electronic Industry Association (EIA), Bell Systems Corporation, modem manufacturers and computer terminal manufacturers in 1970.
It has the characteristics of less signal lines, flexible baud rate selection, negative logic transmission and long transmission distance.
One of the computer communication interfaces. Usually the RS232 interface appears in the form of 9 pins (DB-9) or 25 pins (DB-25). Generally, there are two sets of RS232 interfaces on personal computers, which are called COM1 and COM2 respectively.
The connector uses a DB-25 25-pin socket. The RS232 interface connecting some equipment and PC, because it does not use the transmission control signal of the other party, only needs three interface lines, namely "send data", "receive data" and "signal ground". Therefore, the 9-core plug socket of DB-9 is adopted, and the transmission line is shielded twisted pair.
Intelligent instrument is developed with the maturity of single chip microcomputer technology in the early 1980s, and now the world instrument market is basically monopolized by intelligent instrument. The reason is the need of enterprise informatization, and one of the necessary conditions for enterprises to select instruments is to have a networking communication interface.At first, it was a simple process quantity of data analog signal output, and then the instrument interface was RS232 interface, which could realize point-to-point communication, but this way could not realize networking function. The subsequent RS485 solved this problem.
RS485 has no specific physical shape, and the interface is adopted according to the actual situation of the project.
Working principle of RS232 interface:
Two key problems must be solved in the intelligent converter, that is, how to obtain the power required by the power supply and the RS485 and RS422 interface drive from the RS232 line, and how to intelligently control the transceiver enable of RS485 and RS422.
Working principle of RS485 interface:
Coping with common-mode interference with differential transmission
When the RS485 chip receives a differential voltage greater than 200 mV, the RO pin outputs a low level, while the DI pin itself can output a differential voltage of 1.5V. Therefore, there is enough margin to support long-distance transmission and interference of various environmental noises.
In order to eliminate signal reflections in communication cables, twisted pairs are typically used to avoid interference. For longer connections, it is common practice to shunt termination resistors at the beginning and end of the RS485 bus, since the characteristic impedance of most twisted-pair cables is around 120 Ω.
1. RS232: The signal level value of the transmission level signal interface is high (the signal "1" is "-3 V to -15 V", and the signal "0" is "3 to 15 V"), which is easy to damage the chip of the interface circuit. Because it is not compatible with TTL level (0 ~ "< 0.8 V", 1 ~ "> 2.0 V"), it is necessary to use level conversion circuit to connect with TTL circuit. In addition, the anti-interference ability is poor.
2. RS485: The logic "1" of the transmission differential signal is represented by the voltage difference between the two lines being + (2-6) V; Logic "0" is represented by a voltage difference of- (2 — 6) V between the two lines. The interface signal level is lower than that of RS-232, so that the chip of the interface circuit is not easy to be damaged, and the level is compatible with the TTL level, so that the interface signal level can be conveniently connected with the TTL circuit.
1. RS232: The transmission distance of RS232 is limited. The standard value of the maximum transmission distance is 15m, and only point-to-point communication is available. The maximum transmission rate is 20kB/s.
2. RS485: The maximum wireless transmission distance of RS485 is 1200m. The maximum transmission rate is 10Mbps, and the maximum communication distance can only be achieved at the transmission rate of 100Kb/S.
The special cable with impedance matching and low attenuation can reach 1800 meters! Over 1200 meters, repeaters (up to 8) can be added, so that the transmission distance is close to 10 Km.
RS232: Three-core twisted pair, three-core shielded wire, etc. can be used.
RS485: Two-core twisted pair, two-core shielded wire, etc. can be used.Ordinary twisted pair can be used in low-speed, short-distance and interference-free occasions. On the contrary, in high-speed and long-line transmission, RS485 special cable with impedance matching (generally 120Ω) (STP-120Ω (for RS485 & CAN) a pair of 18 AWG) must be used. Armored twisted pair shielded cable (ASTP-120Ω (for RS485 & CAN) one pair of 18AWG) shall also be used in harsh interference environment.
RS232: The RS232 interface is only allowed to be connected to one transceiver on the bus, and cannot support multi-station transceiver capability, so it can only be used for point-to-point communication, and does not support multi-point communication.
RS485: The RS485 interface allows the connection of up to 128 transceivers on the bus. That is to say, it has the ability of multi-station communication, so that users can easily establish equipment network by using a single RS485 interface.
