First of all, understand the following questions to quickly investigate the fault is of great help:

1, is the newly installed access control on the communication, or after installation for a period of time, suddenly not on the communication?

2, has been unable to communicate or sometimes communication is normal, sometimes communication is not on?

3, there is no non-standard behavior in the construction process of the line or whether the special electrical equipment other than the access control conventional equipment is connected?

4,are all controllers unable to communicate, or is it just one or several controllers with communication problems?

What tools do I need to bring? 

a, spare 485 converter,

b, spare to replace the test access controller

c, multimeter

d, laptop with serial port

Chapter 1 All controllers fail to communicate. Possible causes of failure are

1.RS485/232 converter damaged

The normal converter characteristics: the power lamp is steady on, when there is no communication, RX TX light is not bright, normal communication, RX TX light flashing. Note: A normal converter must have such characteristics, but it does not mean that having such characteristics can be considered a normal converter.

Abnormal characteristics of the converter: only power, no communication, not connected to the computer and controller, RX or TX light flashing or steady on. The power light is not working (the power supply is broken or the converter is broken) and so on.

Recommended detection method: Replace it with a spare converter.

2.The line of the 485 converter to the first controller is interrupted or 485+ 485- is reversed or short-circuited.

Or it's a virtual connection, or it's a disconnect. Recommended detection method: Use the resistance or buzzer mode of a multimeter to power off devices such as the controller and test them.

3.The computer serial port failure:

(1) the serial port number is set wrong. (The general computer is the original serial port is COM1 and COM2, if it is USB or other communication methods to the serial port may be a serial number between COM3 -COM255.)

Recommended test method: Use a laptop or other computer to compare the test.

(2) The computer serial port is damaged.

Recommended test method: Use a laptop or other computer to compare the test.

(3) The computer serial port is disabled. Recommended test method: Use a laptop or other computer to compare the test.

Many new computers, especially original brand machines, will disable serial ports in CMOS or [devices] when they leave the factory, so please let go of these Settings for people who are proficient in computers.

(4) The computer serial port is occupied by other products or other application software suggested detection methods: if it is occupied, there will be text prompts in the information box below our software.

Solutions are: if the computer has two serial ports, the two software use different serial ports, if there is only one, you can take turns to use, do not use at the same time. Or purchase devices such as USB serial converter and multiple serial cards to expand the number of serial ports.

(5) The serial port is obtained through the converter, and there are problems with quality and performance.

Some computers, especially notebooks, have no serial port. Need to buy USB serial port converter multi-serial port card and other devices to expand the serial port, the purchase of the need to pay attention to the recommended purchase of brand equipment, because these devices, although the function is similar, but the performance difference will be relatively large, some conversion equipment has a large delay delay may affect normal communication.

Chapter 2 Only one or several controllers do not communicate, or the communication is unstable, sometimes the communication is normal, sometimes there is a problem, then the possible causes of failure:

1, whether the controller SN number is filled in incorrectly.

2, whether the construction is not standardized: 

the construction is not standardized is the main reason for the instability of communication, and it is not easy to find the cause and solve the problem. Therefore, there should be enough normative awareness during construction, otherwise it will be more difficult to regret later.

3, are passive converters used? 

We believe that the access control system should prohibit the use of passive converters, passive converters have low cost, poor communication quality, short communication distance, poor load capacity, and almost no anti-interference ability.

4. Is there a star connection?

 485 communication mode, requiring each controller and converter are hand in hand down the string, the middle can not fork, nor can there be a star-shaped structure of the line. Because the interference generated by the line itself is very large, it is easy to cause communication instability. The construction personnel can not have a lucky heart, that: the load is not much, the line is not long, as if it can be, and it is lucky to save trouble for a time. Even if the communication can be normal during debugging, it is difficult to guarantee when the communication can not be normal during use.

5. Is there no twisted pair? 

485 communication line is the principle of differential mode transmission, so must take twisted pair (network cable is composed of 4 pairs of twisted pair, so take network cable, choose one of the pairs can, be careful not to be self-defeating, the network cable pair in one, trying to thick line to improve the quality of communication method is counterproductive.) It is quite dangerous to make ordinary power lines or parallel lines 485 communication lines, because the longer the distance between these parallel lines, the greater the interference generated by the line itself, and the worse the anti-interference ability, until it is unable to communicate.

6. Is there a real ground cable for the 220V AC power supply to the controller? 

The 220V ground cable that supplies power to the controller is connected to the chassis. The accumulated static electricity, leakage lightning, and other interfering electrical signals can be removed through the ground cable. If there is no ground cable, the interference will affect the communication quality and even the normal running of the controller. Make lightning protection design ineffective or reduce the effect.

7. Is the communication line too long?

485 communication line nominal is 1200 meters, in fact, according to the actual line quality, installation environment, controller communication chip performance, converter performance and other differences, is not up to this communication distance. Generally, the best equipment and environment can reach 500-800 meters, if the distance is too long, you can consider buying 485 Repeaters from the converter manufacturer to extend the transmission distance.

8. Is the equipment loaded too much?

The number of load controllers of 485 lines is specified, and the load capacity of different controller communication chips and converters is not the same. There are 32, 64, 128, 255 and other different levels, the relevant manufacturers will be marked in the product manual, the number of marks is not necessarily the actual load capacity, the length of the communication distance, the quality of the wire, the quality of the environment will reduce the load capacity. Therefore, it is recommended to calculate its true load capacity by 50% of the parameters indicated by the manufacturer.

9. Is there a certain line 485+ 485- wrong connection?

10, is there strong external interference?

It is recommended to keep a distance of more than 30 cm between the strong wire and the strong wire.

11. Is there any high-power equipment or strong-current cabinet nearby?

Grounding through metal tube shield is an effective method to resist external interference. However, the cost is relatively high, the construction is complex, the general access control is PVC pipe, you can consider the use of shielded network cable or twisted pair cable communication line.

Note:If the following measures are adopted, the anti-interference ability of the system can be improved, but if the construction is not standardized, these anti-interference measures can not solve the problem, it is futile. Therefore, to ensure that the construction standard is to reduce the communication failure rate, to facilitate the investigation of the fault causes of the prerequisite.

Measures: The communication line adopts shielded network cable or shielded twisted pair cable. When the system communication is interfered with, the shielding layer is connected to the GND terminal of all controllers, which may eliminate the interference. If the system has no interference, it is not necessary to connect it in advance, because in some cases, the shielded ground has side effects.

Parallel a 120 ohm resistor to the 485+ 485- terminal of the last controller.