-Extended reading.
When choosing industrial communication cable, the impedance (Z0 =120Ω) should be matched first, and then different models and specifications should be configured according to the specific communication speed, transmission distance, wiring environment and laying mode.
Many people use cables with PVC insulation media such as RVVPS, RVVP, RVSP and KVVP to transmit digital signals, which is wrong! Explain by letter:
RVV copper core PVC insulated and sheathed unshielded flexible cable.
Copper core PVC insulated and sheathed copper wire braided total shielded flexible cable.
RVSP copper core PVC insulated sheathed copper stranded wire braided shield wire
Copper core PVC insulated and sheathed copper wire braided total shielded control cable.
As we all know, although the price of PVC is low, it is a high attenuation material. In addition, the cable itself only considers the primary parameters such as resistance, withstand voltage and current carrying capacity, so if it is considered according to the transmission line equation, these can not meet the communication requirements. Low speed and short distance may be possible, but high speed, long line and multi-node bus communication is difficult. In this case, the field bus cable with impedance matching should be used.
Fieldbus cable is designed according to the transmission line equation, and has transmission elements with secondary parameters such as characteristic impedance and attenuation value.
General Fieldbus Series Cables ※
(PatentNo.: 20 10 2 0559 128.9 Beware of counterfeiting)
Twisted-pair shielded cable with characteristic impedance of120Ω is widely used in RS485/422, CANBUS and other buses. This series of cables has many specifications. Please provide cable laying environment, communication rate, maximum relay-free transmission distance and other parameters, and we will recommend the most suitable product according to the specific situation. The general recommendations are as follows:
Unarmored twisted pair shielded cable STP- 120ω (used for RS485 &；; CAN) A pair of 20 AWG, with an outer diameter of about 7.7mm and a blue sheath. Suitable for indoor, pipeline and general industrial environment. When in use, one end of the shielding layer is grounded!
Unarmored twisted pair shielded cable STP- 120ω (used for RS485 &；; CAN) a pair of 18 AWG, with an outer diameter of about 8.2mm and a gray sheath. Suitable for indoor, pipeline and general industrial environment. When in use, one end of the shielding layer is grounded!
Armored twisted-pair shielded cable astp-120Ω (forrs485 &; CAN) a pair of 18 AWG, with an outer diameter of about 12.3mm and a black sheath. It can be used in places with serious interference, frequent rodent damage and lightning protection and explosion protection requirements. When in use, it is recommended that both ends of the armor layer are grounded, and one end of the innermost shielding layer is grounded!
Frequency converter, power cable, transformer, high-power motor, etc. It is often accompanied by low frequency interference, which can not be solved by cables with high conductivity materials as shielding layers, including imported cables. Only the shielding layer made of high magnetic permeability materials (such as steel strip and steel wire) can effectively suppress low frequency interference.
The most commonly used method is to put a steel pipe or an armored cable made of high permeability material-ASTP-120Ω (used for RS485 &；; CAN) A pair of 18 AWG, with an outer diameter of about 12.3mm, can be used in places with serious interference, frequent rodent damage and lightning protection and explosion protection requirements. When in use, it is recommended that both ends of the armor layer are grounded, and one end of the innermost shielding layer is grounded!
Lightning protection of outdoor cables 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 Lightning Protection Design of Buildings (GB50057-94): ...... In the protected space, when shielded cables are used, the shielding layer should be at least equipotentially connected at the junction of both ends and the lightning protection area. When the system only requires equipotential connection at one end, two layers of shielding should be adopted, and the outer shielding should be treated according to the above requirements.
GB 502 17-2007 Code for Cable Design has a similar statement. Search for "Industrial Automation Series Integrated Cable Solution" and enter the China Industrial Control Network Forum to check and exchange.