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How to choose RS485 communication cable?

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Firstly, the impedance (Z0 = 120 Ω) matching is satisfied, and then different models and specifications are configured according to the specific communication rate, transmission distance, wiring environment and laying mode.
Many people use PVC insulated cables such as rvvps, RVVP, RVSP, kvvp to transmit digital signals. This is wrong! According to the letter:
RVV -- copper conductor PVC insulated and sheathed flexible cable without shielding.
Rvvps -- copper conductor PVC insulated and sheathed copper wire braided total shielding flexible cable.
RVSP -- copper conductor PVC insulated stranded copper wire braided shielding wire without sheath.
As we all know: Although the price is low, PVC is a high attenuation material. In addition, the cable itself only considers the resistance, withstand voltage, current carrying capacity and other primary parameters, so if considering according to the transmission line equation, these can not meet the communication requirements. Low speed and short distance may be possible, but it is difficult to be competent for high speed, long line and multi node bus communication. In this case, impedance matching fieldbus cable should be used.
The fieldbus cable is designed according to the transmission line equation and has the requirements of secondary parameters such as characteristic impedance and attenuation value.

The twisted pair shielded cable with characteristic impedance of 120 Ω is widely used in RS485 / 422, CANbus and other buses. There are many specifications of this series of cables. Please provide the cable laying environment, communication rate, maximum repeater free transmission distance and other parameters. We will recommend the most appropriate product according to the specific situation. The general recommendations are as follows:
Non armored twisted pair shielded cable stp-120 Ω (for RS485 & can) one pair 20 AWG, cable outer diameter about 7.7mm, blue sheath. Suitable for indoor, pipeline and general industrial environment. When using, one end of shielding layer is grounded!

Non armored twisted pair shielded cable stp-120 Ω (for RS485 & can) one pair 18 AWG, cable outer diameter about 8.2mm, gray sheath. Suitable for indoor, pipeline and general industrial environment. When using, one end of shielding layer is grounded!

The armored twisted pair shielded cable astp-120 Ω (for RS485 & can) one pair 18 AWG, the outer diameter of the cable is about 12.3mm, and the black sheath is used. It can be used in places with serious interference, frequent rodent damage, lightning protection and explosion-proof requirements. When using, it is recommended to ground both ends of armor layer and one end of innermost shielding layer!

Frequency converter, power cable, transformer, high-power motor and so on are often accompanied by low-frequency interference, which can not be solved by cables with high conductivity materials as shielding layer, including original imported cables. Only the shielding layer made of high permeability materials (such as steel strip and steel wire) can effectively suppress low frequency interference.
The most common method is to cover the cable with steel pipe or armored cable made of high permeability material - astp-120 Ω (for RS485 & can) one pair 18 AWG, and the outer diameter of the cable is about 12.3 mm. It can be used in places with serious interference, frequent rodent damage, lightning protection and explosion-proof requirements. When using, it is recommended to ground both ends of armor layer and one end of innermost shielding layer!

Lightning protection of outdoor laying cable is very important! The equivalent interference frequency of lightning is about 100k, which also belongs to low-frequency interference.
Article 6.3.1 of code for design of lightning protection of buildings (GB 50057-94):... In the space requiring protection, when shielded cable is used, its shielding layer shall be at least at both ends and shall be connected at the junction of lightning protection area. When the system requires equipotential connection at one end only, two layers of shielding shall be adopted, and the outer shielding shall be treated according to the above requirements.

"GB 50217-2007 cable design code" also has similar expression, search "industrial automation series integrated cable solutions" can be found.
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