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1. What kind of communication line should be used for 485 bus? How many devices can be connected to a bus?
RVSP shielded twisted pair must be used. The specifications of the shielded twisted pair used are related to the distance of the 485 communication line and the number of connected devices, as shown in the following table. 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. Number of communication distance devices Communication line specifications 1-400m 1-32 units 0.5mm2 400-800m 1-16 units 0.5mm2 400-800m 17-32 units 0.75mm2 800-1200m 1-8 units 0.5mm2 800-1200m 9-21 units 0.75mm2 800-1200m 22-32 1.0mm2 engineering companies are mostly accustomed to using Category 5 network cables or super 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 small, which will reduce the transmission distance and reduce the equipment that can be connected. (3) The network wire is a single-strand copper wire, which is easier to break compared to a multi-core wire.
2.Why ground
The 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, etc., and connect them to the ground reliably from one point.
3. Can the electric lock and the controller/card reader use the same power supply?
can not. When the electric lock does not operate, the ripple voltage of SKPS is only 40-50mV; once it is activated, that is, when the electric lock is opened and closed, the ripple voltage will rise to 100mV-300mV, and the ripple will pass through the ground Wire entering the controller and card reader will cause the communication chip and CPU to heat up, resulting in unstable communication, and even burn the chip in serious cases. Moreover, at the moment of power-off and power-on of the electric control lock, the coil inside the electric control lock will charge and discharge to generate a pulse of up to 850mA. If there is no parallel diode at both ends of the electric control lock, the ripple signal will also be transmitted. Controller and card reader. It is recommended that a controller and all card readers connected to it share a SKPS power supply; each electric lock under the controller uses a separate SKPS power supply.
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, so it should be removed. In the access control system, there are two places where the 485 bus is applied. One is the controller connected from the computer to the bottom, and the other is the 485 card reader connected from the controller to the bottom.
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. The methods to eliminate common mode interference include: (1) Use shielded twisted-pair wires and effective grounding (2) Use galvanized pipes for shielding where there is a strong electric field. Bundle the wires together (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 to the 485 bus?
Generally, there is no need to increase the terminal resistance. Only when the 485 communication distance exceeds 300 meters, the terminal resistance should be added at the beginning and end of the 485 communication. Especially when the number of devices on the 485 bus is small. When the number of equipment is large (such as more than 22). Generally, there is no need to increase the terminal resistance, because the terminal resistance will reduce the load capacity of the 485 bus. When you need to increase the terminal resistance, just set the terminal resistance jumper on the controller. If the other end is connected to a computer, short-circuit the brown and white of the 485 converter at the same time.
9.How to extend the communication distance of 485
One of the specifications of the 485 network is a length of 1.2 kilometers and a number of 32 nodes. If this limit is exceeded, a 485 hub must be used to expand the network distance or the number of nodes. Using the 485 hub, a large 485 network can be divided into several network segments. The 485 hub is like a “bridge” between the 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 a 485 hub to extend the network distance. Diagram: Use a 485 hub to solve the 485 bifurcation problem, as shown in the figure: Use a 485 hub to construct a star-shaped 485 network. 485 hub is an extension of the concept of 485 repeaters. It not only solves multiple bifurcations At the same time, it also solves the problem of isolation between network segments, that is, a certain network segment has a problem (such as a short circuit, etc.), which will not affect other network segments, thereby greatly improving the security and stability of large-scale networks. We can experience the benefits of the star-shaped 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 composed of a 485 hub will also be a direction for the development of the 485 network.
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