What is RS485 and how should the 120 ohm matching resistor of RS485 be connected?
The RS485 bus is a standard that defines the electrical characteristics of drivers and receivers in a balanced digital multi-point system. It adopts half-duplex working mode and supports multi-point data communication. The RS485 bus network topology generally adopts a bus-type structure with terminal matching. That is, a bus is used to connect each node in series. RS485 adopts balanced transmission and differential reception, so it has the ability to suppress common mode interference. In addition, the bus transceiver has high sensitivity and can detect voltages as low as 200mv, so the transmission signal can be recovered from kilometers away. The most common application of RS485 is communication between programmable logic controllers in industrial environments.
When some customers test RS485 equipment in the office, the data communication is completely normal. However, when used in industrial sites, there may be situations where communication is unsuccessful. How did this problem arise?
When testing in the office, the wire generally used is only a few meters. At the scene, it may be a few hundred meters of wire, although it is a shielded wire, it may still be because the RS485 level does not match, resulting in unsuccessful communication. After adding 120 ohm resistors at both ends, the problem is solved perfectly.
The RS485 chips of different devices are usually different, and there are chips of different types of loads, which are usually not directly visible to these engineers. So that is to say, the maximum number of connected devices for different devices on the bus is uncertain. For the same number of device connections, please refer to the device description requirements.
