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.
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.
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.
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.