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Measures to improve the immunity of electrical fast transient pulse sets on the RS-485 bus are analyzed

RS-485 bus has the advantages of simple structure, long communication distance, high communication speed, and low cost, and is widely used in industrial communication, power monitoring, instrumentation, and other industries. Due to the harsh industrial control environment, there will be more interference coupling in communication lines, affecting the reliability of the RS-485 bus, and even damaging the RS-burst interference. This is a common type of 485 transceiver chips. We usually use electrical fast transient burst (EFT) immunity tests to simulate this disturbance and verify the reliability of the system.


1. Sources of burst disturbances

Lightning often occurs in industrial control environments. Instantaneous disturbances caused by actions with inductive loads such as short circuits and switching actions are short-term high-energy pulse disturbances, with clusters of pulses, relatively short rise times, and high pulse repetition rates. These disturbances are coupled to the RS-485 bus. Because these pulses are not a single pulse, but a series of pulses, they accumulate on the RS-485 bus, causing the disturbance voltage amplitude to exceed the noise capacity of the RS-485 transceiver, resulting in communication errors. At the same time, due to the short cycle of these pulse disturbances and the short interval between each pulse, when the first pulse disturbance does not disappear, the second pulse is immediately followed by the RS parasitic capacitance and the 485 bus RS. For the junction capacitor of the 485 transceiver, charging begins before discharging. The parasitic capacitor is usually small and has low energy, which can easily damage the RS-485 transceiver and affect the reliability of RS-485 bus communication.


2. Principle of pulse group disturbance

The voltage of a burst disturbance source depends on the inductance of the load circuit, the speed of load disconnection, and other factors. Taking the switching action as an example, due to the relatively close distance between the moving and stationary contacts when the switch is opened, the electrical induction reverse potential in the circuit is sufficient to break through the air gap between the contacts, and the circuit begins to open. However, this discharge process is very short. The circuit will generate a leading edge pulse of ns magnitude, with a width of tens of ns, and the amplitude of high voltage small pulses exceeding thousands of volts. When the above pulses end, the circuit begins to repeat the inductive load to generate a reverse potential and discharge through the air gap between the dynamic and static contacts of the switch.

This process will continue until the energy stored in the inductive performance load is low enough to no longer generate the above discharge process. These interferences are coupled to the RS-485 bus, forming large interferences that affect the reliability of communication.



3. Measures to improve the anti-interference ability of electrical fast pulse trains

Electrical fast transient burst immunity is a common mode interference that can be suppressed by using filters to suppress absorption or isolation.

① Use shielded twisted pair

As shown in Figure 1, in practical applications, RS-shielded twisted pair can be used for 485 communication lines, and the shielding layer can be grounded at a single point, effectively suppressing electrical fast pulse group disturbances coupled to communication lines.


国产电磁兼容仪器设备

Figure 1. Using shielded twisted pairs

② Add RS-485 bus to ground TVS

Add TVS tubes between A and B ground, coupled to RS-When the electrical fast transient burst disturbance voltage range on the 485 bus is high, the interference voltage will be interfered with for the purpose of TVS protection clamping the RS-485 transceiver.

③ · RS-magnetic bead series 485 bus

Because magnetic beads are equivalent to resistors at high frequencies, they convert high-frequency energy into thermal energy and consume RS-beads connected in series to the electrical fast transient pulse group signal coupled to the 485 bus. On the 485 bus, magnetic beads consume energy from electrical fast pulse group disturbances, increasing the anti-interference ability of the RS-485 bus.


4. Examples of improving EFT anti-interference level

As shown in Figure 2, the AB interfaces of the RSM3485PHT module in two circuits are connected in series with magnetic beads, which can reach IEC61000-4-4 ± 4kV, and can effectively reduce the communication error rate. Only connecting magnetic beads in series in the circuit is very cheap and effective. If required, the EFT immunity level is high, and the number of magnetic beads can be appropriately increased.


国产电磁兼容仪器设备

Figure 2. Communication Line Series Magnetic Beads


keywords: EFT
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