In the industrial control environment, lightning, short circuit, switch action and other actions with inductive load will often occur, and these disturbances are short and high-energy pulse disturbances, which have the characteristics of pulse group occurrence, short rise time of pulse, high repetition frequency of pulse.
These interferences will be coupled to the RS485 bus. Because these pulses are not a single pulse, but a series of pulses, they will accumulate on the RS485 bus, making the voltage amplitude of the disturbance exceed the noise tolerance of the RS485 transceiver, causing communication errors.
At the same time, due to the short cycle of these pulse disturbances and the short interval between the occurrence of each pulse, when the first pulse disturbance has not disappeared, the second pulse will follow. For the parasitic capacitance on the RS485 bus and the junction capacitance of the RS485 transceiver, the charging will start before the discharge is completed, and usually the parasitic capacitance is small, and the smaller energy can reach a higher voltage, which is easy to damage the RS485 transceiver, It affects the reliability of RS485 bus communication.
Generation principle of pulse group disturbance
The voltage of the burst disturbance source depends on the inductance of the load circuit, the speed of load disconnection and other factors. Take the switch action as an example, because the distance between the moving and stationary contacts is relatively close at the moment of switch opening, the back electromotive force induced by the inductance in the circuit is enough to break down the air gap between the contacts, and the circuit begins to conduct, but the time of this discharge process is very short. At this time, the circuit will generate a high-voltage small pulse with a front pulse of ns level, a width of tens of ns level, and a amplitude of more than thousands of volts. When the above pulse ends, the circuit begins to repeat the process of back electromotive force generated by the inductive load and discharge through the air gap between the moving and stationary contacts of the switch.
This process will continue until the energy stored in the inductive performance load is low enough to produce the above discharge process. These interferences will be coupled to the RS485 bus, forming large interferences and affecting the reliability of communication.
Measures to improve the anti-interference ability of electric fast pulse group
Electrical fast pulse group immunity is common mode interference, which can be suppressed by filtering, absorption or isolation. It can be roughly summarized into the following five methods.
【1】 RS485 bus isolation
(1) Ensure equipment and personal safety – the impact of high voltage
RS485 is used for communication between equipment. In many cases, the R&D personnel do not know what type of equipment the customer uses to communicate with. If the other party uses the resistance and capacitance voltage reduction principle of a few yuan to reduce 220V to 12V, and there is no isolation from the power grid, the test, commissioning and use will be very dangerous, or the insulation of high-voltage equipment is damaged. If the RS485 line has high voltage, it will threaten the safety of equipment and personnel.
(2) Avoiding the influence of potential difference
In many practical applications, the communication distance can reach several thousand meters, and the distance between nodes is very long. Designers often directly connect the reference ground of each node to the local ground as the return ground of the signal. It seems to be a normal and reliable practice. The actual ground is not the ideal “0″ potential. The ground is also a conductor and has impedance.
When a large current flows through the earth, there will also be a potential difference between the two ends of the earth flowing through the current. For example, in Figure 1 below, because AB is far away, the potential between the reference ground is not zero, and the impedance of the ground wire will not be zero. Due to the effect of the current loop, the voltage at the A end is Vs, and the voltage at the B end becomes Vc+Vs.
(3) Avoid data abnormity and device damage – the impact of ground loop
Since there is a potential difference between the ground between nodes, why not connect the ground of two nodes directly with a wire? absolutely wrong! This can only make the situation more serious. This long wire will form a great earth loop with the earth! I believe you know from your school days that a closed coil will generate current in a changing magnetic field.
50Hz AC power lines and large motors are all sources of AC magnetic field. If the bus is close to or passes through these places, the ground loop will generate current of several amperes or even hundreds of amperes. The common-mode voltage generated by the current flowing through the ground loop will affect the normal communication of the bus. In addition to the stable magnetic field source, the transient interference of some power lines such as surges, lightning strikes, high-frequency noise, etc. may be picked up by this giant “ring antenna”, and cause communication abnormalities.
【2】 Increase ferrite magnetic ring to absorb interference
Adding ferrite magnetic ring at the entrance of the equipment can effectively absorb interference, and increasing the number of turns of the communication line in the ferrite magnetic ring can increase the absorption effect of interference, as shown in Figure 2, adding ferrite magnetic ring near the RS-485 interface of the equipment to be tested.
【3】 Use shielded twisted pair
As shown in the figure, in practical applications, RS485 communication line can use shielded twisted pair, and the shielding layer can be grounded at a single point, which can effectively inhibit the electrical fast pulse group disturbance coupling to the communication line.
【4】 Add RS-485 bus to ground TVS
When TVS tubes are added between A and B and the earth, and the amplitude of electrical fast pulse group disturbance voltage coupled to the RS485 bus is high, the disturbance voltage will be clamped by TVS to protect the RS485 transceiver.
【5】 RS-485 bus series magnetic beads
Because the magnetic bead is equivalent to a resistance at high frequency, it will convert high frequency energy into heat energy and consume it. Therefore, the magnetic beads are connected in series on the RS485 bus. When the electric fast pulse group signal is coupled to the RS485 bus, the energy of the electric fast pulse group disturbance will be consumed by the magnetic beads, improving the anti-interference ability of the RS485 bus.
Post time: Mar-13-2023