With mature and developed SCM technology in early 80’s, the world’s instrument market is basically monopolized by smart meters, which is attributed to demands of enterprise information. One of the essential conditions for enterprises to select meters is to have a network communication interface. The initial data analog signal output is a simple process, then the instrument interface is RS232 interface, which can achieve point-to-point communication, but this way cannot achieve networking function, then the emergence of RS485 solves this problem.
RS485 is a standard that defines the electrical characteristics of drivers and receivers in balanced digital multipoint systems. The standard is defined by the Telecommunications Industry Association and the Electronics Industry Union. Digital communication networks using this standard can effectively transmit signals over long distances and in the environment of high electronic noise. RS-485 makes possible the configuration of connecting local networks as well as multiple branch communication links.
RS485 has two kinds of wiring of two wire system and four wire system. Four wire system can only achieve point-to-point communication mode, rarely used. Two wire system wiring mode is commonly used with bus topology structure and can be connected to 32 nodes at most in the same bus.
In the RS485 communication network, the main-sub communication is generally used, that is, a main meter is connected with multiple sub meters. In many cases, the connection of RS-485 communication link is simply connected with a pair of twisted pair of the “A” and “B” end of each interface, while ignoring the signal ground connection. This connection method in many occasions can work normally, but it has buried a great hidden danger. One of the reasons is common mode interference: RS – 485 interface adopts differential mode transmission method and do not need to detect the signal against any reference, but detect the voltage difference between two wires, which might lead to the ignorance of the common mode voltage range. The RS485 transceiver common-mode voltage ranges between – 7V and + 12V and the entire network can work normally, only when it meets the above conditions,; When the common mode voltage of the network line exceeds this range, the stability and reliability of communication will be affected, and even the interface will be damaged. The second reason is the EMI problem: the common-mode part of the output signal of the sending driver needs a return path. If there is no low resistance return path (signal ground), it will return to the source in the form of radiation, and the whole bus will radiate electromagnetic waves outward like a huge antenna.
Typical serial communication standards are RS232 and RS485, which define voltage, impedance, etc., but do not define the software protocol. Different from RS232, RS485 features include:
1. Electrical characteristics of RS-485: logic “1″ is represented by the voltage difference between two lines as + (2 — 6) V; Logical “0″ is represented by the voltage difference between the two lines as – (2 — 6) V. When the interface signal level is lower than RS-232-C, it is not easy to damage the chip of the interface circuit, and the level is compatible with the TTL level, so it is convenient to connect with the TTL circuit.
2. The maximum data transmission rate of RS-485 is 10Mbps.
3. RS-485 interface is strong, that is, good anti-noise interference.
4. The maximum transmission distance of RS-485 interface is 4000 feet standard value, in fact it can reach 3000 meters (theoretical data, in practical operation, the limit distance is only up to about 1200 meters), in addition, RS-232-C interface only allows to connect 1 transceiver on the bus, that is, the single station capacity. The RS-485 interface on the bus is allowed to connect up to 128 transceivers. That is, with multi-station capability, users can use a single RS-485 interface to easily set up a network of devices.
Because RS-485 interface has good anti-noise interference, the above advantages of long transmission distance and multi-station capability make it the preferred serial interface. Because the half-duplex network composed of RS485 interface generally only needs two wires, the RS485 interface adopts shielded twisted pair transmission. The RS485 interface connector uses the 9-core plug block of DB-9, and the intelligent terminal RS485 interface uses DB-9 (hole), and the keyboard interface RS485 connected with the keyboard uses DB-9 (needle).
Post time: Mar-15-2021