RS-485 bus has the advantages of simple structure, long communication distance, high communication speed and low cost. It is widely used in industrial communication, power monitoring and instrumentation industries. Due to the harsh industrial control environment, there will be more interference coupling in the communication line, affecting the reliability of the RS-485 bus, and even damaging the RS-485 transceiver chip. Among them, pulse group disturbance is a common one. We typically use the Electrical Fast Burst (EFT) immunity test to simulate this disturbance and verify the reliability of the system.
1. Source of horror harassmentIn the industrial control environment, there are often transient disturbances caused by lightning, short circuit, switching action and other inductive loads. These disturbances are short-lived high-energy pulse disturbances, with bursts of pulses appearing, and the rise time of the pulses is relatively short. The pulse repetition frequency is high. These disturbances are coupled to the RS-485 bus. Since these pulses are not a single pulse, but a series of pulses, they accumulate on the RS-485 bus, causing the amplitude of the disturbance to exceed the noise capacity of the RS-485 transceiver. Limit, causing communication errors. At the same time, due to the short period of these pulse disturbances, the interval between occurrences of each pulse is short. When the first pulse disturbance has not disappeared, the second pulse follows, for the parasitic on the RS-485 bus. Capacitance and the junction capacitance of the RS-485 transceiver start charging again after the discharge has not been completed, and usually the parasitic capacitance is small, and the smaller energy can reach a higher voltage, which easily damages the RS-485 transceiver. Affect the reliability of RS-485 bus communication.
2, pulse group disturbance generation principleThe voltage of the pulse group disturbance source depends on the inductance of the load circuit, the speed at which the load is disconnected, and the like. Taking the switching action as an example, since the distance between the moving and static contacts is close when the switch is turned on, the back electromotive force induced by the inductance in the circuit is enough to break the air gap between the contacts, and the circuit starts to conduct, but this discharging process The time is very short. At this time, the circuit will generate a high-voltage small pulse with a leading edge pulse of ns, a width of several tens of ns, and a magnitude of several kilovolts. When the above pulse is over, the circuit begins to repeat the process of generating an anti-electromotive force by the inductive load and discharging through the air gap between the switching static and dynamic contacts. This process will continue until the energy stored in the inductive performance load is low enough to again produce the above discharge process. These interferences are coupled to the RS-485 bus, creating large interferences that affect the reliability of the communication.
3. Measures to improve the anti-interference ability of electric fast pulse groupElectrical fast pulse group immunity is common mode interference and can be suppressed by filtering, absorbing or isolating.
· RS-485 bus isolation
For example, Zhiyuan Electronics' isolated RS-485 transceiver module RSM485PHT (automatic transceiver), RSM485PCHT, RSM485PCT and RSM485ECHT, etc., integrates signal isolation and power isolation functions, and can reach IEC 61000 without external protection circuit. 4-4 ±2kV (B grade, RS-485 port is tested under floating conditions). If a higher level is required, the following protection measures can be added according to the actual situation. As shown in Figure 1, the RSM test diagram is used for the RSM3485PHT isolated transceiver module. If the protection circuit is not added outside the module, it can pass IEC 61000-4-4 ±2kV (Class B).
Figure 1 EFT test chart of RSM3485PHT module
· Increase the absorption interference of ferrite magnetic ring
Adding a ferrite magnetic ring at the inlet end of the device can effectively absorb the interference, and increasing the number of turns of the communication line in the ferrite magnetic ring can increase the interference absorption effect, as shown in FIG. 2, in the RS-485 interface of the device under test. Add a ferrite ring nearby.
Figure 2 Communication line increases ferrite ring
· Use shielded twisted pair
As shown in the figure, in practical applications, the RS-485 communication line can use shielded twisted pair, and the shield layer is connected to the ground at a single point, which can effectively suppress the coupling of the electrical fast bursts to the communication line.
Figure 3 Using shielded twisted pair
· Increase RS-485 bus to ground TVS
When the TVS tube is added between the A to the earth and the B to the earth, and the amplitude of the electrical fast pulse group disturbance voltage coupled to the RS-485 bus is high, the interference voltage is clamped by the TVS to achieve protection of the RS-485 transceiver. the goal of.
· RS-485 bus series magnetic beads
Since the magnetic beads are equivalent to the resistor at high frequency, the high-frequency energy is converted into heat energy, so the magnetic beads are connected in series on the RS-485 bus, and the electric fast pulse group signal is coupled to the RS-485 bus. The energy of the pulse group disturbance will be consumed by the magnetic beads, which improves the anti-interference ability of the RS-485 bus.
4. Examples of improving the EFT immunity levelAs shown in the circuit shown in Figure 4, the two RSM3485PHT module AB interfaces are connected in series with magnetic beads, which can reach IEC 61000-4-4 ±4kV, and can effectively reduce the communication error rate. Only the magnetic beads are connected in series in the circuit. It is cheap and effective. If the required EFT immunity level is higher, the number of beads can be increased appropriately.
Figure 4 Communication line series magnetic beads
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