[RS-422 and RS-485] RS-422 and RS-485 serial interface standard

RS-232 Serial interface standard

at present RS-232 yes PC Serial interface is the most widely used in computer and communication industry .RS-232 It is defined as a single ended standard that increases the communication distance in low rate serial communication .RS-232 Unbalanced transmission mode is adopted , The so-called single ended communication . Because the difference between the transmission level and the reception level is only 2V to 3V about , Therefore, its common mode suppression ability is poor , Plus the distributed capacitance on the twisted pair , The maximum transmission distance is about 15 rice , The highest speed is 20kb/s.RS-232 It's for point-to-point （ That is, only one pair of 、 Hair equipment ） Designed for communication , The drive load is 3～7kΩ. therefore RS-232 Suitable for communication between local devices .

======================================================================

RS-422 And RS-485 Serial interface standard

     1． Balanced transmission

     RS-422、RS-485 And RS-232 Dissimilarity , The data signal adopts Differential transmission The way , Also known as balanced transmission , It uses a pair of twisted pairs , Define one line as A, The other line is defined as B, Usually , Send drive A、B The positive level between is +2～+6V, It's a logical state , The negative level is in -2～-6V, It's another logical state . Another signal is C, stay RS-485 There's another one of them “ Can make ” End , And in the RS-422 It's available but not available in .

“ Can make ” The terminal is used to control the disconnection and connection between the transmitting driver and the transmission line . When “ Can make ” When the end works , The transmit driver is in a high impedance state , Referred to as “ The third state ”, That is, it is different from logic “1” And “0” The third state of .

    The receiver also makes provisions relative to the transmitter , closed 、 The originator will AA And BB Corresponding connection , When at the receiving end AB There is more than +200mV At the same time, when the level of , Output positive logic level , Less than -200mV when , Output negative logic level . The level range of receiver receiving balance line is usually in 200mV to 6V Between .

====================================================================

2．RS-422 Electrical regulations

   RS-422 The full name of the standard is “ Electrical characteristics of balanced voltage digital interface circuit ”, It defines the characteristics of the interface circuit . Typical RS-422 It's a four wire interface . There's actually a signal ground wire , common 5 Root line . Its DB9 Connector pin definition . Because the receiver uses high input impedance and transmit driver ratio RS232 Stronger driving power , Therefore, simultaneous interpreting of multiple receiving nodes is allowed on the same transmission line , Up to 10 Nodes . It's a master device （Master）, The rest are slave devices （Salve）, There is no communication between slave devices , therefore RS-422 Support point to many two-way communication . The input impedance of the receiver is 4k, So the maximum load capacity of the originator is 10×4k+100Ω（ Termination resistance ）.RS-422 The four wire interface uses separate transmit and receive channels , So you don't have to control the direction of the data , Any necessary signal exchange between devices can be done in software mode （XON/XOFF handshake ） Or hardware （ A single pair of twisted pairs ） Realization .

   RS-422 The maximum transmission distance of is 1219 rice , The maximum transmission rate is 10Mb/s. The length of the balanced twisted pair is inversely proportional to the transmission rate , stay 100kb/s Below the speed , To reach the maximum transmission distance . Only in a very short distance can we get the highest transmission rate . commonly 100 The maximum transmission rate on a meter long twisted pair is only 0 1Mb/s.

   RS-422 A terminating resistor is required , It is required that the resistance value is about equal to the characteristic impedance of the transmission cable . In the moment distance transmission, there is no need to terminate the resistance , That is, in general 300 There is no need to terminate the resistance below meters . The terminating resistor is at the far end of the transmission cable .

==================================================================== 

3．RS-485 Electrical regulations

   because RS-485 It's from RS-422 On the basis of , therefore RS-485 Many electrical regulations are related to RS-422 similar . For example, the balanced transmission mode is adopted 、 All of them need to be connected with terminating resistor on the transmission line .RS-485 Two line and four line modes can be adopted , Two wire system can realize real multi-point two-way communication , And with four wire connections , And RS-422 It can only realize point to many communication , That is, there can only be one master （Master） equipment , The rest are slave devices , But it is better than RS-422 There are improvements , Whether four or two-wire connection, the bus can be connected to 32 Devices .

   RS-485 And RS-422 The difference also lies in its common mode output voltage is different ,RS-485 yes -7V to +12V Between , and RS-422 stay -7V to +7V Between ,RS-485 The minimum input impedance of the receiver is 12k.RS-485 The drive can be used in RS-422 Network applications .

===================================================================  

RS-485 Relevant electrical regulations .

    RS-485 And RS-422 equally , The maximum transmission distance is about 1219 rice , The maximum transmission rate is 10Mb/s. The length of the balanced twisted pair is inversely proportional to the transmission rate , stay 100kb/s Below the speed , It is possible to use the specified maximum cable length . Only in a very short distance can we get the highest transmission rate . commonly 100 The maximum transmission rate of meter long twisted pair is only 1Mb/s

    RS-485 need 2 Two terminating resistors , Its resistance should be equal to the characteristic impedance of the transmission cable . In the moment distance transmission, there is no need to terminate the resistance , That is, in general 300 There is no need to terminate the resistance below meters . The terminating resistors are connected at both ends of the transmission bus .