Common failures of RS485 communication, solutions, and precautions for wiring installation [should be collected]

With the unfolding of the 'Industry 4.0' strategy, the development of computer technology, communication technology, and IT technology has penetrated into the industrial control field. The most important manifestations are industrial fieldbus technology and industrial Ethernet technology. Among them, industrial fieldbus technology, especially the widespread use of Ethernet technology, has brought profound changes to automation technology.

RS485/MODBUS is a popular industrial networking method, which is characterized by simple and convenient implementation, and there are many meters that support RS485. Nowadays, instrument vendors have also switched to support RS485/MODBUS. The reason is simple. The conversion interface of RS485 is not only cheap but also has a wide variety. At least in the low-end market, RS485/MODBUS will still be the most important industrial networking method.

RS485 has two-wire system and four-wire system. The four-wire system can only realize the point-to-point communication mode, which is rarely used now. Nowadays, the two-wire connection mode is mostly used. The method is a bus topology structure that can connect up to 32 nodes on the same bus.

The standard 485 network generally adopts the hand-in-hand bus structure to route, try not to use star connection and irregular branch connection. The star structure will produce reflected signals, which will affect the 485 communication. The communication line should be as far away as possible from interference sources such as high-voltage wires, fluorescent lamps, etc. When the communication line cannot be avoided from interference sources such as power lines, the communication line should be perpendicular to the power line, not parallel, and cannot be bundled together, and use high-quality twisted-pair cables. Line.

Common failures and solutions of RS485

1. What should be paid attention to in RS485 wiring?

The following suggestions are made to reduce communication failures.

(1) It is recommended that users use and purchase the 485 converter provided by the manufacturer or the 485 converter of the recommended brand designated by the manufacturer.

(2) The manufacturer will do a lot of testing work on the 485 converter that it is supporting, and will require the 485 converter manufacturer to carry out production and quality testing according to its fixed performance parameters, so it is compatible with the access control equipment Have better compatibility. Don't be greedy to buy a 485 converter from a brand-name manufacturer at a low price.

(3) Carry out the construction strictly in accordance with the 485 bus construction specification, and put an end to any fluke psychology.

(4) If the communication distance is too long, such as over 500 meters, it is recommended to use a repeater or 485 HUB to solve it.

(5) If there are too many loads, such as more than 30 units on a bus, it is recommended to use 485 HUB to solve the problem.

(6) Use scientific and reserved solutions for 485 bus projects with longer lines and more loads.

(7) On-site debugging with complete debugging equipment. For on-site debugging, you must carry a few converters that can connect to long distances and multiple loads, a commonly used computer notebook, a multimeter for testing the open circuit, and a few 120 ohm terminal resistors.

2. The common communication failures with 485 bus structure are as follows?

(1) No communication, no response.

(2) Data can be uploaded, but data cannot be downloaded.

(3) The system prompts that there is interference during communication, or the communication indicator keeps flashing when not communicating.

(4) Sometimes it can communicate, sometimes it can’t communicate, some commands can be communicated, and some commands cannot be communicated.

3. What are the debugging methods for failure?

Before debugging, make sure that the equipment wiring is correct and the construction conforms to the specifications. The following test methods can be used according to the problems encountered.

(1) Common ground method:

Use 1 wire or shielded wire to connect the GND ground of all 485 devices, so as to avoid the potential difference between all devices that affects communication .

(2) Intermediate segment disconnection method:

Check whether the equipment is overloaded, the communication distance is too long, or a certain device affects the entire communication line by disconnecting from the middle. Influence etc.

(3) Terminal resistance method:

Connect 120 ohm terminal resistance in parallel to the 485+ and 485- of the last 485 device to improve the communication quality.

(4) Changing the converter method:

Bring a few converters with you so that you can rule out whether the quality of the converter is affecting the communication quality.

(5) Pull wire alone:

Pull a wire separately to the device, so that it can be used to rule out whether it is the wiring that caused the communication failure.

(6) Notebook debugging method:

First make sure that the computer notebook you carry with you is a device with normal communication, and use it to replace the client computer for communication. If it is normal, it means that the client The serial port of your computer may be damaged or injured.