The difference between RS-232 and RS485 communication methods


1. Different transmission methods. RS-232 adopts unbalanced transmission mode, which is called single-ended communication. While RS485 adopts balanced transmission mode, that is, differential transmission mode.

2, the transmission distance is different. RS-232 is suitable for communication between local devices, and the transmission distance generally does not exceed 20m. And the transmission distance of RS-485

For tens of meters to thousands of meters.

3. RS-232 only allows one-to-one communication, while the RS-485 interface allows up to 128 transceivers to be connected on the bus.

 

RS-232 is defined as a single-ended standard for increasing communication distance in low-speed serial communication.

RS: is the English abbreviation of Recommended Standard

RS-232 adopts an unbalanced transmission method, which is called single-ended communication. The data signals of the receiving and transmitting ends are relative to the signal ground. For example, the data sent from the DTE equipment is 2 feet relative to the 7 feet (signal ground) when using the DB25 connector. Level.

 

A typical RS-232 signal swings between positive and negative levels. When sending data, the transmitter driver outputs a positive level of +5 to +15V and a negative level of -5 to -15V. When there is no data transmission, the line is TTL, from the beginning of data transmission to the end, the line level is from TTL level to RS-232 level and back to TTL level.

 

The typical working level of the receiver is +3 to +12V and -3 to -12V. Since the difference between the sending level and the receiving level is only about 2V to 3V, its common mode rejection capability is poor. Coupled with the distributed capacitance on the twisted pair, the maximum transmission distance is about 15 meters, and the maximum rate is 20kb/ s. RS-232 is designed for point-to-point (that is, only one pair of receiving and sending devices) communication, and its driver load is 3 to 7kΩ. So RS-232 is suitable for communication between local devices.

 

RS-485 is different from RS-232. The data signal adopts the differential transmission method, also known as balanced transmission. It uses a pair of twisted pairs, one of which is defined as A, and the other is defined as B. Usually, the sending driver The positive level between A and B is +2~+6V, which is a logic state, and the negative level is -2~6V, which is another logic state. There is another signal ground C, and there is an "enable" terminal in RS-485. The "enable" terminal is used to control the disconnection and connection of the sending driver and the transmission line. When the "enable" terminal is active, the transmit driver is in a high-impedance state, which is called the "third state", that is, it is the third state that is different from logic "1" and "0". The receiver also makes relative regulations to the sending end. The receiving and sending ends connect AA and BB correspondingly through balanced twisted pairs. When there is a level greater than +200mV between the receiving ends AB, it outputs a positive logic level, which is less than -200mV. When the output is negative logic level. The level on the receiver balanced line is usually between 200mV and 6V.

 

Both rs232 and rs485 are serial communication

rs232 is full-duplex, that is, it can send and receive at the same time.

rs485 is half-duplex, it can only receive or send data at a certain time, otherwise, the bus will be paralyzed

 

rs232 is 1-to-1, rs485 can be 1-to-many, often 485 adopts master-slave mode, one master and several slaves form a network

 

The transmission level of rs232 and rs485 is different. rs232 is a level mode. The communication distance is about 15m, which is actually seven or eight meters. 485 is a differential mode of transmission. There are only two lines. , 700m or so will not work.

 

The following is the reference network resources

The difference between RS232 and RS485

 

1. RS-232-C is a serial physical interface standard formulated by EIA (Electronic Industry Association). RS is the abbreviation of "Recommended Standard" in English, 232 is the identification number, and C is the number of revisions. The RS-232-C bus standard has 25 signal lines, including a main channel and an auxiliary channel. In most cases, the main channel is mainly used. For general duplex communication, only a few signal lines can be implemented, such as one A transmit wire, a receive wire, and a ground wire. The data transfer rates specified by the RS-232-C standard are 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600, 19200 baud per second. The RS-232-C standard stipulates that the driver is allowed to have a capacitive load of 2500pF, and the communication distance will be limited by this capacitance. For example, when a communication cable of 150pF/m is used, the maximum communication distance is 15m; if the capacitance per meter of the cable 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, and there are problems such as common ground noise and inability to suppress common mode interference, so it is generally used for communication within 20m.

 

2. RS-485 bus, when the communication distance is required to be tens of meters to thousands of meters, the RS-485 serial bus standard is widely used. RS-485 adopts balanced transmission and differential reception, so it has the ability to suppress common mode interference. In addition, the bus transceiver has high sensitivity and can detect voltages as low as 200mV, so the transmission signal can be recovered from kilometers away. RS-485 adopts half-duplex working mode, only one point is in the sending state at any time, therefore, the sending circuit must be controlled by the enable signal. RS-485 is very convenient for multi-point interconnection, which can save many signal lines. The application of RS-485 can be networked to form a distributed system, which allows up to 32 drives and 32 receivers in parallel.

 

In the past, the communication between PC and smart devices mostly used RS232, RS485, Ethernet, etc., which mainly depended on the interface specification of the device. However, RS232 and RS485 can only represent the physical medium layer and link layer of communication. If you want to achieve bidirectional access to data, you must write your own communication application programs, but most of these programs cannot comply with ISO/OSI specifications, and can only achieve A single function, suitable for a single device type, the program is not universal. In the device network formed by RS232 or RS485 devices, if the number of devices exceeds 2, RS485 must be used as the communication medium. If the devices on the RS485 network want to communicate with each other, only through the "Master" device transfer can be achieved. This The master device is usually a PC, and only one master device is allowed in this device network, and the rest are all slave (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, conflict detection, and link maintenance.


Discover more RS485/RS232 module in RenhotecIC!