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What is RS485

  • Author:Snoppy Huang
  • Source:CPS encyclopedia
  • Release on:2016-07-11
What is RS485 
      Intelligent instrument with the early 1980s, SCM technologies mature and developed, and now the world equipment market basically smart meter monopoly. The reason is the business information needs of enterprises in the selection of instruments in which a necessary condition is to have a network communication interface. Originally a simple process data analog signal output quantity, then meter interface is a RS232 interface, which can point to point communication, but this approach can not be networked. Ensuing RS485 solve this problem.
   1. RS-485 electrical characteristics: differential signal negative logic, logic "1" to a voltage difference between the two lines is + (2 ~ 6) V represents; a logical "0" to the voltage difference between the two lines is - ( 2 ~ 6) V FIG. Interface signal level than the RS-232-C is reduced, it is not easy to damage the chip interface circuit, and the level compatible with TTL level, can be easily connected with TTL circuits.
   2.The maximum data transfer rate of 2. RS-485 is 10Mbps.
   3. RS-485 interface using a combination of drive and differential receiver balanced, enhanced resistance to common mode interference, noise immunity that is good.
   4. RS-485 maximum communication distance of about 1219m, the maximum transmission rate of 10Mbps, the transmission rate is inversely proportional to distance transmission, the transmission rate at 100KbpS, we can achieve the maximum communication distance, if necessary transmission over longer distances, You need to add 485 repeaters. RS-485 bus generally supports up to 32 nodes, 485 if you use a special chip that can reach 128 or 256 nodes, the biggest can support up to 400 nodes.
RS485 / 422 primarily in Video Surveillance
       In data communications, computer networks and distributed control systems in industrial, often need to use serial communication to achieve the purpose of remote information exchange. Currently, there are a variety of interface standards can be used for serial communications, the most commonly used interfaces RS-232, RS-422, RS-485. RS232 serial interface standard is the first in a short distance, low baud rate serial communication has been widely used. Subsequently developed RS-422 and RS-485 electrical standard balance transfer, compared to RS-232 unbalanced transmission system has greatly improved in the electrical specifications. But in general, RS-232, RS-422 and RS-485 are initially developed by the Electronic Industries Association (EIA) and published, RS-232 released in 1962, named for the EIA-232-E, as an industrial standards to ensure compatibility between products from different manufacturers. RS-422 evolved from the RS-232, it is to compensate for the lack of RS-232 and raised. To improve the RS-232 communication distance is short, low rate of defects, RS-422 communication interface defines a balance, transfer rate up to 10Mb / s, the transmission distance is extended 4000 feet (at a rate of less than 100kb / s) to, and allows the connection of up to 10 receivers on a balance bus. RS-422 is a single send, receives more than one way, the balanced transmission standard, was named TIA / EIA422-A standard. For the expansion of the scope of application, EIA and in 1983 enacted in RS-422 based on the RS-485 standard, an increase of multi-point, two-way communication capability, which allows multiple transmitters connected to the same bus, while increasing the transmitter drive capability and conflict protection features, expanded bus common mode range, named after the TIA / EIA-485-a standard. RS-232, RS-422 and RS-485 standard only on the electrical characteristics of the interface make provisions without involving connectors, cables or agreement, on this basis, the user can create their own high-level communication protocol.
RS-422 and RS-485 in the monitoring system is mainly used to achieve PTZ camera selection and control and sub-control management to achieve transmission of signaling.