The maximum transmission distance of RS485 without relay is 1200m, which exceeds the number of repeaters that can be added (up to 8) and the transmission distance is close to 10Km.
RS485 is a master-slave structure, so the transmission rate is high, up to 12Mbps.
CAN is a multi-master structure, so it has good fault tolerance, but the speed is inferior.
Although both CAN and RS485 use twisted-pair shielded bus cables with characteristic impedance of120Ω, the CAN current is also large, which is an intrinsically safe circuit.
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The following suggestions are expected to help:
1. Special bus cable with impedance matching and low attenuation (patent number: 20 10 2 0559 128.9) is more conducive to ensuring communication. The general recommendations are as follows:
Ordinary twisted pair shielded cable STP-120Ω (used for RS485 &；; CAN) A pair of 20 AWG with an outer diameter of about 7.7 mm, suitable for indoor, pipeline and general industrial environment. When in use, one end of the shielding layer is grounded!
Ordinary twisted pair shielded cable STP-120Ω (used for RS485 &；; CAN) a pair of 18 AWG, with an outer diameter of about 8.2mm It is 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, 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!
2. When the transmission distance exceeds 300m, the terminal resistance should be increased (generally120Ω).
3. Frequency converters, power cables, transformers, high-power motors, 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 &；; Yes) a pair of 18 AWG.
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 Integrated Cable Solution" and enter China Industrial Control Network for more information.