Design of RS485 interface long-distance data communication circuit with 3.3V power supply

　　In the fields of industrial control, power communication, intelligent instruments, etc., serial communication is usually used for data exchange. The original RS232 interface often causes signal transmission errors due to electrical interference due to factors such as the external application environment. In addition, the RS232 interface can only realize point-to-point communication, does not have networking function, and its maximum transmission distance is only 15 meters, which cannot meet the requirements of long-distance communication. RS485 solves these problems. The data signal adopts differential transmission mode, with a maximum transmission distance of about 1219 meters, allowing multiple transmitters to be connected to the same bus. Considering energy saving and low power consumption, the system voltage is changed from the traditional 5V to 3.3V, so the 3.3V powered RS485 interface came into being.

　　RS-485 Standard Overview

　　RS-485 data signals are transmitted in differential mode, and the receiving and transmitting ends connect AA and BB correspondingly through a balanced twisted pair. When line A is higher than line B (VA-VB> +200mV), the receiving end outputs a logic high level (RO=1); when line A is lower than line B (VA-VB-200mV), the receiving end outputs a logic low level (RO=0). When the logic level of the driver's input end is high (DI=1), the line A level is higher than the line B level; when the logic level of the driver's input end is low (DI=0), the line A level is lower than the line B level. See Figure 1.

　　The RS-485 interface uses differential transmission to transmit signals. The common-mode voltage range that the transceiver can withstand is generally -7V to +12V. Once the common-mode voltage exceeds this range, it will affect the reliability of communication and even damage the interface. Since each system will have an independent ground loop, under long-distance communication conditions, the ground potential difference VGPD between systems will be very large. The output common-mode voltage of the transmitter is VOC, so the common-mode voltage VCM at the input of the receiver is VOC+VGPD. The RS-485 standard stipulates that VOC is less than or equal to 3V, but the amplitude of VGPD can reach more than ten volts or even tens of volts, and may be accompanied by strong interference signals, causing the common-mode input VCM of the receiver to exceed the normal range and generate interference current on the signal line. The solution to this problem is:

　　a. Isolate the system power supply and the RS-485 transceiver power supply through an isolated DC-DC, as shown in Figure 2;

　　Figure 2: Low voltage 3.3V isolated power supply solution

　　b. Isolate the signal through optocoupler to reduce the impact of common mode voltage.

　　When this method is used, the signal lines and power lines of the bus transceiver are isolated from the power supply of the local signal.

　　Optocoupler isolation circuit

　　Optocouplers are often the main factor limiting the baud rate of communication data. For low-speed transmission, PS250, TIL117, etc. can be used. In high-speed circuit design, high-speed optocouplers such as 6N137 and 6N136 can be considered to optimize circuit parameter design. The schematic diagram of optocoupler isolation is shown in Figure 3. In Figure 3, if the resistors R3 and R4 are selected to be larger, the speed at which the light-emitting tube of the optocoupler enters the saturation state from the cutoff will be slower; if they are selected to be too small, the exit from saturation will be slower. Different types of optocouplers and drive circuits make the values ​​of these two resistors slightly different, and the selection of resistance values ​​is usually determined by experiments.

　　Figure 3: Schematic diagram of optocoupler isolation