RS485 communication interface has many advantages such as low cost, simple structure, high reliability and strong anti-interference ability. Therefore, 485 communication interface circuit occupies an important position in smart energy meter communication, and its technical performance directly affects whether its communication is smooth. The practical problem encountered in the bus mode networking is that since the bus terminals A and B of the ordinary 485 chip are differentiated in polarity, that is, the A and B terminals have a high level and a low level, which is usually Said positive (+) and negative (-). In the construction of the project, the connection between the electric energy meter and the electric energy meter, the connection between the electric energy meter and the collecting terminal, must be connected with the positive end and the positive end, and the negative end and the negative end are connected to realize normal communication. However, in the construction process of the networking, there is often a phenomenon in which the positive and negative terminals are reversed, resulting in poor communication.
Figure 1 Smart Energy Meter
According to the large number of such problems in the networking in recent years, the construction and construction has brought a lot of maintenance and repair costs. The State Grid Corporation recently announced the implementation of the enterprise standard "Q/GDW354-2012 Smart Energy Meter Functional Specification" 4.9 .1 "RS485 communication" section clearly states:
The RS485 interface must be electrically isolated from the internal circuitry of the meter and has a fail-safe circuit.
The RS485 interface should meet the electrical requirements of DL/T 645-2007 and can withstand the test of AC voltage 380V without damage for 2 minutes.
The RS485 interface communication rate can be set. The standard rate is 1200bps, 2400bps, 4800bps, and 9600bps. The default value is 2400bps.
RS485 interface communication follows the DL/T 645-2007 protocol and its filing documents.
The RS485 interface can be used for communication within 3 seconds after the power meter is powered on.
The RS485 interface should ensure that the positive and negative wires can communicate normally on the 485 bus.
The above regulations clearly put forward the requirement that "the RS485 interface must be electrically isolated from the internal circuit of the energy meter and have a fail-safe circuit", that is to say, the RS485 chip should be powered by a separate isolated power supply, and must pass with the internal MCU circuit of the electric energy meter. The optocoupler or isolation chip is electrically isolated.
The above provisions also clearly stated that "RS485 interface should be able to ensure that the positive and negative wiring can communicate normally on the 485 bus", that is to say, the 485 bus no longer has the polarity, that is, no matter how the two ends of the bus are connected, Both can achieve normal communication, which means that a polarity-free connection is achieved. There are two ways to implement a non-polar connection:
The first way is to install a polarity judgment circuit on the bus front end of a common polar RS485 chip. When the network communication is performed, the polarity determination circuit determines the polarity of the bus, and the positive polarity connection positive polarity is automatically realized, thereby achieving normal communication. This method is less reliable and will increase part of the cost, and the practical application is less.
The second way is to use microelectronics technology to make a non-polar 485 communication module. This non-polar 485 communication module has the advantages of high reliability and low cost, and is a new communication interface module that replaces the ordinary RS485 chip.
Based on the latest published Q/GDW354-2012 Smart Energy Meter Functional Specification, Zhou Ligong Zhiyuan Electronics has introduced the non-polar RSM485N isolated transceiver module as shown in Figure 2, which is small in size and convenient in application design. The module integrates power isolation, electrical isolation, RS-485 transceiver and bus protection devices. The communication baud rate can support up to 250kbps and can connect up to 256 nodes.
Figure 2 Non-polarity isolated transceiver module for smart energy meter
Using the non-polar RSM485N isolated transceiver module to realize the smart energy meter communication network can eliminate the troubles of construction and maintenance. The RS485 bus can be arbitrarily wired without professional construction personnel, greatly improving the networking efficiency. It can be used in a completely non-polar communication network, as shown in Figure 3, and also for non-polar and polar hybrid RS485 networks.
Figure 3 Non-polar RS-485 communication network
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