2-core twisted-pair shielded special bus cable, just connect SD+ RD- respectively. SG is the signal ground, it doesn’t need to be connected. One end of the cable shielding layer is connected to the earth, and the grounding resistance is ≯ 4Ω! The other end of the cable is connected to the RS232/RS485 converter → computer COM port.
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To select industrial communication cables, first meet the impedance (Z0=120Ω) matching, and then configure different models and specifications according to the specific communication rate, transmission distance, wiring environment, and laying method.
Many people use RVVPS, RVVP, RVSP, KVVP and other PVC insulated cables to transmit digital signals. This is wrong! Explanation according to letters:
RVV - Copper core conductor PVC insulated and sheathed flexible cable without shielding.
RVVPS - Copper core conductor PVC insulated and sheathed copper wire braided total shielded flexible cable.
RVSP - Copper core conductor, PVC insulated twisted copper wire braided shielded wire without sheath.
KVVP - Copper core conductor PVC insulated and sheathed copper wire braided total shielded control cable.
It is generally known that although the price is low, polyvinyl chloride (PVC) is a high-attenuation material. In addition, the cable itself only considers primary parameters such as resistance, withstand voltage, and current carrying capacity. Therefore, if considered according to the transmission line equation, these cannot meet the communication requirements. It may be possible for low speed and short distance, but it is difficult to perform high-speed, long-term, and multi-node bus communication. In this case, impedance-matched fieldbus cables should be used.
Fieldbus cables are designed according to the transmission line equation and have transmission elements with secondary parameter requirements such as characteristic impedance and attenuation value.
※ Universal fieldbus series cables
(Patent number: 2010 2 0559128.9 Beware of counterfeiting)
Twisted-pair shielded cables with a characteristic impedance of 120Ω are widely used RS485/422, CANBUS and other buses, this series of cables have 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!
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-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. Search "Industrial Automation Series Comprehensive Cable Solutions" to enter the China Industrial Control Network forum to view and communicate. 13064880370