Practical problems and solutions of the hottest 48

Practical problems and solutions of 485 bus in the field

1. The communication of 485 bus has good corporate reputation and the distance between household registration tablets can reach 1200 meters

according to the 485 bus structure theory, under the premise of ideal environment, the transmission distance of 485 bus can reach 1200 meters. The condition is that the communication wire is of high quality and up to standard, the baud rate is 9600, and only one 485 device is loaded, so the communication distance can reach 1200 meters. Therefore, the actual stable communication distance of 485 bus is often less than 1200 meters. If there are many load 485 devices, the wire impedance does not meet the standard, the wire diameter is too thin, the quality of the converter is poor, the lightning protection of the equipment is complex, the baud rate is increased, and other factors will reduce the communication distance

2. 485 bus can carry 128 devices for communication

in fact, not all 485 converters can carry 128 devices. It should be judged according to the model of the chip in the 485 converter and the model of the 485 device chip, and its load capacity can only be determined according to the chip with lower index. Generally, the load capacity of 485 chips has three levels - 32, 128 and 256.. In addition, 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 wire quality, the worse the converter quality, the insufficient power supply of the converter (passive converter), and the stronger the lightning protection, all of which will reduce the real load

3, 485 bus is the simplest, most stable and most mature industrial bus structure

this concept is wrong. 485 bus is an economical and traditional industrial bus mode used for equipment connection.. Its communication quality needs to be debugged and tested according to construction experience, which can be guaranteed. Although 485 bus is simple, it must be wired in strict accordance with the installation and construction specifications

second, the construction must be carried out in strict accordance with the construction specifications

the construction of 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 with each other

2. Multi strand shielded twisted pair must be laid for wiring. Multi strand is for standby, shielding is for debugging in special cases, and twisted pair is because 485 communication adopts the principle of differential mode communication, so twisted pair has good anti-interference performance. It is wrong not to use twisted pair

3. 485 bus must be hand-in-hand bus structure, and star connection and bifurcation connection must be resolutely avoided

4. The AC and chassis powered by the equipment must be truly grounded and well grounded. In many places, there are triangles on the surface. Some people have predicted that more than 80% of cars will use plastic sockets in the future. In fact, there is no grounding at all. A good grounding can prevent equipment from being struck by lightning and surge. When static electricity accumulates, it can cooperate with the lightning protection design of the equipment to better release energy and protect 485 bus equipment and related chips from damage

5. In order to avoid strong current interference, 485 bus should avoid walking together with strong current

III. several debugging methods are recommended:

before debugging, it is necessary to ensure that the equipment wiring is correct and the construction meets the specifications. The following debugging methods can be adopted according to the problems encountered

1. Common ground method: connect GND of all 485 devices with one wire or shielded wire, so as to avoid potential difference affecting communication between all devices

2. Terminal resistance method: connect 120 ohm terminal resistance to 485+ and 485- of the last 485 device to improve communication quality

3. Middle section disconnection method: check whether the equipment is overloaded, the communication distance is too long, the influence of a certain equipment on the whole communication line, etc. by disconnecting from the middle. 3. Open the main interface of the fatigue testing machine software

4. Separate wire pulling method: simply pull a wire to the equipment alone, which can be used to eliminate whether it is wiring. Our Jinan experimental machine factory is the national leader in technology, service and after-sales. We sincerely welcome your visit and consultation, which has caused communication failures

5. Converter replacement method: carry several converters with you, so that you can rule out whether it is the quality problem of the converter that affects the communication quality

6. Notebook debugging method: first ensure that the laptop 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

IV. suggestions and advice

several common communication failures with 485 bus structure are as follows

1. No communication and no response

2. Data can be uploaded, but not downloaded

3. The system prompt is disturbed during communication, or the communication indicator light keeps flashing when there is no communication

4. Sometimes it can communicate, sometimes it can't communicate, some instructions can be connected, and some instructions can't be connected

in order to reduce communication failures, the following suggestions and advice are put forward for reference

1. It is recommended that users use and purchase 485 converters provided by the access control manufacturer or 485 converters of recommended brands designated by the manufacturer

2. The access control manufacturer will do a lot of testing work on its supporting 485 converter, and will require the 485 converter manufacturer to carry out production and quality inspection according to its fixed performance parameters, so it has good compatibility with access control equipment. Never be greedy for cheap to buy 485 converters from non brand manufacturers

3. Strictly follow the construction specifications of 485 bus, and put an end to any fluke

4. Adopt scientific and reserved solutions for 485 bus projects with long lines and more loads

5. If the communication distance is too long, such as over 500 meters, it is recommended to use repeater or 485hub to solve it

6. If there are too many loads, such as more than 30 on a bus, 485hub is recommended to solve the problem

7. Carry out on-site commissioning and complete commissioning equipment. For on-site debugging, you must carry several converters that can connect long distance and multi load, a common computer notebook, a multimeter with open test path, and several 120 ohm terminal resistors