Precautions for RS485 wiring
Source: https://vedard.com/blog/precautions-for-rs485-wiring/
Date: 2022/05/01
I. RS-485 communication
1. The communication distance of RS485 bus can reach 1200 meters？
RS485 bus transmission only in the ideal environment is possible to make the transmission distance up to 1200 meters. It requires high-quality communication wire, baud rate 2400, only a 485 equipment, which can make communication distance reached 1200 meters, and can communicate does not mean that every communication is normal. Therefore, the actual stable communication distance of 485 bus is far less than 1200 meters. Load much RS485 equipment, wire impedance is not up to standard, wire diameter is too thin, poor quality converter, equipment lightning protection, baud rate and other factors will reduce the communication distance.
2. RS485 bus can carry 128 devices for communication？
In fact, not all 485 converters can take 128 devices. According to the type of 485 converter chip and the type of 485 device chip to judge. Whichever is lower will apply. Generally, the load capacity of 485 chips has three levels: 32, 128 and 256. Theoretical claims are often unattainable in practice. The longer the communication distance, the higher the baud rate, the thinner the wire diameter, the worse the quality of the wire, the worse the quality of the converter, the insufficient power supply of the converter (passive converter), the stronger the lightning protection these will greatly reduce the real load.
3. RS485 bus is one of the most simple and stable and mature industrial bus structure？
This is wrong. 485 bus is an economical traditional industrial bus for device networking. Communication quality needs to be tested and debugged according to construction experience. Although the 485 bus is simple, it must be wired strictly in accordance with the installation and construction specifications.
II． Several requirements for RS485 construction
1. 485+ and 485- data lines must be twisted-pair to each other
2. Use shielded twisted-pair cables. Multiple strands are for reserve; Shielding is for debugging in special cases; Twisted-pair communication because 485 uses the principle of differential mode communication, twisted-pair interference is the best. It is extremely wrong not to use twisted pair cable. security equipment
3. 485 bus must be hand in hand bus structure, resolutely avoid star connection and bifurcation connection. The ac power supply of the device and the chassis must be properly grounded. There are many places where there are triangular sockets on the surface, but there is no grounding at all, so be careful. When the device is properly grounded, the device cannot be struck by lightning. Avoid laying with strong electricity, so as not to interfere with it.
II. Several debugging methods
1. Common ground method: use 1 line or shielding line to connect all 485 equipment GND, so that you can avoid the impact of communication between all equipment potential difference.
2. Terminal resistance method: Connect 120 ohm terminal resistance in parallel on 485+ and 485- of the last 485 device to improve communication quality.
3. Middle section disconnection method: check whether the equipment load is too much, communication distance is too long, the impact of an equipment damage on the whole communication line and other reasons by disconnecting from the middle.
4. Separate wire method: a separate simple and temporary wire to the device, which can be used to rule out whether wiring caused a communication failure.
5. Replace converter method: carry several converters, so you can rule out whether the quality of the converter affects the quality of communication.
6. Notebook debugging method: first ensure that the portable computer notebook is normal communication equipment, replace the customer’s computer, to communicate, if it works well, it indicates that the serial port of the customer’s computer may be damaged or injured.