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Seven basic questions about RS-485 wiring

by:Rika Sensors     2021-11-01
Seven basic questions about RS-485 wiring
RS485 is a standard that defines the electrical characteristics of drivers and receivers in a balanced digital multipoint system, and has a very wide range of applications in the field of environmental monitoring. Through RS-485 wiring, not only can multi-point two-way communication be carried out on a pair of twisted pairs, but also its noise suppression capability, data transmission rate, cable length and reliability are unmatched by other standards. When it is necessary to communicate in a noisy environment and a certain distance, RS-485 wiring is a good choice.

What exactly is RS-485 wiring, and what function does it have? This article will introduce in detail through seven questions and answers.

1. What kind of communication line should be used for the 485 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. 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.

2. How many devices can be connected to a 485 bus?

Generally speaking, the 485 bus we commonly use can connect 128 devices for communication.

However, not all 485 converters can carry 128 devices. It should be judged according to the model of the chip in the 485 converter and the model of the 485 device chip, and the load capacity should be determined according to the chip with a lower index. Generally, there are three levels of 485 chip load capacity-32 units, 128 units and 256 units.

In addition, the theoretical nominal rating is often not reached in fact. The longer the communication distance, the higher the baud rate, the thinner the wire diameter, the worse the wire quality, the worse the quality of the converter, and the power supply of the converter. Insufficient (passive converter), stronger lightning protection, these will reduce the real load quantity.

3. Why should RS-485 be grounded?

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 methods to eliminate common-mode interference is to use the shielding layer of the 485 communication line as a line to connect sensors, computers and other network equipment together, and to reliably connect to the earth from one point.

4. How should the RS-485 communication line be routed?

RS-485 wiring should be as far away as possible from interference sources such as high-voltage wires and fluorescent lamps. 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, or tied together. Use high-quality twisted-pair cabling.

5. Is there a connection point between the device and the device 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.

6. 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. The connection method of the 120Ω matching resistance of the dome terminal is as follows: The 120Ω matching resistance of the dome terminal can be connected by the dial switch on the chassis of the dome. ,As shown below. When the dome camera leaves the factory, the 120Ω matching resistance is not connected by default. You can connect the 120Ω matching resistance to the line by turning the 10th bit of the DIP switch to ON. On the contrary, if the 120Ω matching resistance is not connected, just turn the 10th bit to OFF.

7. When there is no relay, what line should be selected to ensure the transmission distance?

(1) Ordinary twisted pair shielded cable STP-120Ω (for RS485u0026CAN) one pair 20 AWG, the outer diameter of the cable is about 7.7mm. Suitable for indoor, pipeline and general industrial environment. When in use, one end of the shielding layer should be grounded!

(2) Ordinary twisted-pair shielded cable STP-120Ω (for RS485u0026CAN) one pair 18 AWG, the outer diameter of the cable is about 8.2mm. Suitable for indoor, pipeline and general industrial environment. When in use, one end of the shielding layer should be grounded!

(3) Armored twisted-pair shielded cable ASTP-120Ω (for RS485u0026CAN) one pair 18 AWG, the outer diameter of the cable is about 12.3mm. It can be used in places with severe interference, frequent rodent damage, and lightning protection and explosion-proof requirements. When in use, it is recommended to ground both ends of the armor layer and ground one end of the inner shield



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