485 bus structure debugging method for access control system

Concept 1: 485 bus communication distance can reach 1200 meters.

Actually, the 485 bus structure theoretically can only achieve a transmission distance of 1200 meters under the premise of an ideal environment. It generally refers to the high quality standards of communication wires, and the baud rate is 9600. Only one 485 device can make the communication distance reach 1,200 meters. Communication does not mean that each communication is normal. Therefore, the actual stable communication distance of the 485 bus is far less than 1200 meters. There are many loads of 485 equipment, the wire resistance is not up to standard, the wire diameter is too small, the quality of the converter is bad, and the equipment Lightning protection, baud rate increase and other factors will reduce the communication distance.

Concept 2: 485 bus can carry 128 devices for communication.

In fact, not all 485 converters can bring 128 devices. According to the model used in the 485 converter chip and the model used by the 485 device chip to determine. Who is low on who. General 485 chip load capacity has three levels 32 sets of 128 sets of 256 sets.

The theoretical nominal value is often inaccessible. The longer the communication distance, the higher the baud rate, the finer the wire diameter, the worse the wire quality, the poorer the quality of the converter, and the insufficient power supply of the converter (passive conversion ), the stronger the lightning protection, these will greatly reduce the actual load.

Concept 3: The 485 bus is the simplest, most stable and mature industrial bus structure.

This concept is wrong. It should be: The 485 bus is an economical, traditional industrial bus method for networking devices. The communication quality is tested and debugged according to construction experience. The 485 bus is simple but must be strict Installation and construction specifications for wiring.

Strictly several construction specifications:

485+ and 485- data lines must be mutually twisted pair. Wiring must be shielded with twisted pair twisted pairs. Multiple strands are for standby. Shielding is for debugging when special conditions occur. Twisted-pair is because 485 communication adopts differential mode. Communication principle, the best anti-interference twisted pair. Not using twisted pair, is extremely wrong.

485 bus must be hand-in-hand bus structure, and resolutely eliminate the star connection and bifurcated connection.

The equipment-powered AC and chassis must be grounded and grounded. There are many places with triangular sockets on the surface. In fact, there is no grounding at all. Be careful. When the grounding is good, you can ensure that the equipment is shocked by lightning. The lightning protection design releases energy well. Protecting the 485 bus device and related chips from damage.

Avoid going with strong electricity to avoid interference by strong electricity.

Several common communication failures:

Communication is not, no response.

Can upload data but cannot download data.

During communications, the system prompts you to be disturbed. When not communicating, the communication light flashes.

Sometimes communication is possible, sometimes communication is not available. Some instructions can be connected and some instructions cannot be communicated.

Recommend several debugging methods:

First of all, we must ensure that the equipment wiring is correct and strictly conform to the common grounding method: Use 1 line or shielded wire to connect the GND ground of all 485 equipments, so as to avoid the potential difference between all equipments that affect communication.

Terminating resistor method: Connect 485+ and 485- on the last 485 device with 120 ohm terminating resistor to improve communication quality.

Intermediate segmentation disconnection method: by disconnecting from the middle to check whether the device is overloaded, the communication distance is too long, the impact of a device damage on the entire communication line and other reasons.

Separate pull method: Separately pull a line briefly to the device, this can be used to rule out whether the wiring caused communication failure.

Change the converter method: Take several converters with you, so you can rule out whether the quality of the converter affects the quality of the communication.

Notebook debugging method: first ensure that your portable computer laptop is a normal communication device, replace the client computer, to communicate, if you can, that the client's computer serial port may be damaged or injured.

Make several suggestions and advice:

It is recommended that users use and purchase the 485 converter provided by the access control manufacturer or the 485 converter designated by the manufacturer to recommend the brand.

Access control manufacturers will do a lot of testing work with their supporting 485 converters, and will individually require 485 manufacturers to install their fixed performance parameters for production and quality testing, so they have good compatibility with their access control devices. Buy cheaper manufacturers of 485 converters.

Strictly install the construction specification of the 485 bus to carry out construction and eliminate any chances.

Use active scientific reserved solutions for situations where the line has long loads.

If the communication distance is too long, it is recommended to use a repeater or 485HUB to solve the problem if it exceeds 500 meters.

If the load is too much, it is recommended to use 485HUB to solve the problem if more than 30 buses on one bus.

On-site commissioning with Qi debugging equipment.

On-site commissioning must be carried with several portable multimeters to ensure long-distance and multi-load conversion of a common computer laptop test path, several 120 ohm terminators.