Several issues that should be noticed in 485 wiring project

 Chapter one clarifies several concepts
Concept 1: The communication distance of the 485 bus can reach 1200 meters.
In fact, it is only possible to make the transmission distance reach 1200 meters under the premise of ideal environment in theory of 485 bus structure. Generally, it means that the quality of the communication cable is up to the standard, and the baud rate is 9600. Only one 485 device can make the communication distance reach 1200 meters, and being able to communicate does not mean that every communication is normal. Therefore, the actual stable communication distance of the 485 bus is far from 1200 meters. There are many load 485 devices, the impedance of the wire is not up to standard, the wire diameter is too thin, the quality of the converter is poor, the lightning protection of the device, the increase of the baud rate and other factors will reduce the communication distance.
Concept 2: The 485 bus can carry 128 devices for communication.
Not all 485 converters can carry 128 devices. It should be judged according to the model used in the 485 converter chip and the model used in the 485 device chip. Whoever lowers it. Generally, there are three levels of 485 chip load capacity: 32 units, 128 units, and 256 units. The theoretical nominal is often not actually achieved. The longer the communication distance, the higher the baud rate, the thinner the wire diameter, the worse the wire quality, the worse the converter quality, the insufficient power supply to the converter (passive converter), and the stronger the lightning protection. These will greatly reduce the real load Quantity.
Concept 3: The 485 bus is the simplest, most stable, and most mature industrial bus structure.
This concept is wrong. It should be: 485 bus is an economical traditional industrial bus method for device networking. The communication quality needs to be tested and debugged according to construction experience. Although the 485 bus is simple, it must be installed strictly for wiring.
Chapter Two Strict Construction Rules
485+ and 485- data lines must be twisted with each other.
Multi-stranded shielded twisted-pair wires must be laid for wiring. Multi-stranded wires are for backup purposes. Shielding is for debugging in special situations. Twisted-pairs are based on the principle of differential mode communication. Not using twisted pair is extremely wrong.
485 bus must be a hand-in-hand bus structure, resolutely eliminate star connection and bifurcation connection. The AC power supplied by the device and the chassis must be truly grounded and well grounded. There are many places with triangular sockets on the surface, but they are not grounded at all. Be careful. When grounded properly, you can ensure that the device is struck by lightning.
Surge shock, when the static electricity accumulates, it can cooperate with the lightning protection design of the equipment to release the energy well. Protect 485 bus equipment and related chips from harm. Avoid walking with strong electricity to avoid strong electricity from interfering with it.
Chapter Three Several Common Communication Failures
No communication and no response. Data can be uploaded, but not downloaded. When communicating, the system prompts interference. When not communicating, the communication indicator blinks constantly. Sometimes communication is possible, sometimes communication is not available. Some instructions can pass, some instructions cannot pass.
Chapter 4 Suggested Debugging Methods
First of all, make sure that the equipment is wired correctly and strictly in compliance with the specifications.
Common ground method: Use one wire or shielded wire to connect the GND ground of all 485 devices. This can avoid the potential difference between all devices that affects communication.
Terminal resistance method: Connect 120 ohm terminal resistance in parallel to 485+ and 485- of the last 485 device to improve the communication quality.
Intermediate segment disconnection method: Check whether the equipment is overloaded, the communication distance is too long, and the damage of a certain device to the entire communication line by disconnecting from the middle.
Single wire drawing method: It is simple and easy to temporarily pull a wire to the device, which can be used to rule out whether the wiring has caused a communication failure.
Replace the converter method: Carry a few converters with you, which can rule out whether the quality of the converter affects the communication quality.
Notebook debugging method: first ensure that the computer notebook you carry is a device with normal communication. Replace the client computer for communication. If possible, it means that the serial port of the client computer may be damaged or injured.
Chapter 5 Make a few suggestions and advice
It is recommended that users use and purchase 485 converters provided by access control manufacturers or 485 converters recommended by manufacturers. The access control manufacturers will do a lot of testing work on the matching 485 converters, and will separately require the 485 manufacturers to install their fixed performance parameters for production and quality inspection, so they have better compatibility with their access control equipment. Do not try to buy 485 converters from other manufacturers.

The construction is strictly carried out in accordance with the construction specifications of the 485 bus, and no luck is avoided.
For the case of longer lines and more loads, adopt active and scientific reserved solutions.
If the communication distance is too long, it is recommended to use a repeater or 485HUB to solve the problem if it exceeds 500 meters. If there are too many loads, it is recommended to use 485HUB to solve the problem if there are more than 30 units on a bus.
On-site debugging with all debugging equipment.
On-site debugging must carry several converters to ensure long-distance and multi-load converters, a commonly used computer notebook, a multimeter for testing open circuit, and several 120 ohm termination resistors.

For more news and events on surge protective device, please pay attention to our official


April. 20, 2020