RS485 is a standard that defines the electrical characteristics of drivers and receivers in balanced digital multipoint systems, and has a very wide range of applications in the field of environmental monitoring. RS-485 cabling can not only carry out multi-point bidirectional communication on a pair of twisted pairs, but also its noise suppression capability, data transmission rate, cable length and reliability are unmatched by other standards. RS-485 cabling is a good choice when communication is required in noisy environments and at a distance.
7 RS-485 wiring and functions #
1. What kind of communication line should be used for RS485 bus?
RVSP shielded twisted pair must be used. The specification of the shielded twisted pair used is related to the distance of the 485 communication line and the number of connected devices. The use of shielded twisted pair helps to reduce and eliminate the distributed capacitance generated 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 commonly used 485 bus 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, 128 and 256.
In addition, the theoretical nominal is often unattainable in practice. The longer the communication distance, the higher the baud rate, the thinner the wire diameter, the worse the wire quality, the worse the converter quality, and the insufficient power supply of the converter (passive power supply). converter), the stronger the lightning protection, these will reduce the number of real loads.
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 seriously damaged. Common mode interference will increase the above common mode voltage. One of the effective means to eliminate common mode interference is to use the shielding layer of the 485 communication line as a line to connect the devices in the network such as sensors and computers together, and connect them to the ground reliably from one point.
4. How should the RS-485 communication line be routed?
The RS-485 wiring should be kept away from high-voltage wires, fluorescent lamps and other sources of interference as far as possible, and the communication wire should be perpendicular to the power wire, not parallel, or bundled together, and use high-quality Twisted pair wiring.
5. Can there be contacts between devices on the 485 bus?
In the same network system, use the same cable to minimize the number of contacts in the line. Make sure that the joints are well welded and tightly wrapped to avoid loosening and oxidation. A single, continuous signal path is guaranteed as a bus.
6. Under what circumstances should the terminating resistor be added on the 485 bus?
Under normal circumstances, it is not necessary 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 devices 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 dialing the code switch on the dome chassis. ,As shown below. When the dome is shipped from 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 position of the DIP switch to ON. Conversely, if the 120Ω matching resistor is not connected, then 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 RS485&CAN) one pair 20 AWG, the cable outer diameter is about 7.7mm. Suitable for indoor, pipeline and general industrial environments. When in use, one end of the shielding layer is grounded!
(2) Ordinary twisted pair shielded cable STP-120Ω (for RS485&CAN) one pair 18 AWG, the cable outer diameter is about 8.2mm. Suitable for indoor, pipeline and general industrial environments. When in use, one end of the shielding layer is grounded!
(3) Armored twisted pair shielded cable ASTP-120Ω (for RS485&CAN) one pair 18 AWG, cable outer diameter 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 that both ends of the armored layer be grounded, and one end of the inner shield is grounded.
