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1. Several concepts about 485 bus:
1. The communication distance of 485 bus can reach 1200 meters.
According to the 485 bus structure theory, under the premise of an ideal environment, the 485 bus transmission distance can reach 1200 meters. The condition is that the communication wire is high-quality and up to the standard, the baud rate is 9600, and only one 485 device is loaded, so that the communication distance can reach 1200 meters, so the actual and stable communication distance of the 485 bus is usually less than 1200 meters. If there are many load 485 devices, the wire impedance is not up to the standard, the wire diameter is too thin, the quality of the converter is poor, the equipment lightning protection is complicated, and the baud rate increases, etc., will reduce the communication distance.
2. The 485 bus can communicate with 128 devices.
In fact, not all 485 converters can carry 128 devices. It is judged based on the model of the chip in the 485 converter and the model of the 485 device chip, and the load capacity can only be determined according to the chip with a lower index. Generally, the load capacity of 485 chips has three levels-32 units, 128 units and 256 units. . In addition, the theoretical nominal rating is often actually not reached. The longer the communication distance, the higher the baud rate, the thinner the wire diameter, the worse the wire quality, the worse the quality of the converter, and the insufficient power supply of the converter (passive Converter), the stronger the lightning protection, these will reduce the real load quantity.
3. 485 bus is the simplest, most stable and most mature industrial bus structure
This concept is wrong. The 485 bus is an economical and traditional industrial bus mode used for device networking. . The communication quality needs to be debugged and tested based on construction experience to be guaranteed. Although the 485 bus is simple, it must be wired in strict accordance with the installation and construction specifications.
2. The construction must be strictly in accordance with the construction specifications
The construction of the 485 bus system must be carried out in strict accordance with the construction specifications, and the following points should be paid special attention to.
1. 485+ and 485-data lines must be twisted pair each other.
2. The wiring must be multi-strand shielded twisted pair. Multi-strand is for backup, shielding is to facilitate debugging when special circumstances occur, and twisted pair is because 485 communication adopts the principle of differential mode communication, and the anti-interference of twisted pair is better. It is wrong not to use twisted pair.
3. The 485 bus must have a hand-in-hand bus structure, and resolutely avoid star connection and bifurcation connection.
4. The AC power supply and the chassis of the equipment must be grounded and well grounded. There are many places where there are triangular sockets on the surface, but they are not grounded at all. Good grounding can prevent the equipment from being struck by lightning and surges. When static electricity accumulates, it can cooperate with the equipment’s lightning protection design to better release energy, and protect the 485 bus equipment and related chips from harm.
5. In order to avoid interference from strong electricity, the 485 bus should be avoided walking with strong electricity.
Three, several debugging methods are recommended:
Before debugging, make sure that the wiring of the equipment is correct and the construction conforms to the specifications. The following debugging methods can be used according to the problems encountered.
1. Common ground method: Use 1 wire or shielded wire 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: Connect 120 ohm terminal resistance to the 485+ and 485- of the last 485 device in parallel to improve the communication quality.
3. Intermediate segment disconnection method: Check whether the equipment is overloaded, the communication distance is too long, or the influence of a certain device on the entire communication line by disconnecting from the middle.
4. Separate wire drawing method: Simply draw a wire to the device separately, which can be used to rule out whether the wiring caused the communication failure.
5. The method of replacing the converter: Carry several converters with you so that you can rule out whether the quality of the converter is affecting the communication quality.
6. Notebook debugging method: First, ensure that the computer notebook you carry with you is a device with normal communication, and use it to replace the client’s computer for communication. If it is normal, it indicates that the serial port of the client’s computer may be damaged or injured.
Four, suggestions and advice
Several common communication failures with 485 bus structure are as follows.
1. No communication, no response.
2. Data can be uploaded, but data cannot be downloaded.
3. The system prompts that it is disturbed during communication, or the communication indicator keeps flashing when not communicating.
4. Sometimes it can communicate, sometimes it can’t communicate, some commands can be communicated, and some commands cannot be communicated.
In order to reduce communication failures, the following suggestions and advices are provided for reference.
1. It is recommended that users use and purchase the 485 converter provided by the access control manufacturer or the 485 converter of the recommended brand designated by the manufacturer.
2. Access control manufacturers will do a lot of testing work on the 485 converters that are matched with them, and will require the 485 converter manufacturers to carry out production and quality testing according to their fixed performance parameters, so it has better compatibility with the access control equipment. Don’t try to buy a 485 converter from a brand-name manufacturer cheaply.
3. Carry out the construction strictly in accordance with the 485 bus construction specifications, and put an end to any fluke psychology.
4. Use scientific and reserved solutions for the 485 bus project with long lines and heavy loads.
5. If the communication distance is too long, such as over 500 meters, it is recommended to use a repeater or 485 HUB to solve it.
6. If there are too many loads, such as more than 30 units on a bus, it is recommended to use 485 HUB to solve the problem.
7. On-site debugging with complete debugging equipment. For on-site debugging, you must carry a few converters that can connect to long distances and multiple loads, a commonly used computer notebook, a multimeter for testing the circuit break, and a few 120 ohm terminal resistors.
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