Because of its simple wiring, stable and reliable, 485 bus is widely used in anti-theft alarm, video surveillance, access control intercom, etc., but in the 485 bus wiring process, due to a lot of inaccurate concepts lead to many problems, In a variety of sites, the 485 bus is relatively prone to failure. Now the 485 bus is prone to failure and can be troubleshooting methods are as follows:
1. Since the 485 signal uses a pair of unbalanced differential signals, it means that each device in the 485 network must be connected to the ground through a signal loop to reduce the noise on the data line. Therefore, the data line is preferably a twisted pair cable. Make up, and add the shielding layer as the ground wire outside, connect the 485 equipment of 485 network, and reliably ground at one point.
2, in the industrial scene, the scene is very complex, there is a high common-mode voltage between each node, 485 interface uses a differential transmission mode, there is resistance to common-mode interference, but when the common-mode voltage is greater than +12V or less -9V exceeds the limit receiving voltage of the 485 receiver. The receiver will not work and may even burn the chip and the device together. The 485 optically isolated repeater can be used in the 485 bus to completely isolate the 485 signal and power supply, thereby eliminating the effects of common-mode voltage.
3,485 bus With the extension of the transmission distance, echo reflection signals will be generated. If the transmission distance of the 485 bus exceeds 100 meters, it is recommended that the terminating resistor at the beginning and end of the 485 communication be 120 ohms during construction. Related wiring method can refer to the following figure:
4,485 bus 485 nodes to minimize the distance between the trunk, the general recommendation 485 bus using hand-bus topology. The star structure will produce reflected signals, affecting the quality of 485 communications. If in the construction process, 485 nodes must be required to exceed the distance from the trunk of the 485 bus over a certain distance, the 485 bus hub product of Shenzhen Lekly Electronics Co., Ltd. can make a bifurcation of the 485 bus. Satisfy the use of star topology requirements during construction.
5, the factors that affect the load capacity of 485 bus: communication distance, quality of wire, baud rate, power supply of converter, lightning protection of 485 equipment, choice of 485 chip. If there are more 485 devices on the 485 bus, it is recommended to use a 485 converter with a power supply. When the passive 485 converter is stealing power from the serial port, the power supply capacity is not enough and the load capacity is not enough. If you choose a good wire, if you can use the baud rate as low as possible, choose a high load capacity of the 485 chip, can increase the load capacity of the 485 bus. The lightning protection in the 485 equipment will absorb the voltage and cause the 485 bus load capacity to decrease. Removing the lightning protection can increase the 485 bus load capacity. If the relevant factors cannot be changed during on-site construction, it is recommended to use an RS485 hub to provide the load capacity of the 485 bus.
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