RS-485 brief introduction

RS-485 It is a common communication protocol in industrial control environment ,RS yes Recommended Standard Abbreviation , It means recommendation standard .RS-485 Also known as ANSI/TIA/EIA-485,1983 year RS-485 The communication interface is used by the Electronic Industry Association (Electronics Industries Association EIA) Approved as a communication interface standard . Half duplex , Differential signal transmission , Have strong anti-interference ability 、 Long transmission distance .

RS485 Characteristics

1、 Interface level is low , Not easy to damage the chip . RS 485 Electrical characteristics of ： Logic “ 1 ” Take the voltage difference between the two lines as +(2~6)V

Express ; Logic “ 0 ” In two lines The voltage difference between is -2 To -6V Express . Interface signal level ratio RS232 To reduce the ,

A chip that is not easy to damage the interface circuit , And this level is related to TTL Level compatible , It is convenient to communicate with TTL Circuit connection .

2、 Long transmission distance . The longest communication distance can reach 1200 rice .

3、 High transmission rate . The transmission distance is 10 When rice RS485 Maximum data transmission rate Can be up to 35Mbps , stay 1200m when , transmission

Speed up to 100Kbps .

4、 Strong anti-interference ability . RS 485 The interface is a combination of a balanced driver and a differential receiver , Anti common mode interference capability

enhance , That is, it has good noise immunity .

5、 Multiple nodes are supported . General maximum support 32 Nodes , If you use special 485 chip , You can achieve 128 A or 256 Nodes , The biggest can support 400 Nodes .

RS-485 The physical layer

contrast CAN Communication network , It can be found that their network structure composition is similar , Each node consists of a communication controller and

It consists of a transceiver , stay RS-485 In communication network , The serial port controller in the node uses RX And TX The signal line is connected to the receiver

On the hair apparatus , The transceiver is connected to the network bus through a differential line , Serial port controller and transceiver are generally used TTL Signal transmission

transport , The transceiver and bus use differential signals to transmit signals . When sending data , Design of serial port controller TX The signal is converted by the transceiver

The differential signal is transmitted to the bus , When receiving data , The transceiver converts the differential signal on the bus into TTL The signal goes through RX

The pin is transmitted to the serial port controller .

RS-485 The maximum transmission distance of the communication network can reach 1200 rice , Can be mounted on the bus 128 Communication nodes , And because the RS-485 network

There is only a pair of differential signal lines , It uses differential signals to express logic , When AB The voltage difference between the two lines is -6V~-2V It means

Logic 1, When the voltage difference is +2V~+6V To express logic 0, Only one signal can be expressed at a time , So its communication is half double

In the form of , It is associated with RS-232 The characteristics of the communication protocol are shown in the figure below

RS-485 And RS-232 The difference is only reflected in the physical layer , Their protocol layer is the same , It is also the form of using serial port packets

Data transmission . And because the RS-485 It has powerful networking function , On top of the basic agreement, people also formulated MODBUS Association

discussion , It is widely used in industrial control network . The basic protocol mentioned here refers to the protocol of serial port basic packet format ( Based on data bits ), and MODBUS The protocol is a high-level application protocol that combines basic data packets into a communication frame format ( Based on packets or bytes ).

When I have time, I will do a separate issue to explain to you MODBUS Related knowledge .