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

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In the fields of industrial control, power communication, smart meters, etc., serial communication is usually used for data exchange. The initial RS232 interface, due to external application environment and other factors, often caused by electrical interference caused by signal transmission errors. In addition, the RS232 interface can only achieve point-to-point communication, does not have networking capabilities, and its maximum transmission distance is only 15 meters, which can not meet the long-distance communication requirements. RS485 solves these problems. The data signal is differentially transmitted. The maximum transmission distance is about 1219 meters, allowing multiple transmitters to be connected to the same bus. Considering energy saving, low power consumption, etc., the system voltage is changed from the traditional 5V to 3.3V, so the 3.3V-powered RS485 interface came into being.

Overview of the RS-485 standard

The RS-485 data signal adopts differential transmission mode, and the receiving and transmitting ends connect AA and BB through a balanced twisted pair. When line A is higher than line B level (VA-VB + 200mV), the output of the receiver is logic high (RO = 1); when line A is lower than line B (VA-VB-200mV) The receiver output is logic low (RO=0). When the input logic level of the driver is high (DI=1), the line A level is higher than the line B level; when the input logic level of the driver is low (DI=0), the line A level is low. At line B level. see picture 1.

The RS-485 interface transmits signals in a differential manner. Generally, the transceiver can withstand a common-mode voltage range of -7V to +12V. Once the common-mode voltage exceeds this range, communication reliability will be affected and the interface will be damaged. Since each system will have an independent ground loop, the ground potential difference VGPD between systems will be large under long-distance communication conditions. The output common-mode voltage of the transmitter is VOC, then the common-mode voltage of the receiver input is VCM=VOC+VGPD. The RS-485 standard specifies that the VOC is less than or equal to 3V, but the amplitude of VGPD can reach ten or even tens of volts, and may With strong interference signals, the receiver's common-mode input VCM is out of the normal range and generates interference current on the signal line. The solution to this type of problem is:

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

Figure 2: Low-voltage 3.3V isolated power supply scheme

b. Isolate the signal by optocoupler to reduce the effect of common mode voltage.

With this method, the signal lines and power lines of the bus transceiver are isolated from the power supply of the local signals.

Optocoupler isolation circuit

Optocoupler is 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 optocoupler isolation diagram is shown in Figure 3. In Figure 3, if the resistors R3 and R4 are selected to be larger, the speed of the photocoupler's LED will be slowed down by the cut-off state. If the selection is too small, the exit saturation will be slower. Different types of optocouplers and drive circuits make the values ​​of these two resistors slightly different. The selection of resistance values ​​is usually determined by experiments.

Figure 3: Photocoupler isolation diagram