RS232 and RS485 interfaces are typical serial communication standards. SONY Autofocus cameras are often used to transmit data. For the working principles and differences between the two interfaces, please refer to the details.
I. What is RS-232:
RS-232 (also known as EIA RS-232) is one of the commonly used serial communication interface standards. It was developed in 1970 by the EIA in collaboration with Bell Systems, modem manufacturers, and computer terminal manufacturers.
It has the characteristics of fewer signal lines, flexible baud rate selection, negative logic transmission and long transmission distance.
One of the computer communication interfaces, usually the RS-232 interface in the form of 9 pins (DB-9) or 25 pins (DB-25), generally personal computers will have two groups of RS-232 interfaces, respectively called COM1 and COM2.
The connector uses the DB-25 connector. Some devices connect to the PC RS-232 interface, because do not use the other side of the transmission control signal, only three interface lines, namely “send data”, “receive data” and “signal ground”. Therefore, the 9-core plug of DB-9 is adopted, and the transmission line is shielded twisted pair.
Two, what is RS-485:
RS485 does not have a specific physical shape. It uses an interface based on the actual project situation to realize networking functions.
Three, the working principle of the two interfaces:
Working principle of the 232 interface:
The intelligent converter must solve two key problems, that is, how to obtain the power from the RS-232 line and the power required to drive the RS-485RS-422 interface, and how to intelligentlycontrol the RS-485RS-422 transceiver enableability.
Working principle of 485 interface:
Differential transmission is used to deal with common-mode interference
When RS-485 chip receives differential voltage greater than 200mV, RO pin output low level, and itself DI can output 1.5V differential voltage. Therefore, there is enough margin to support long-distance transmission and various environmental noise interference.
Working principle of RS-485 chip
In order to eliminate signal reflection in the communication cable, twisted pair is generally used to avoid interference. For longer connections, it is common practice to attach terminal resistors at both the beginning and end of the RS-485 bus, since most twisted pair cables have characteristic impedance of around 120Ω.
Iv. Comparison of electronic characteristics of the two interfaces:
1. RS232: The signal level value of the transmission level signal interface is high (signal “1” is “-3V to -15V”, signal “0” is “3-15V”), which is easy to damage the chip of the interface circuit, and because it is incompatible with the TTL level (0~ “<0.8v”,1~ “>2.0V”), it is necessary to use the level conversion circuit to connect to the TTL circuit. In addition, the anti-interference ability is poor.
2, RS485: transmission differential signal logic “1” to the voltage difference between the two lines is + (2-6) V; Logical “0” is represented by the voltage difference between the two lines as – (2-6) V. If the interface signal level is lower than RS-232, it is not easy to damage the chip of the interface circuit, and the level is compatible with the TTL level, which can facilitate the connection with the TTL circuit.
V. Comparison of communication distance between the two interfaces:
1. RS232: RS232 transmission distance is limited, the maximum transmission distance standard value is 15 meters, and only point-to-point communication, the maximum transmission rate is 20kB/s.
2. RS485: The maximum wireless transmission distance of RS485 is 1200 meters. The maximum transmission rate is 10Mbps, and the maximum communication distance can be reached only at the transmission rate of 100Kb/S.
Using impedance matching, low attenuation dedicated cable can reach 1800 meters! Over 1200 meters, Repeaters can be added (up to 8), so that the transmission distance is close to 10Km.
Six, the communication line difference between the two interfaces:
RS232: Can use three core twisted pair wire, three core shielding wire, etc.
RS485: Two-core twisted pair cable, two-core shielded cable, etc. In low speed, short distance, no interference CAN be used in the occasion of ordinary twisted pair, on the contrary, at high speed, long line transmission, must use impedance matching (generally 120Ω) RS485 special cable (STP-120Ω (for RS485 & CAN) a pair of 18AWG), And in bad interference environment should also use armored type twisted-shielded cable (ASTP-120Ω (for RS485 & CAN) a pair of 18AWG).
Seven, two interface can support multi-point communication:
RS232: The RS232 interface can connect only one transceiver on the bus and cannot support multi-station transceiver capability. Therefore, it can only support point-to-point communication, but cannot support multi-point communication.
RS485: The RS485 interface allows the connection of up to 128 transceivers on the bus. It has the ability of multi-station communication, so that users can use a single RS485 interface to easily establish the equipment network.
RS485 port on the HDMI 4K interface board