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The functions and differences of RS232 and RS485 interfaces commonly used in auto focus cameras

RS232 and RS485 interface is a typical serial communication standard, SONY auto focus camera is often used in the transmission of data, for the two functions and differences, please see detailed article.

I. What is RS-232:

RS-232 (also known as EIA RS-232) is one of the most commonly used serial communication interface standards, which was developed by the American Electronic Industry Association (EIA) in conjunction with Bell Systems, modem manufacturers, and computer terminal manufacturers in 1970.

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, the RS-232 interface usually appears in the form of 9 pins (DB-9) or 25 pins (DB-25), and the general personal computer will have two groups of RS-232 interfaces, respectively called COM1 and COM2.

The connector uses the DB-25 25-pin plug holder. Some devices are connected to the PC RS-232 interface, because they do not use the other party’s transmission control signal, only three interface lines, that is, “send data”, “receive data” and “signal ground”. Therefore, the 9-core plug holder of DB-9 is adopted, and the transmission line is shielded twisted pair.

Second, what is RS-485:

RS485 has no specific physical shape, and the interface used according to the actual situation of the project can achieve networking functions.

Three, the working principle of the two interfaces:

1, RS232 interface working principle:

The work of RS-232 can be understood in terms of the protocol format. Since RS-232 is a point-to-point asynchronous communication protocol, it sends data in a single direction. Here, no clock is needed to synchronize the transmitter and receiver. The data format starts with the start bit and then sends 7 bits of binary data, the parity bit and the stop bit in turn.

2, the working principle of RS485 interface:

Differential transmission is used to deal with common-mode interference

When the RS-485 chip receives a differential voltage greater than 200mV, the RO pin outputs a low level, and the DI itself can output a differential voltage of 1.5V. Therefore, there is enough margin to support long-distance transmission and various environmental noise interference.

Four, RS232 and RS485 electronic characteristics comparison:

1. RS232: The signal level value of the transmission level signal interface is high (signal “1” is “-3V to -15V”, signal “0” is “3 to 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 a level conversion circuit to connect with 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; The logical “0” is represented by the voltage difference between the two lines as – (2-6) V. 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, and it can be easily connected with the TTL circuit.

Five, the communication distance comparison of 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: RS485 maximum wireless transmission distance of 1200 meters. The maximum transmission rate is 10Mbps, and the maximum communication distance can be achieved at the transmission rate of 100Kb/S.

Dedicated cables with impedance matching and low attenuation can reach 1800 meters! More than 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 shielded wire, etc.

RS485: can use two core twisted pair wire, two core shielded wire, etc. In low speed, short distance, no interference occasions CAN use ordinary twisted pair, on the contrary, in high-speed, long line transmission, you must use impedance matching (generally 120Ω) RS485 special cable (STP-120Ω (for RS485 & CAN) a pair of 18AWG), And in harsh interference environments should also use armored type twisted pair shielded cable (ASTP-120Ω (for RS485 & CAN) a pair of 18AWG).

Seven, two interfaces can support multi-point communication:

RS232: The RS232 interface is only allowed to connect 1 transceiver on the bus, and cannot support multi-station transceiver capability, so it can only be 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, it has multi-station communication capability, so that users can easily establish equipment networks by using a single RS485 interface.

Due to the superiority of RS485, the interface board independently developed by Xuanjian Technology is equipped with RS485 interface to facilitate the control and data transmission of the auto focus camera, if the additional RS232 interface is required, it can also be customized according to the specific needs of customers.

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