1 Introduction

RS-485 is a long-standing wired interface standard that allows for relatively high speed, robust, and long-distance communications. Many RS-485 systems can be subject to high voltage surges due to events such as lightning strikes on the system and the systems must be able to survive during one of these events. Most classical RS-485 transceivers are not able to handle these high voltage transients on the bus facing pins without sustaining damage – so more complicated protection schemes are used in the design to increase the robustness in the face of high voltage transients. At the core of all these protection schemes is a device that can clamp the bus voltage, a protection diode. In the past designers mostly looked to fully discrete designs that range from clamping diodes and current limiting resistors to more complex configurations that can include TBUs (transient blocking units) and MOVs (metal oxide varistors) as well.

As RS-485 transceivers have evolved, additional protection features such as integrated surge diodes have been implemented in some special use RS-485 devices, as can be seen in the THVD2419, THVD2429, as well as the THVD14x9 family of devices. However, as diodes start to shunt current the voltage on the line can increase, the clamping voltage of the line is directly related to the current shunted through the diode. With many RS-485 devices having asymmetric min and max voltage ratings that are under +/-15V finding the correct diode and current limiting resistor that not only protect the device during surges, but allow for high quality communication during normal operation can be the bane of designers working on a relatively simple protection scheme.

This limitation can be worked around with TI’s new line of integrated surge devices in the THVD24x9x family including devices like THVD2419 and the THVD2429. These devices not only boast an integrated surge diode for protection, but also offer high fault protection on the bus facing pins – with the pins being tolerant of up to ±42V with respect to the circuit ground

To fully understand the benefits of this new line of devices three main points can be covered. First, an understanding of discrete surge designs for RS-485 and what design challenges designers face. Second, how the integrated surge simplifies the system design. Finally, why the higher stand-off voltage separates a design using the THVD24x9x from other surge protected RS-485 designs.