1、 Introduction to RS485 bus
RS485 bus is a common serial bus standard, which adopts the way of 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 kilometers, RS485 bus is the most widely used bus. It is also widely used in multi node working system.
2、 Introduction to typical circuit of RS485 bus
RS485 circuit can be divided into isolated type and non isolated type. The isolated type has better performance in anti-interference and system stability than the non isolated type, but the non isolated type can also be used in some occasions. Let’s talk about the typical non isolated circuit first. The non isolated circuit is very simple. Only one RS485 chip is directly connected with the serial communication port of MCU and an I / O control port. As shown in Figure 1:
Figure 1. Typical 485 communication circuit diagram (non isolated type)
Of course, the above figure is not a complete 485 communication circuit diagram. We also need to add a 4.7K pull-up bias resistor on line a; Add a 4.7K pull-down bias resistor on line B. R16 in the middle is the matching resistance, generally 120 Ω. Of course, this depends on the cable you use for transmission. (matching resistance: 485) in the whole communication system, we usually add matching resistance to the first node and the last node for the transmission stability of the system. Therefore, we generally set a jumper 120 Ω resistance at each node during design. Whether it is used or not is set by field personnel. Of course, how to distinguish the first node from the last node The last node needs to be answered by experts on site.) we generally choose 6.8v TVs, which we will explain further later.