Preface     

485 communication instability problems (specifically manifested as sometimes can communicate on, sometimes communication is not) RS485 in the connection with too many devices, communication distance is too long, twisted-pair cable quality is poor, wiring is not standardized and so on, will lead to communication instability problems.

Solution:

A few concepts about 485 bus.

1. 485 bus communication distance can reach 1200 meters.

According to the theory of 485 bus structure, under the premise of ideal environment, 485 bus transmission distance can reach 1200 meters. The condition is that the communication cable quality standards, baud rate of 9600, only load a 485 device, in order to make the communication distance of 1200 meters, so usually the actual stability of the 485 bus communication distance often does not reach 1200 meters. If the load of 485 devices, wire impedance is not in line with the standard, wire diameter is too thin, poor quality converters, equipment lightning protection complex and baud rate increase and other factors will reduce the communication distance.

2. 485 bus can take 128 devices for communication.

In fact, not all 485 converters are able to take 128 devices, according to the 485 converter chip model and 485 device chip model to determine, only according to the indicators of the lower chip to determine its load capacity. General 485 chip load capacity has three levels - 32 units, 128 units and 256 units. In addition to the theoretical nominal often actually can not be reached, the longer the communication distance, the higher the baud rate, the finer the wire diameter, the poorer the quality of the wire, the poorer the quality of the converter, the converter power supply is insufficient (passive converter), the stronger the lightning protection, which will reduce the true number of loads.

3. 485 bus is one of the simplest, most stable, most mature industrial bus structures

This concept is wrong. 485 bus is a kind of equipment for networking, economical, traditional industrial bus way. Its communication quality needs to be based on the construction experience of debugging and testing can be guaranteed. 485 bus, although simple, but also must be strictly in accordance with the installation and construction specifications for wiring.

Must be strictly in accordance with the construction specification construction

In the 485 bus system construction must be strictly in accordance with the construction specification construction, in particular, the following points should be noted.

1. 485 + and 485 - data lines must be mutually twisted.

2. The wiring must be cloth multi-stranded shielded twisted pair. Multi-stranded is for backup, shielding is to facilitate the emergence of special circumstances when the debugging, twisted because 485 communication using differential mode communication principle, twisted better anti-interference. It is wrong not to use twisted pair.

3. 485 bus must be used hand in hand bus structure, and resolutely avoid star connection and fork connection.

4. Equipment power supply AC and chassis must be real grounded, and well grounded. There are many places on the surface of the triangle socket, in fact, there is no grounding, good grounding can prevent the equipment from being struck by lightning, surge impact. Static electricity can be accumulated with the equipment lightning protection design better release of energy to protect the 485 bus equipment and related chips from injury.

5. In order to avoid interference with strong electricity, 485 bus should be avoided and strong electricity go together.

Recommend several debugging methods.

Before debugging, we must first ensure that the equipment is wired correctly, and the construction is in line with the norms. Can be used according to the problems encountered in the following debugging methods.

1. A common ground method: with a line or shielded wire will be all 485 devices connected to the GND ground, so that you can avoid the existence of all the equipment affects the communication potential difference between.

2. Terminating resistor method: in the last 485 device 485 + and 485- and connected to the 120 ohm terminating resistor to improve the quality of communication.

3. The middle section disconnect method: by disconnecting from the middle to check whether the equipment load is too much, the communication distance is too long, the impact of a device on the entire communication line.

4. A separate line method: a separate simple pull a line to the device, which can be used to rule out whether the wiring caused by communication failures.

5. Replace the converter method: carry a few converters, so you can rule out whether the quality of the converter affects the quality of communication.

6. The laptop debugging method: first ensure that their own portable computer laptop is a normal communication device, use it to replace the customer's computer for communication, if normal, it shows that the customer's computer serial port may be damaged or injured.

The recommendations and advice using the 485 bus structure of several common communication failures are the following.

1、Communication is not on, no response.

2、Can upload data, but not download data.

3、Communication system prompted by the interference, or not communicate with the communication indicator also keeps flashing.

4、Sometimes you can communicate, sometimes you can not communicate, some commands can pass, some commands can not pass.

In order to reduce the communication failure to put forward the following suggestions and advice for reference.

1. It is recommended that users use and purchase access control manufacturers to provide 485 converter or manufacturer-specified recommended brand of 485 converter.

2. Access control manufacturers will do a lot of testing work with its matching 485 converter, and will require 485 converter manufacturers in accordance with its fixed performance parameters for production and quality testing, so it has a better compatibility with access control equipment. Do not buy cheap 485 converter miscellaneous manufacturers.

3. In strict accordance with the construction specifications of the 485 bus construction, to eliminate any chance.

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

5. If the communication distance is too long, such as more than 500 meters, it is recommended to use repeaters or 485HUB to solve the problem.

6. If the number of loads is too much, such as a bus more than 30 units, it is recommended to use 485HUB to solve the problem.

7. Site commissioning with all the commissioning equipment. On-site commissioning must carry a few can be connected to long-distance and multi-load converter, a commonly used computer laptop, test path break multimeter, a few 120 ohm terminal resistor.     

Theory and practice is always a gap, the theory is just a reference, the specific project application, not contrary to the basic theory of the premise, specific problems and specific analysis!     

In a variety of field, the 485 bus is very widely used, but the 485 bus is more prone to failure, now the 485 bus is prone to failure and can be eliminated these faults are listed below:　　

1. As the 485 signal uses a pair of unbalanced differential signals, meaning that each device in the 485 network must be connected to ground through a signal loop to reduce the noise on the data line, so the data line is best made up of twisted-pair cable, and outside the shielding layer as a ground, the 485 network in the 485 device is connected to the network, and at a point reliably grounded.　　
2. In the industrial field, the scene is very complex, there is a very high common mode voltage between the nodes, 485 interface is used in differential transmission mode, there is anti-common-mode interference ability, but when the common mode voltage is greater than +12V or less than -9V, more than the limit of the 485 receiver receiving voltage. The receiver will not be able to work, and may even burn the chip and together with the device.　　
3. 485 bus with the extension of the transmission distance, will produce echo reflection signal, if the transmission distance of the 485 bus if more than 100 meters, it is recommended that the construction of the 485 communication in the beginning of the end and the end of the end of the 120 ohm termination resistor.　　
4. 485 bus in the 485 node to minimize the distance between the trunk, it is generally recommended that the 485 bus using hand in hand bus topology. The star structure will produce reflected signals, affecting the quality of 485 communication. If in the construction process must require 485 nodes from the 485 bus trunk distance more than a certain distance, the use of 485 repeaters can make a 485 bus bifurcation.　　
5. Factors affecting the load capacity of the 485 bus: communication distance, the quality of the wire, baud rate, converter power supply capacity, lightning protection of 485 devices, 485 chip selection. If there are more 485 devices on the 485 bus, it is recommended to use the 485 converter with power supply, passive 485 converter due to power theft from the serial port, power supply is not very sufficient, the load capacity is not enough. Choose a good cable, if possible to use the lowest possible baud rate, choose a high load capacity of the 485 chip, can improve the load capacity of the 485 bus. 485 device lightning protection in the lightning protection tube will absorb the voltage, resulting in a reduction in the load capacity of the 485 bus, remove the lightning protection can improve the load capacity of the 485 bus.