RS485 Construction Suggestions

I. Several Concepts about 485 bus:
1. The communication distance of the 485 bus can reach 1200 meters.
According to the 485 bus structure theory, the transmission distance of the 485 bus can reach 1200 meters under the ideal environment. The condition is that the communication wire is of high quality and the baud rate is 9600. Only one 485 device can be loaded to make the communication distance reach 1200 meters, therefore, the actual stable communication distance of the 485 bus is usually less than 1200 meters. If there are more than 485 devices under load, the wire impedance is not compliant with the standard, the wire diameter is too small, the converter quality is poor, the equipment lightning protection is complicated, and the baud rate is improved, and other factors will reduce the communication distance.
2. The 485 bus can carry 128 devices for communication.
In fact, not all 485 converters can contain 128 devices. It should be determined based on the chip model of the 485 converter and the chip model of the 485 device, the load capacity can only be determined by chips with low indicators. Generally, 485 chips have three levels of load capacity: 32, 128, and 256 .. In addition, in theory, the nominal name is often not actually reached, the longer the communication distance, the higher the baud rate, the finer the wire diameter, the poorer the wire quality, the poorer the converter quality, the insufficient power supply of the converter (Passive Converter), and the stronger the lightning protection, these will reduce the number of real loads.
3. The 485 bus is the simplest, most stable, and most mature industrial bus structure.
This concept is incorrect. The 485 bus is a cost-effective and traditional industrial bus used for device networking .. The communication quality can be ensured through debugging and testing based on the construction experience. Although the 485 bus is simple, it must be cabled in strict accordance with the installation and construction specifications.
2. Construction in strict accordance with construction specifications
The construction of the 485 bus system must be strictly in accordance with the Construction Specifications. Pay special attention to the following points.
1. 485 + and 485-data lines must be twisted to each other.
2. multi-strand shielded twisted pair wires must be deployed for cabling. Multiple strands are used for backup, and shielding is used to facilitate debugging in special cases. The reason is that the 485 Communication adopts the principle of differential mode communication, and the dual twisting has better anti-interference performance. It is wrong not to use twisted pair wires.
3. The 485 bus must use a hand-held bus structure to resolutely avoid star and branch connections.
4. The AC power supply and chassis of the equipment must be grounded and grounded properly. There are many triangular sockets on the surface. In fact, there is no grounding at all. Good grounding can prevent the impact of lightning strikes and surge on the equipment. When static electricity is accumulated, it can work with the lightning protection design of the equipment to release energy well, to protect 485 bus devices and related chips from harm.
5. To avoid interference from strong electricity, the 485 bus should avoid interference with strong electricity.
3. Several debugging methods are recommended:
Before commissioning, ensure that the wiring of the equipment is correct and the construction complies with the specifications. You can use the following debugging methods based on the problem.
1. Co-location method: Use one line or a shielded line to connect the Gnd of all 485 devices, so as to avoid potential differences affecting communication between all devices.
2. Terminal resistance method: the terminal resistance of the last 485 devices is 485 + and 485-connected with 120 Ohm to improve the communication quality.
3. Center segmentation disconnection method: checks whether there are too many device loads, the communication distance is too long, and the impact of a certain device on the entire communication line.
4. Independent cable pulling method: simply pull a line to the device, which can be used to determine whether the wiring causes a communication fault.
5. Change the converter method: carry several Converters with you, so that you can determine whether the converter quality problem affects the communication quality.
6. Laptop debugging method: ensure that the laptop you carry with you is a normal communication device, and use it to replace the client's computer for communication. If it is normal, it indicates that the client's computer serial port may be damaged or injured.
4. suggestions and advice
There are several common communication faults using the 485 bus structure.
1. Unable to communicate, no response.
2. Data can be uploaded but cannot be downloaded.
3. The system prompts interference during communication, or the communication indicator does not stop flashing when the communication fails.
4. Sometimes it can communicate, sometimes it cannot communicate, some commands can communicate, and some commands cannot communicate.
Provide the following suggestions and suggestions to reduce communication faults.
1. We recommend that you use and purchase the 485 converter provided by the access control manufacturer or the 485 converter of the recommended brand specified by the manufacturer.
2. Access Control manufacturers will perform a lot of tests on the 485 converter they provide, and require the 485 converter manufacturer to perform production and quality tests according to its fixed performance parameters, therefore, it has good compatibility with access control devices. Do not purchase a 485 converter from a brand manufacturer at a low cost.
3. strictly follow the construction specifications of the 485 bus to prevent any luck.
4. Use scientific and reserved solutions for 485 bus projects with long lines and heavy loads.
5. If the communication distance is too long, such as over 500 meters, we recommend that you use a repeater or 485hub.
6. If the number of loads is too large, for example, if there are more than 30 servers on a bus, we recommend using 485hub to solve the problem.
7. On-site debugging with debugging equipment. On-site debugging must be carried with you several transformer that can be connected to long distance and multi-load, a commonly used computer notebook, A multimeter with test circuit breaker, and several 120 Ohm terminal resistors.