We are in the field of industrial control, instrumentation and power communication is the need for long-distance data exchange. We know that in the previous use of RS232 serial communication due to the use of point-to-point communication, the maximum transmission distance is only a dozen meters away, and in the transmission of data RS232 transmission method will be subject to electromagnetic interference caused by the signal transmission error. In order to make up for the shortcomings of RS232 communication, RS485 communication methods on the call. I'll talk about the RS485 communication method.
RS485 communication
We know that RS485 communication is in the RS232 interface is not enough to introduce a new interface standard, this communication connection has the following characteristics.
The first is that it uses a differential transmission method in the signal transmission method, using this transmission method can inhibit the interference of common mode signals, especially in the industrial field environment is more complex, there will be a variety of electromagnetic interference, the use of differential communication can effectively improve the reliability of data transmission.
RS485 communication connection is relatively simple, it has only two signal lines, generally used D + and D - to indicate that when the transmission of "1" signal, the voltage difference between the two lines is +0.2V to +6V; when the transmission of "0" signal, the voltage difference between the two lines is -0.2V. The voltage difference is -0.2V to -6V, which shows that they belong to the typical differential communication mode.
The second point is that the RS485 communication connection is tens of times faster than the RS232 transmission speed, we know that the RS232 transmission of data speed is generally only one or two hundred kilobits per second, while the RS485 communication rate can reach the maximum transmission speed of 10 megabits per second or more.
The third point is that the physical structure of the RS485 communication connection method inside the use of balanced driver and differential receiver, through the combination of these two, can improve the anti-interference ability of RS485 communication.
The fourth point is that the RS485 communication bus can be networked to realize multi-machine communication. Multiple transceivers are allowed to be mounted on the bus, for example, through the RS485 dedicated chip can be hung 32, 64, 128, 256 and other different devices drive.
The fifth point from the RS485 interface is very simple, in communication with the microcontroller, you can use a RS485 converter, you can directly connect with the microcontroller's UART serial port. Its communication protocol is also used is asynchronous serial communication protocol, due to the use of differential communication, so receiving data and sending data can not be done at the same time, so we say, RS485 is also a half-duplex communication.
Application of RS485
RS485 is a half-duplex communication mode, which is used in many industrial control fields, such as programmable controller ( PLC ) communication, inverter communication and touch screen ( HMI ) communication, etc. This communication mode is used. Even in the microcontroller communication can also be used in RS485 communication, communication, we only need to use a commonly used RS-485 converter between the host computer and the microcontroller can be, as shown in the figure below. In order to improve the anti-interference ability in communication, you need to be close to the MAX485 chip pin A and B between the parallel connection of a resistor, the resistance value of this resistor can be from 100 ohms to 1 kilo-ohm are available.