485 cabling problem

The first chapter clarifies several concepts:
Concept One: 485 bus communication distance can reach 1200 meters.
In fact, only 485 bus structure theoretically under the premise of ideal environment can make the transmission distance reached 1200 meters. Generally refers to the communication wire quality standards, baud rate of 9600, only a 485 equipment to enable communication distance to reach 1200 meters, And being able to communicate does not mean that every communication is normal. Therefore, the 485 bus is usually the actual stable communication distance of up to 1200 meters. Load 485 equipment, wire impedance is not up to standard, line diameter is meticulous, the converter quality is poor, equipment lightning protection, baud rate heightening and so on factors will reduce the communication distance.

Concept two: 485 bus can communicate with 128 devices.
In fact, not all 485 converters can carry 128 devices. It is to be judged by the model used in the 485 converter chip and the model used by the 485 device chip. The average 485 chip load capacity has three levels 32 128 units 256 units. The theoretical nominal is often not in fact achieved. The longer the communication distance, the higher the baud rate, the finer the wire diameter, the worse the quality of the wire rod, the worse the quality of the converter, the less the power supply (passive converter), and the stronger the lightning protection will greatly reduce the actual load.---http://bianceng.cn ( Learn computer)

Concept three: 485 bus is one of the simplest and most stable and most mature industrial bus structure.
This concept is wrong. It should be: 485 bus is a kind of economical traditional industrial bus mode for equipment networking. Communication quality needs to be tested and debugged according to the construction experience. 485 bus Although simple, but must strictly install the construction code for wiring.

The second chapter strictly several construction norms:
The 485+ and 485-bar data lines must be twisted to each other.
Wiring must be a number of strands shielded twisted-pair, many stocks are in order to spare, shielding is in order to have special circumstances debugging, twisted is because 485 communication using differential mode communication principle, twisted pair of anti-interference is the best. It is extremely wrong not to use twisted-pair wires.
485 bus must be hand-held hand-type bus structure, resolutely eliminate star-type connection and fork connection.
Equipment power supply of AC and chassis must be real grounding, and good grounding.
There are many places on the surface of the triangle plug, in fact, there is no grounding, be careful. When the grounding is good, can ensure that the equipment is lightning surge impact electrostatic accumulation can be matched with the equipment of the Lightning protection design to release energy better. Protects 485 bus devices and related chips from damage.
Avoid the power to go together, so as not to interfere with the strong electricity.

The third chapter is a few common communication failures:
No communication, no response.
You can upload data, but you cannot download data below.
When communicating, the system prompts for interference. Or no communication, the communication led flashing.
Sometimes it can be communicated, sometimes it can't be communicated. Some instructions can be used, and some instructions may not be able to pass.

The fourth chapter recommends several debugging methods:
First of all to ensure that the equipment wiring is correct, and strictly conform to specifications.

Common Ground Method: The gnd of all 485 devices is connected with 1 lines or shielding lines, which avoids the potential difference between all devices affecting communication.

Terminal resistance: 485+ and 485-on the last 485 devices and 120 ohm terminal resistors to improve communication quality.

Intermediate segment Disconnection method: To check whether the equipment load too much communication distance is too long the impact of a device damage on the entire communication line and other reasons.

Separate Pull method: simple to temporarily pull a line to the device, which can be used to troubleshoot whether the wiring caused the communication failure.

Replacement Converter method: Take a few converters with you, so that you can rule out whether the quality of the communication is affected by the problem of the converter.

Notebook Debugging Method: First of all to ensure that they carry the computer notebook is the normal communication equipment, replace the client computer, to carry out communications, if you can, it indicates that the client's computer serial port may be damaged or injured.

The fifth chapter puts forward several suggestions and advice:
It is recommended that users use and purchase 485 converters provided by access control manufacturers or 485 converters designated by the manufacturer to recommend brands.
Access control manufacturers will be associated with the 485 converter to do a large number of testing work, and will require 485 manufacturers to install their fixed performance parameters for production and quality testing, so and its access control equipment with better compatibility. Do not covet the cheap purchase of the 485 converters of the factory.

In strict accordance with the construction Code of 485 bus construction, to eliminate any luck psychology.

Active scientific and reserved solutions are adopted for the more long line load.
If the communication distance is too long, it is advisable to use repeater or 485HUB to solve the problem if more than 500 meters are used.
If you have too many loads, it is recommended that you use 485HUB to solve the problem if you have more than 30 units on a bus.

On-site debugging with the debugging equipment.
On-site commissioning must carry several 120 Ohm terminal resistors with a common computer notebook test path that can be used to connect a number of long distance and multiple load converters.