485 bus isolation and stability solutions - Database & Sql Blog Articles

SMD aluminum electrolytic capacitor

Manufacturers produce direct sales LED lamp bead patch lighting products 0805 white light super bright indicator light dedicated

The probe capacitor polarity needle has a head diameter of 5.8. The total length of the 7-pin and 12-pin needles is 5.

First, several concepts about the 485 bus:
1, 485 bus communication distance 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 quality of the communication wire is up to 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 stable communication distance of the 485 bus often reaches less than 1200 meters. If there are many 485 load devices, the wire impedance is not up to standard, the wire diameter is too thin, the converter quality is poor, the lightning protection of the device is complicated, and the baud rate is increased, etc., which will reduce the communication distance.
2, 485 bus can bring 128 devices for communication.
In fact, not all 485 converters can carry 128 devices. According to the model of the 485 converter and the model of the 485 device chip, the load capacity can only be determined according to the chip with lower index. The general 485 chip load capacity has three levels - 32, 128 and 256. . In addition, the theoretical nominal is often not achieved. 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, and the insufficient power supply of the converter (passive Converters, the stronger the lightning protection, these will reduce the actual load.
3, 485 bus is one of the simplest, most stable, and most mature industrial bus structure. This concept is wrong. The 485 bus is an economical, traditional industrial bus method for device networking. . The communication quality needs to be debugged and tested according to the construction experience. Although the 485 bus is simple, it must be wired in strict accordance with the installation and construction specifications.
Second, must be strictly in accordance with the construction specifications construction in the 485 bus system construction must be strictly in accordance with the construction specifications, in particular, the following points should be noted.
1, 485+ and 485-data lines must be twisted together.
2, the wiring must be clothed with multiple strands of shielded twisted pair. Multi-strand is for standby, shielding is for the convenience of debugging in special situations. Twisting is because 485 communication adopts the principle of differential mode communication, and the anti-interference of twisted pair is better. It is wrong to not use twisted pair.
3, 485 bus must be hand-held bus structure, and resolutely avoid star connection and fork connection.
4. The AC and chassis of the equipment supply must be grounded and grounded. There are many places where there are triangular sockets on the surface. In fact, there is no grounding at all. Good grounding can prevent the equipment from being struck by lightning or surge. When the static electricity is accumulated, the lightning protection design of the device can be used to release the energy better, and the 485 bus device and related chips are protected from damage.
5, in order to avoid strong interference to it, 485 bus should avoid walking with strong electricity.
Third, recommend several debugging methods:
Before commissioning, first make sure that the equipment is wired correctly and the construction is in compliance. The following debugging methods can be used according to the problems encountered.
1. Common ground method: Connect the GND grounds of all 485 devices with one wire or shielded wire. This can avoid the potential difference between all devices affecting communication.
2. Terminating resistance method: Connect the 120 ohm terminating resistor on the 485+ and 485- of the last 485 device to improve the communication quality.
3. Intermediate segment disconnection method: Check whether the equipment is overloaded, the communication distance is too long, and the influence of a certain equipment on the entire communication line is made by disconnecting from the middle.
4, separate cable method: a simple pull a line to the device, this can be used to eliminate whether the wiring caused communication failure.
5, replace the converter method: carry a few converters with you, this can eliminate whether the quality of the converter affects the communication quality.
6, notebook debugging method: First ensure that the computer notebook that you carry with you is a normal communication device, use it to replace the client computer for communication, if normal, it means that the serial port of the customer's computer may be damaged or injured.
Fourth, the recommendations and advice to use the 485 bus structure common types of communication failures are as follows.
1. Communication is not on, no response.
2, you can upload data, but you can not download data.
3. When the system prompts interference during communication, or the communication indicator does not stop flashing when not communicating.
4, sometimes can communicate, sometimes communication is not on, some instructions can pass, some instructions can not pass.
The following suggestions and advice are provided for reference to reduce communication failures.
1. It is recommended that the user 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, the access control manufacturer will do a lot of testing work with its supporting 485 converter, and will require 485 converter manufacturers to carry out production and quality testing according to their fixed performance parameters, so it has good compatibility with access control equipment. Don't be tempted to buy a 485 converter from a brand-name manufacturer.
3, in strict accordance with the construction specifications of the 485 bus construction, put an end to any luck.
4. Adopt a scientific and reserved solution for the 485 bus project with long line and heavy load.
5. If the communication distance is too long, such as over 500 meters, it is recommended to use a repeater or 485HUB to solve.
6. If there are too many loads, such as more than 30 on a bus, it is recommended to use 485HUB to solve the problem.
7, on-site debugging with debugging equipment. On-site debugging must carry a few converters that can be connected to long distances and multiple loads, a commonly used computer notebook, a multimeter with a test path open circuit, and several 120 ohm terminating resistors.