Photoelectric isolation RS485 typical circuit diagram

　　1. Introduction to RS485 bus

　　RS485 bus is a common serial bus standard, which adopts balanced transmission and differential reception, so it has the ability to suppress common mode interference. When the communication distance is required to be from tens of meters to thousands of meters, RS485 bus is the most widely used bus. It is also widely used in multi-node working systems.

　　2. Introduction to typical RS485 bus circuit

　　RS485 circuits can be generally divided into isolated and non-isolated types. The isolated type has better performance than the non-isolated type in terms of anti-interference and system stability, but the non-isolated type can also be used in some occasions. Let's first talk about the typical circuit of the non-isolated type. The non-isolated circuit is very simple. It only requires an RS485 chip to be directly connected to the serial communication port and an I/O control port of the MCU. As shown in Figure 1:

　　Figure 1. Typical 485 communication circuit diagram (non-isolated type)

　　Of course, the above diagram is not a complete 485 communication circuit diagram. We also need to add a 4.7K pull-up bias resistor on line A; and a 4.7K pull-down bias resistor on line B. The middle R16 is a matching resistor, which is generally 120Ω. Of course, this depends on the cable you use for transmission. (Matching resistor: In the entire 485 communication system, for the transmission stability of the system, we generally add matching resistors to the first node and the last node. Therefore, when we design, we usually set a jumpable 120Ω resistor at each node. Whether to use it or not is set by the on-site personnel. Of course, how to distinguish the first node from the last node is still to be answered by the experts on site.) We generally use 6.8V TVS, which we will explain in more detail later.