More than isolation – isolated RS485/422 transceivers for harsh environments

“RS485/422, as a multi-node, easy networking communication interface, is widely used in instrumentation, security, transportation, industrial equipment and other fields. Because the actual application field environment of RS485/422 is relatively complex, and the operator’s wiring method or use of transmission cables is incorrect, it often causes RS485/422 to be greatly interfered in the actual use process, especially the common mode. Noise, ground potential differences, wiring errors, and high-voltage transients such as electrostatic discharge (ESD), electrical fast transients (EFT), and lightning surges can disrupt communications and even cause permanent damage.Harsh environments are bound to affect device availability.

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by Kayden Wang

RS485/422, as a multi-node, easy networking communication interface, is widely used in instrumentation, security, transportation, industrial equipment and other fields. Because the actual application field environment of RS485/422 is relatively complex, and the operator’s wiring method or use of transmission cables is incorrect, it often causes RS485/422 to be greatly interfered in the actual use process, especially the common mode. Noise, ground potential differences, wiring errors, and high-voltage transients such as electrostatic discharge (ESD), electrical fast transients (EFT), and lightning surges can disrupt communications and even cause permanent damage. The harsh environment inevitably puts forward higher requirements on the reliability and robustness of the device.

–icoupler isolation technology improves reliability

RS-485/422 differential communication networks are often used in industrial and instrumentation applications in harsh environments. These networks can communicate at ranges up to 1200m. When communicating over long distances in harsh environments, the following problems may occur:

a. Ground loop current interference due to different ground potentials of different nodes;

b. Induced ground noise caused by motors, inductive switching loads and other high-noise electrical equipment;

c. Harmful electrical surges.

Galvanic isolation is an ideal solution if there is no guarantee that the ground potential of different nodes in the system will not exceed the common-mode range of the transceiver. Galvanic isolation does not prevent the flow of information, but prevents current surges. The occurrence of such faults can be avoided through power isolation and internal signal isolation. After using the isolated transceiver, the formation of ground loops can be effectively prevented, and the bus reference ground can fluctuate with the fluctuation of the common mode voltage. The common mode voltage is all borne by the isolation band. The common-mode voltage is no longer visible to the bus signal, ensuring that the bus can communicate stably and reliably.

For the suppression of common mode noise, many schemes used photoelectric isolation in the past, but this scheme has disadvantages such as large volume, many circuit separation components, complex circuit, and unstable system. The single-chip RS485/422 isolator introduced by ADI adopts ADI’s unique icoupler magnetic isolation technology to realize pulse coupling modulation through a transformer in the chip, which is compared with the light emitting diode (LED) and photodiode used in the optocoupler difference. Compared to traditional opto-isolation devices, icoupler magnetic isolation devices eliminate the uncertain current transfer ratio, nonlinear transfer characteristics, and drift over time and temperature issues associated with optocouplers; power consumption is reduced by 90%; no need External drivers or discrete devices, especially in terms of volume and integration, have incomparable advantages over optoelectronic isolation.

Figure 1: Principle of ADM Magnetic Isolation

– more than isolation

In addition to isolation technology, harsh use environments and high reliability requirements require transceivers to have better EMC immunity and protection.

Kayden Wang, an application engineer from Excelpoint Shijian, a technology distributor who has cooperated with ADI for more than 30 years, introduced to us an RS485/422 transceiver, the ADM2795E, which is suitable for harsh industrial environments and high reliability requirements. This is a 5 kV rms signal isolated RS-485/422 transceiver that integrates Analog Devices’ iCoupler® technology, combining a triple-channel isolator, RS-485 transceiver and IEC Electromagnetic Compatibility (EMC) transient protection Integrated in a monolithic package. Features an extended common-mode input range of ±25 V for improved data communication reliability in high-noise environments. A wide temperature enhanced model from -55°C to +125°C is also available.

Figure 2: ADM2795E functional block diagram

► IEC certified EMC performance

Programmable logic controller (PLC) communication ports in industrial automation typically use the RS-485 interface, and these ports may experience high common-mode noise, ground potential differences, wiring errors, and high-voltage transients such as electrostatic discharge (ESD), Electrical Fast Transient (EFT) and Lightning Surge (Surge).

The ADM2795E provides a complete system-level solution that meets IEC-61000 surge, EFT, and ESD standards, as well as immunity to conducted, radiated, and magnetic field disturbances common in industrial environments. The integration of isolation robustness and EMC protection significantly saves printed circuit board (PCB) space for communication port interfaces.

Relevant EMC certified performance of the ADM2795E:

• RS-485 A and B bus pins provide level 4 EMC certified protection

•IEC 61000-4-5 surge protection (±4 kV)

•IEC 61000-4-4 EFT protection (±2 kV)

•IEC 61000-4-2 ESD protection

• Contact discharge: ±8 kV

• Air gap discharge: ±15 kV

•IEC 61000-4-6 Conducted RF Immunity (10 V/m rms)

• Certified IEC 61000 isolation barrier immunity

•IEC 61000-4-2 ESD, IEC 61000-4-4 EFT, IEC 61000-4-5 Surge, IEC 61000-4-6 Conducted RF Immunity, IEC 61000-4-3 Radiated Immunity, IEC 61000-4-8 Magnetic Field Immunity

Figure 3: The ADM2795E’s integrated IEC 61000-4-5 certified surge solution provides designers with significant PCB area savings

The passing of IEC 61000-4-6 conducted radio frequency immunity, IEC 61000-4-3 radiated immunity, and IEC 61000-4-8 magnetic field immunity guarantees the products to meet the increasingly stringent electromagnetic compatibility requirements.

► Excellent lightning protection performance of communication interface

Lightning protection design has always been a headache for some engineers. Using the bypass protection method uses transient suppression components (such as TVS, MOV, gas discharge tubes, etc.) to bypass the hazardous transient energy to the ground. The transient interference within the range cannot last for a long time, and a good channel to connect to the ground is required, which is difficult to implement. And some TVS designed for lightning protection are not small in size, and the price is also high. The ADM2795E-EP uses a combination of isolation and internal protection. In this method, the isolation interface isolates large-scale lightning disturbances, and the protection device protects the isolation interface from excessive transient voltage breakdown.

Some Electronic equipment with high reliability requirements may suffer from lightning strikes during use. The DO-160G standard “Environmental Conditions and Test Procedures for Airborne Equipment” is an environmental test standard for general avionics hardware. Many aircraft manufacturers specify DO-160G Section 22 “Lightning Stroke Induced Transient Susceptibility” as a requirement for critical systems.

• Lightning protection requirements for related electronic equipment

The DO-160G Section 22 lightning protection standard simulates transient voltages and currents in avionics induced by the magnetic field produced by a direct lightning surge through the fuselage of an aircraft. Table 1 shows that for Wave 3 and Wave 4/Wave 1, commercial aircraft typically require DO-160G Section 22 Level 1 to Level 4 lightning protection. Aircraft equipment is divided into three categories, each with an associated electromagnetic compatibility (EMC) environment. The harshest EMC environments are located in Class A and Class B areas.

Table 1: DO-160G Section 22 Typical Lightning Protection Requirements for Commercial Aircraft

The ADM2795E-EP is an enhanced version of the ADM2795 that reduces system failures when operating in harsh application environments where high reliability is required. The ADM2795E-EP has the following important features:

• Has enhanced robustness against interference of DO-160G EMC.

• Operating temperature range: −55°C to +125°C.

• Leadframe: To mitigate tin whisker issues, the ADM2795E-EP features a nickel/palladium/gold (NiPdAu) leadframe finish.

• Production: Enhanced products manufactured through a single-process process baseline.

The ADM2795E-EP integrates fully certified DO-160G EMC protection on the RS-485 bus pins, providing lightning protection as described in Section 22. The ADM2795E-EP also provides ±15 kV electrostatic discharge (ESD) air-gap discharge protection as described in Section 25. For the lightning protection requirements of Section 22, the ADM2795E-EP protects against waveform 3, waveform 4/waveform 1, and waveform 5A: for GND2 testing, class 4 requirements can be achieved with 33 Ω or 47 Ω current limiting resistors; for GND1 Test, the isolation barrier can reach the level 4 requirements. The waveform shape and rise/fall times of the DO-160G standard are significantly longer than those specified in the IEC 61000-4-5 standard, as shown in Figure 4. The DO-160G Section 22 lightning protection standard involves very high energies, and the reliability design of the ADM2795E-EP protects the device from these extreme transients.

Figure 4: DO-160G Section 22 Waveform 1 and Waveform 5A and IEC61000-4-5 Surge Waveform

► Fault overvoltage protection and ±42 V miswire protection

The ADM2795E is fault protected over the entire extended common-mode operating range of ±25 V. And the driver output and receiver input are short-circuit protected from any voltage in the C42V to +42 VAC/DC peak range. The maximum current in the fault state is ±250 mA. The RS-485 driver includes a foldback current-limiting circuit that reduces the driver current when voltages exceed the transceiver’s ±25 V common-mode range limit. This current reduction due to the foldback feature enables better power dissipation and thermal management.

The ADM2795E is protected against high voltage miswiring when operating on a bus without RS-485 termination or bus bias resistors installed. A typical wiring error is when the high voltage 24VAC/DC power supply is connected directly to the RS-485 bus pin connector. The ADM2795E can withstand miswiring faults up to ±42V peak relative to GND2 on the RS-485 bus pins without damage. Miswiring protection is ensured on the A and B bus pins, and when the connector and bus pins are hot-plugged.

– Support case

Kayden Wang has encountered similar problems in the process of helping Shijian customers before. The customer experienced RS485 communication failure and black screen phenomenon during product debugging. In the process of troubleshooting with the customer, Kayden found that the transceiver used by the customer is a non-isolated RS485 transceiver of a certain brand.

During the startup process of the cross-linked high-power device, the startup current is conducted to the internal ground wire of the product through the distributed capacitance on the cable and the voltage generated by the inductive coupling, resulting in excessive instantaneous voltage between the RS485 bus and the ground wire (this RS485 transceiver can Withstand the maximum range of common-mode voltage (-7V ~ +12V), so that the transceiver sending pin and the reference ground breakdown and short-circuit, resulting in RS485 communication failure.

The RS485 transceiver and the product microcontroller are powered by the same set of +3.3V power supply. The normal range of the power supply is 2.7V~3.6V. At the same time, the internal 27VDC and 3.3V power supply of the product share the same ground. When the power device is disconnected, the induced voltage generated by the coil Conducted to the internal ground of the product through the 27VDC ground wire and the transceiver is damaged, resulting in the voltage difference between the internal 3.3V power supply and the ground of the product being lower than 2.7V, causing the microcontroller to reset for a short time and further causing a black screen.

In view of this fault phenomenon, combined with the customer’s requirements for EMC and lightning protection design, Kayden recommends the customer to use the ADM2795E+ADuM6028 (isolated power supply) solution. As shown in Figure 5, after the test of the test board, a common mode pulse is applied to the output pin and ground wire of the transceiver, and the amplitude adjustment range is (-10V ~ +15V). Power supply and output signal, product communication is normal, and the system is stable. It also successfully passed various EMC tests and lightning protection tests. The customer’s communication port reliability has been greatly improved.

Figure 5: 2.5 Mbps, 5 kV signal and power isolation RS-485 scheme (±42V fault protection and Level4EMC protection)

The ADM2795E integrated complete system-level solution can adapt to various harsh environments. A large number of experimental results and related cases have proved that ADM2795E can not only improve the reliability of RS485/422 communication, but also the circuit is very simple.

The Links:   LM48014F   LQ104V1DG5A