Photoelectric isolation RS485 typical circuit diagram

 1. Introduction to RS485 bus

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

2. Introduction to typical circuits of RS485 bus

RS485 circuits can generally be 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 situations. Let’s talk about the typical non-isolated circuit first. The non-isolated circuit is very simple. It only requires an RS485 chip to be directly connected to the MCU’s serial communication port and an I/O control port. As shown in Figure 1:

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