Chapter I clarifies several concepts:

Concept 1: The communication distance of 485 bus can reach 1200 meters.

In fact, only the 485 bus structure theoretically makes it possible to make the transmission distance up to 1200 meters under the premise of an ideal environment. Generally refers to the communication wire quality standard, baud rate 9600, only a 485 equipment can make the communication distance of 1200 meters, and can communicate does not mean that each communication is normal. Therefore, the actual stable communication distance of the 485 bus is usually far less than 1200 meters. Load 485 equipment, wire impedance is not up to standard, wire diameter is too thin, converter quality is poor, equipment lightning protection, baud rate increase and other factors will reduce the communication distance.

Concept 2: The 485 bus can communicate with 256 devices.

In fact, not all 485 converters are capable of 256 devices. To judge according to the type of chip used in the 485 converter and the type of chip used in the 485 device. The general 485 chip load capacity has four levels :32, 64, 128, 256. The theoretical nominal is often not achieved 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), and the stronger the lightning protection will greatly reduce the number of real loads.

Concept 3: 485 bus is one of the simplest, most stable, most mature industrial bus structure.

This concept is wrong. 485 bus is an economical traditional industrial bus method for device networking. The quality of communication needs to be tested and debugged according to construction experience. Although the 485 bus is simple, it must be wired according to strict installation construction specifications.

Chapter II Strict several construction specifications:

1, 485+ and 485- data lines must be twisted each other. The wiring must be multi-strand shielded twisted pair, multi-strand is for backup, shielding is for debugging in special cases, twisted pair because 485 communication uses the principle of differential mode communication, twisted pair has the best anti-interference. It is extremely wrong not to use twisted pair.

2, 485 bus must be hand-in-hand bus structure, resolutely eliminate star connection and fork connection.

3, the AC power supply of the equipment and the chassis must be truly grounded, and well grounded. There are many places with triangular sockets on the surface, in fact, there is no grounding, be careful. When the device is properly grounded, it can release energy according to the lightning protection design when the accumulated static electricity of the device is caused by lightning surges. Protect 485 bus devices and related chips from damage.

4, avoid walking together with strong electricity, so as not to interfere with it.

Chapter Ⅲ Several common communication failures:

1, no communication, no response.

2, you can upload data, but you can not download data.

3. When communicating, the system prompts interference. Or when there is no communication, the communication light keeps flashing.

4. Sometimes you can communicate, sometimes you can't. Some instructions can be passed, some instructions can not be passed.

Chapter Ⅳ recommends several debugging methods:

Make sure the device is wired correctly and strictly according to specifications.

1, common ground method: use a line or shielded line to connect the GND ground of all 485 devices, so as to avoid the potential difference between all devices that affects communication.

2, terminal resistance method: in the last 485 equipment 485+ and 485- and connected to 120 ohm terminal resistance to improve the quality of communication.

3, the middle section disconnection method: through the middle discontinuity to check whether the equipment load is too much communication distance is too long a equipment damage to the entire communication line and other reasons.

4, separate wire method: separate simple temporary pull a line to the equipment, which can be used to eliminate whether the wiring caused the communication failure.

Replace the converter method: Carry several converters with you, so that you can rule out whether the quality of the converter affects the quality of communication.

5, notebook debugging method: First ensure that the laptop you carry with you is a normal communication device, replace the customer's computer, to communicate, if you can, it shows that the customer's computer serial port may be damaged or injured.

Chapter V offers several suggestions and advice:

1. It is recommended that users use and purchase the 485 converter provided by the access control manufacturer or the 485 converter designated by the manufacturer. Access control manufacturers will do a lot of testing work on their supporting 485 converter, and will separately require 485 manufacturers to install its fixed performance parameters for production and quality testing, so it has better compatibility with its access control equipment. Never buy a cheap 485 converter from an off-brand manufacturer.

2, strict installation of 485 bus construction specifications for construction, to eliminate any fluke.

For the case of long lines and large loads, active scientific and reserved solutions are adopted.

If the communication distance is too long, it is recommended to use a repeater or 485HUB if it exceeds 500 meters to solve the problem.

If there is too much load, it is recommended to use 485HUB if there are more than 30 units on a bus to solve the problem.

Field debugging with complete debugging equipment. Be sure to carry with you several converters to ensure that you can handle long distance and multiple loads before commissioning a common computer laptop test channel break multimeter several 120 ohm terminal resistors.