Differential operation is also called "differential transmission". Generally speaking, the driver sends out two signals with equal values and opposite phases, and the receiver judges whether the logic state is "0" or "1" by comparing the difference between the two voltages. Conductor pairs carrying differential signals are called differential conductors.
Compared with ordinary single-ended signal wiring, the most obvious advantages of differential signal are as follows:
A. Strong anti-interference ability. Because the coupling between the two differential lines is very good, when there is noise interference from the outside, they are coupled to the two lines almost at the same time. The receiver only cares about the difference between the two signals, so it can completely cancel the external * * * mode noise.
B. EMI can be effectively suppressed. Similarly, because the polarities of the two signals are opposite, the electromagnetic fields radiated by them can cancel each other out. The closer the coupling is, the less electromagnetic energy is released to the outside world.
C. timing positioning is accurate. Because the switching change of the differential signal is located at the intersection of the two signals, unlike the ordinary single-ended signal, which is judged by the threshold voltage, it is less affected by the process and temperature, which can reduce the timing error and is more suitable for circuits with low amplitude signals.
RS485/RS422 adopts the combination of balanced driver and differential receiver, which is also called "balanced transmission", and its anti-interference ability is enhanced, that is, it has good anti-noise interference ability. RS485 uses a pair of twisted pairs with characteristic impedance of120Ω, in which one pair is defined as A and the other pair is defined as B..RS422 uses two pairs of twisted pairs with characteristic impedance of120Ω, one pair transmits and the other pair receives.
- ※ - ※ - ※ -
Universal field bus series cable
(PatentNo.: 20 10 2 0559 128.9)
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:
Ordinary 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!
Ordinary 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!
The bus cable of CC-Link is a three-core stranded shielded cable with a characteristic impedance of11010Ω. The domestic model specification is STP- 1 10ω (for CANopen &；; CC-Link)3C×20AWG, when in use, only one end of the shielding layer needs to be 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!
The equivalent interference frequency of lightning is about 100k, which 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 cable design specification has similar expression, please search for "industrial automation series integrated cable solution".