485 issues to be paid attention to during cabling

Chapter 1 clarifies several concepts:
Concept 1:The communication distance of the 485 bus can reach 1200 meters.
In fact, only the 485 bus structure can theoretically make the transmission distance reach 1200 meters under the ideal environment. it generally refers to the high quality standard of the communication wire, with a baud rate of 9600. Only one 485 device can make the communication distance reach 1200 meters, and communication does not mean that every communication is normal. therefore, the actual stable communication distance of the 485 bus is usually less than 1200 meters. many 485 devices are loaded, the wire impedance is not compliant with the standard, the wire diameter is too small, the converter quality is poor, the equipment lightning protection, the baud rate increase and other factors will reduce the communication distance.

Concept 2:The 485 bus can communicate with 128 devices.
In fact, not all 485 converters can contain 128 devices. it depends on the model used by the 485 converter chip and the model used by the 485 device chip. who is low. generally, the load capacity of 485 chips is divided into three levels, 32 sets, 128 sets, and 256 sets. theoretically, the nominal name is often not actually reached. the longer the communication distance, the higher the baud rate, the smaller the wire diameter, the worse the quality of the wire, the worse the quality of the converter, the less power supply of the converter (Passive Converter ), stronger lightning protection will greatly reduce the number of real loads.

Concept 3:The 485 bus is the simplest, most stable, and most mature industrial bus structure.
This concept is incorrect. it should be: 485 bus is a economical and traditional industrial bus method used for device networking. communication quality should be tested and debugged based on construction experience. although the 485 bus is simple, it must be installed in strict construction specifications for cabling.

Chapter 2 strict construction specifications:
485 + and 485-data lines must be twisted to each other.
Multi-strand shielded twisted pair cables must be deployed for the purpose of backup. the shielding is used for debugging in special cases. The 485 Communication adopts the principle of differential mode communication and the best anti-interference performance of the twisted pair cables. it is an extreme mistake not to use twisted pair wires.
The 485 bus must be a hand-held bus structure, and the star connection and the split connection should be resolutely put an end to it.
The AC power supply and chassis of the equipment must be grounded and grounded properly.
There are many triangular sockets on the surface, but there is no grounding at all. Be careful. when the grounding is good, the lightning protection design of the equipment can be used to release energy well when the equipment is hit by lightning surge and static electricity accumulation. protect the 485 bus devices and related chips from harm.
Avoid interference with strong power to avoid interference.

Chapter 3 common communication faults:
Unable to communicate, no response.
Data can be uploaded but cannot be downloaded.
During communication, the system prompts to be disturbed. Or when the communication fails, the communication indicator lights keep flashing.
Sometimes it can communicate, sometimes it cannot communicate. Some commands can communicate, and some commands cannot communicate.

Chapter 4 recommends several debugging methods:
First, ensure that the device wiring is correct and strictly complies with the specifications.

Common 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.

Terminal resistance method: 485 + and 485-on the last 485 devices are connected with a 120 Ohm terminal resistance to improve the communication quality.

Intermediate segmentation disconnection method: checks whether there are too many device loads, the communication distance is too long, and the damage of a device to the entire communication line.

Single Wire pulling method: temporarily pull a line to the device separately, which can be used to eliminate whether the wiring has caused communication faults.

Change the converter method: carry several Converters with you, so that you can eliminate whether the converter quality problem affects the communication quality.

Laptop debugging method: first ensure that the laptop you carry with you is a normal communication device, replace the customer's computer for communication, if you can, it indicates that the client's computer serial port may be damaged or injured.

Chapter 5 provides several suggestions and suggestions:
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.
Access Control manufacturers will perform a lot of test work on the 485 converter they provide, and will separately require 485 manufacturers to install their fixed performance parameters for production and quality testing, therefore, it has good compatibility with its access control equipment. do not purchase the 485 converter from a brand manufacturer at a low cost.

Strictly follow the construction specifications of the 485 bus to prevent any luck.

If the line load is too long, we should adopt an active and scientific reserved solution.
If the communication distance is too long, we recommend that you use a repeater or 485HUB if the communication distance exceeds 500.
If there are too many loads, we recommend that you use 485HUB if there are more than 30 servers on one bus.

Onsite debugging with debugging equipment.
On-site debugging must carry a few with you to ensure that the previous long distance and multi-load converter can be connected to a commonly used computer laptop test path open circuit multimeter a few 120 Ohm terminal resistance.