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Communication Protocol techinical details

RS-485 is a commonly used communication protocol in industrial control environment, which has the characteristics of strong anti-interference ability and long transmission distance. The RS-485 communication protocol is an improvement over the RS-232 protocol. The protocol layer remains unchanged, but only the physical layer has been improved, thus retaining the simplicity of the serial communication protocol application.

Electrical characteristics of RS-485: Logic “0” is represented by the voltage difference between two lines as +(2-6) volts; logic “1” is represented by the voltage difference between the two lines as -(2-6 ) volts. The interface signal level is lower than RS-232-C, which is not easy to damage the chip of the interface circuit, and the level is compatible with the TTL level, which can be easily connected with the TTL circuit.

The maximum data transfer rate of RS-485 is 10Mbps.

The RS-485 interface is strong, that is, the anti-interference is good.

The standard maximum transmission distance of the RS-485 interface is 4000 feet, and the actual distance can reach 3000 meters (theoretical data, in actual operation, the limit distance is only about 1200 meters).

In addition, the RS-232-C interface allows only one transceiver on the bus to be connected, a single station capability. The RS-485 interface allows up to 128 transceivers to be connected to the bus. That said, it has multi-drop capability, so users can easily build a network of devices using a single RS-485 interface.

The RS-485 communication protocol bus is a traditional communication bus. It cannot perform automatic arbitration of the bus, that is, it cannot send data at the same time to avoid bus contention. Therefore, the communication efficiency of the whole system is necessarily low, and the data redundancy is large, and the RS-485 bus is not suitable for applications with high-speed requirements.

At the same time, because there is usually only one host on the RS-485 bus, this bus mode is a typical centralized and decentralized control system. Once the host fails, the communication of the entire system will be limited to a paralyzed state, so it is an important measure to make an online backup of the host.