Usually the number of nodes on the RS485 bus does not exceed 32, but the number of nodes also changes depending on the cables and chips used.
AC220V power cord is best routed separately to prevent low-frequency interference. Otherwise, DC power supply should be used.
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☆ How many devices can be connected to one RS485 cable?
What chip should I look at? And related to the quality of the cable used! The more nodes there are, the farther the transmission distance, and the harsher the electromagnetic environment, the higher the requirements for the selected cable.
Support 32-node chips: SN75176, SN75276, SN75179, SN75180, MAX485, MAX488, MAX490
Support 64-node chips: SN75LBC184
Chips that support 128 nodes: MAX487, MAX1487
Chips that support 256 nodes: MAX1482, MAX1483, MAX3080~MAX3089
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General Fieldbus series cables
(Patent No.: 2010 2 0559128.9)
The twisted-pair shielded cable with a characteristic impedance of 120Ω is widely used in RS485/422, CANBUS and other buses. This series of cables There are many specifications. Please provide the cable laying environment, communication rate, maximum non-relay transmission distance and other parameters. We will recommend the most appropriate product according to the specific situation. The general recommendations are as follows:
Unarmored twisted pair shielded cable STP-120Ω (for RS485 & CAN) one pair 20 AWG, cable outer diameter is about 7.7mm, blue sheath. Suitable for indoor, pipeline and general industrial environments. When in use, one end of the shielding layer should be grounded!
Unarmored twisted pair shielded cable STP-120Ω (for RS485 & CAN) one pair 18 AWG, cable outer diameter is about 8.2mm, gray sheath. Suitable for indoor, pipeline and general industrial environments. When in use, one end of the shielding layer should be grounded!
Armored twisted pair shielded cable ASTP-120Ω (for RS485 & CAN) one pair 18 AWG, cable outer diameter is about 12.3mm, black sheath. It can be used in places with serious interference, frequent rodent infestations, and places with lightning protection and explosion-proof requirements. When in use, it is recommended that both ends of the armor layer be grounded and one end of the innermost shielding layer be grounded!
The CC-Link bus cable is a 3-core twisted shielded cable with a characteristic impedance of 110±10Ω. Domestic model specifications: STP-110Ω (for CANopen & CC-Link) 3C×20AWG. When used, The shield should be connected to ground at one end only!
Frequency converters, power cables, transformers, high-power motors, etc. are often accompanied by low-frequency interference, and this interference cannot be solved by cables shielded with high conductivity materials, including original imported cables. Only shielding layers made of high magnetic permeability materials (such as steel strips and steel wires) can effectively suppress low-frequency interference.
The most common method is to cover the cable with a steel pipe or directly use an armored cable made of high magnetic permeability material - ASTP-120Ω (for RS485 & CAN) one pair 18 AWG, cable outer diameter About 12.3mm.
It can be used in places with serious interference, frequent rodent infestations, and places with lightning protection and explosion-proof requirements. When in use, it is recommended that both ends of the armor layer be grounded and one end of the innermost shielding layer be grounded!
Lightning protection for outdoor laying cables is very important! The equivalent interference frequency of lightning is around 100k, which is also a low-frequency interference.
Article 6.3.1 of "GB50057-94 Design Code for Lightning Protection of Buildings": ...In the space that needs to be protected, when using shielded cables, the shielding layer should be at least at both ends. It is also advisable to make equipotential connections at the junction of lightning protection zones. When the system requires equipotential connections at only one end, two layers of shielding should be used, and the outer shielding should be processed according to the aforementioned requirements.
"GB 50217-2007 Cable Design Specification" also has similar statements. Please search "Industrial Automation Comprehensive Cable Solutions" to enter the China Industrial Control Network Forum to view and communicate!