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Safeguard RS-485 Links Against Electrical Transient Threats

RS-485 has long been a workhorse communication standard for industrial and communications electronics. Much like a tireless marathon runner, it is still widely prevalent today for its advantages in robust, high-noise-immunity communication. From high-end base stations, surveillance, and HVAC control to POS and industrial networks, RS-485 is everywhere.

The harsh environments and long distances over which RS-485 links run mean they are often assaulted with a broad range of electrical transient spikes. As such, these ports need to be well defended from electrical transient stress, and the protection circuitry needs to be as versatile at arresting rogue transients as the RS-485 interface is in its use in end applications. The links must be safeguarded from sub-nanosecond rise time transient threats — electrostatic discharge (ESD) and electrical fast transients (EFT) — and long-duration, high-power surges.

Table 1

Electrical transient threats.

Electrical transient threats.

Circuit protection typology
The centerpiece of a sound RS-485 protection strategy is the use of transient voltage suppression (TVS) diodes. These are nonlinear elements designed to shunt transient energy away from CMOS transceiver circuitry. With their sub-nanosecond response time and low clamping voltages, TVS devices are ideal for safeguarding data circuits from both fast rise time ESD and slow rise time surge events. Advanced TVS diodes can offer very low clamping voltage and high-surge immunity with only minimal capacitance penalty. Though RS-485 at 10 Mbit/s is not a high-speed port by today's standards, the TVS capacitance loading must still be considered, particularly for multi-drop RS-485 applications.

Protection against EFT and ESD
Nearly all RS-485 links need safeguarding against ESD and EFT events. Depending on the application, a subsegment of these links may not need high-surge immunity. For these applications, the protection topology can be targeted to arrest the fast rise time ESD events and lower-level surge threats. Figure 1 shows a dual-line TVS (SM712) device used to protect the RS-485 differential pair interface. In this case, the TVS has an asymmetrical +12V, -7V breakdown voltage, corresponding to the common mode voltage range of RS-485.

With a surge rating of 12 A (8×20 us), this solution is also well suited to protect against EFT and lower=level surge events. The EFT threat, though a fast rise time event, can present considerably higher energy levels than ESD due to the repetitive/burst nature of this transient condition. Thus, it is essential that the junction of the TVS be rated to withstand some level of surge events, not just ESD.

Figure 1

RS-485 protection scheme for ESD/EFT using a Semtech SM712 TVS device.

RS-485 protection scheme for ESD/EFT using a Semtech SM712 TVS device.

Protection against surge events
In many communication environments, the RS485 transceiver needs to be hardened to withstand higher-energy threats than ESD. RS-485 links may run in a harsh outdoor environment or in a telecom system where higher immunity levels are required. The circuit shown below uses a protection typology with higher surge-rated TVS devices (100 A, 8×20 us, 12 pF). The devices are configured so that there is both common mode and differential mode protection. In most cases, the TVS diodes alone will protect against the typical surge conditions encountered. For immunity to extremely high surge events, additional protection on the front end of the interface may be necessary. However, most communications applications can be safeguarded with high surge-rated TVS diodes alone.

Figure 2

RS-485 protection scheme for high-surge immunity using a Semtech TClamp1202P TVS array.

RS-485 protection scheme for high-surge immunity using a Semtech TClamp1202P TVS array.

Even after decades of use, RS-485 remains a prominent link in industrial and communications networks. With a good understanding of the transient threat environment and the types of transients encountered, TVS diodes can be implemented to achieve robust protection against short, fast rise time transients, as well as long, high current surge events.

9 comments on “Safeguard RS-485 Links Against Electrical Transient Threats

  1. Davidled
    June 11, 2014

    This article proposed SM712 device installation in RS-485 cable. I am wondering where this device is located in the cable line. This device would be located either the inside of RS-485 or other side of terminal. Potenial signal distortion could be caused by clamping the voltage.

  2. etnapowers
    June 13, 2014

    I guess that the reliability of the Transition Voltage Suppression diodes is a top priority to guarantee the correct functionality of RS-485 links in an harsh environment.

  3. BIG TOM_#1
    June 17, 2014

    I advise RS-485 system designers to distinguish between node design and bus design.

    In the past items like failsafe biasing, termination, and TVS belonged to the bus design. This changed slightly when Semtech came out with the SM712 TVS because of its relative low capacitance of 75pF. Previous TVS diodes or arrays had capacitances of 1000pF and were better placed strategically along the data link.

    The SM712 power rating of 400W however is rated only for 8/20 us currents pulse (or often falsely quoted 1.2/50us voltage pulse) specified in IEC61000-4-5 for surge protection.

    IEC61000-4-5 however clearly states that for symmetrical (differential) unshielded data lines, the cobination wave generator (CWG) is a 10/700us (voltage) or more correctly 5/320us current pulse. This pulse will exhaust the capabilities of the SM712.

    However, Semtech recently intrduced the TClamp1202, which is rated for a 10/1000us pulse with 300W and for the 8/20 us pulse with almost 2kW (derived from the device's power rating curve). Further more, this device has a junction capacitance of only 12pF typical. Hence it quickly made it into my RS-485 design tool-box.

    Regards,

    Thomas Kugelstadt

    Senior Applications Engineer, Industrial Interfaces, Texas Instruments Inc.

  4. JAYARAMAN KIRUTHI VASAN
    June 21, 2014

    Hi Timothy,

    Nice post. It has become a standard practice to include the TVS diodes in the lines. We always have the circuit as a complete block and cut-paste across all our designs.

    Can these diodes have any effect on the overall termination impedance/resistance, say, if I have around 32 RS485 participants in the network?

    Also, where to best position these diodes – near the connector or near the A,B pins of the RS485 chip?

  5. millikans drop
    June 23, 2014

    Hi Jayaraman,

    Thank you for your comments.  The TVS devices should have minimal impact to the termination scheme as they will appear as a high-impedance in the circuit.  The leakage current under normal operation is on the order of 10s of nA.  Thus, the TVS components should have very little effect on the drive current and the termination characteristics.

    Regarding the TVS placement, generally it is good practice to place the TVS as closely as possible to the connector.  

    Best regards,

    Timothy Puls

  6. millikans drop
    June 23, 2014

    Hi Thomas,

    Thank you for your comments. You are definitely a respected expert on RS-485 circuits and issues of EMC in RS-485 networks.  Thank you for your favorable comments regarding the TClamp1202P.

    Best regards,

    Timothy Puls

  7. chirshadblog
    June 24, 2014

    @jayaraman: Do you think the policies are effective ? I feel the policies are being created but not put into practice 

  8. vasanjk
    July 15, 2014

    Hi Timothy

    I would like to bring in a little bit of specificity into my question.

     

    Is it Connector>>>TVS>>Rest of the circuit or

    Connector(TVS just on connector pin)>>Rest of the circuit ?

  9. vasanjk
    July 15, 2014

    chirshadblog

     

    These principles are very important and are very much in practice.

     

    But there are no thumb rules which can be implemented just like that. Every design is unique and one solution that was best may not be even remotely effective in another design.

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