RS485 And Modbus Communication Protocol Tutorial (2)

3. RS485 has a two-wire system and four-wire system. The four-wire system can only realize point-to-point communication, which is rarely used now. The two-wire system is a bus topology, and a maximum of 32 nodes can be connected on the same bus. In the RS485 communication network, the master-slave communication method is generally used, that is, a master with multiple slaves.

4. In many cases, when connecting the RS-485 communication link, simply use a pair of twisted pairs to connect the "A" and "B" ends of each interface. Ignoring the connection of the signal ground, this connection method can work normally on many occasions, but it has buried a lot of hidden dangers for two reasons:

1) Common mode interference problem: The RS-485 interface adopts the differential mode to transmit signals and does not need to detect the signal relative to a certain reference point. The system only needs to detect the potential difference between the two lines. But people often ignore that the transceiver has a certain common-mode voltage range. The common-mode voltage range of the RS-485 transceiver is -7 to +12V. Only when the above conditions are met, the entire network can work normally. When the common-mode voltage of the network line exceeds this range, it will affect the stability and reliability of communication, and even damage the interface.

2) EMI problem: The common-mode part of the output signal of the transmit driver needs a return path. If there is no low resistance return channel (signal ground), it will return to the source in the form of radiation, and the entire bus will radiate electromagnetic waves like a huge antenna.

5. Since the PC only has an RS232 interface by default, the following method can get the RS485 circuit of the PC host computer:

1) Convert the RS232 signal of the serial port of the PC to the RS485 signal through the RS232/RS485 conversion circuit. For the complex industrial environment, it is best to choose the product with anti-surge and isolation.

2) Through the PCI multi-serial port card, the expansion card whose output signal is RS485 can be directly selected.