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485 communication knowledge

by:Sure     2021-08-11
1. What kind of communication line should be used for the 485 bus? The internationally accepted shielded twisted pair must be used. The type of shielded twisted pair we recommend is RVSP2*0.5 (two-core shielded twisted pair, each core is composed of 16 strands of 0.2mm wire). The use of shielded twisted pairs helps to reduce and eliminate the distributed capacitance generated between the two 485 communication lines and the 1.485 bus generated around the communication line. What kind of communication line should be used for the 485 bus must use the internationally accepted shielded twisted pair Wire. The type of shielded twisted pair we recommend is RVSP2*0.5 (two-core shielded twisted pair, each core is composed of 16 strands of 0.2mm wire). The use of shielded twisted-pair cables helps to reduce and eliminate the distributed capacitance between the two 485 communication lines and the common mode interference generated around the communication lines. Most engineering companies are accustomed to using Category 5 network cables or over Category 5 network cables as 485 communication lines, which is wrong. This is because:
(1) Ordinary network cables have no shielding layer and cannot prevent common mode interference. (2) The network cable is only 0.2mm square, and the wire diameter is too thin, which will reduce the transmission distance and reduce the equipment that can be connected. (3) The network cable is a single-strand copper wire, which is easier to break than a multi-core wire. 2. Why grounded 485 transceiver can work normally when the specified common mode voltage is between -7V and +12V. If it exceeds this range, the communication will be affected, and the communication interface will be damaged severely. Common-mode interference will increase the above-mentioned common-mode voltage. One of the effective ways to eliminate common mode interference is to use the shielding layer of the 485 communication line as a ground wire, connect the equipment in the network such as machines, computers, and so on, and connect them to the ground reliably from one point. 4. How to route the 485 communication line? The communication wires should be kept away from high-voltage wires as much as possible, not parallel to the power wires, let alone bundled together. 5. Why does the 485 bus adopt the hand-in-hand structure instead of the star structure? The star structure will produce reflected signals, which will affect the 485 communication. The length of the branch line from the bus to each terminal device should be as short as possible, generally not exceeding 5 meters. If the branch line is not connected to the terminal, there will be reflected signals, which will cause strong interference to the communication, and it should be removed. 6. Can there be contacts between devices on the 485 bus? In the same network system, use the same type of cable to minimize the number of contacts in the line. Ensure that the joints are well welded and wrapped tightly to avoid loosening and oxidation. Ensure a single, continuous signal channel as a bus. 7. What is common mode interference and differential mode interference? How to eliminate the interference on the communication line? The 485 communication line is composed of two twisted-pair lines. It transmits signals through the voltage difference between the two communication lines, so it is called differential voltage transmission. Differential mode interference is transmitted between two signal lines and belongs to symmetrical interference. The method to eliminate differential mode interference is to add a bias resistor in the circuit and use a twisted pair; common mode interference is transmitted between the signal line and the ground, which is asymmetrical interference. Methods to eliminate common-mode interference include: (1) Use shielded twisted-pair wires and effectively ground them (2) Use galvanized pipes for shielding where there is a strong electric field (3) When wiring, stay away from high-voltage lines, not to mention high-voltage power lines and signals (4) Do not share the same power supply with the electric control lock. (5) Use linear stabilized power supply or high-quality switching power supply (ripple interference less than 50mV)8. Under what circumstances should a terminal resistance be added on the 485 bus? Generally, there is no need to increase the terminal resistance. Only when the 485 communication distance exceeds 100 meters, the terminal resistance should be added at the beginning and end of the 485 communication. 9. How to extend the communication distance of 485 One of the specifications of the 485 network is 1.2 kilometers in length and 32 nodes. If this limit is exceeded, a 485 repeater or 485 hub must be used to extend the network distance or the number of nodes.  Using a 485 repeater or a 485 hub, a large 485 network can be divided into several network segments. A 485 repeater or 485 hub is like a 'bridge' between 485 network segments. Of course, each network segment still follows the above 485 specification, which is 1.2 kilometers in length and 32 nodes. Use 485 repeater to extend the network distance Graphic: Use 485 repeater to solve the 485 bifurcation problem, as shown in the figure: Use 485 hub to construct star-shaped 485 network 485 hub is an extension of the concept of 485 repeater, it not only solves the problem The problem of multiple forks is solved, and the problem of mutual isolation between network segments is also solved, that is, a problem (such as a short circuit, etc.) in one network segment will not affect other network segments, thereby greatly improving the security of large-scale networks And stability. We can experience the benefits of the star wiring network from the development process of the local area network from the bus type to the star type. Similarly, a star-shaped 485 network formed by a 485 hub will also be a direction for the development of the 485 network.
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