Transient protection for RS485 bus nodes

The huge cost of industrial network downtime caused by functional failure due to electrical overstress makes it necessary to protect network nodes, especially the electrical transients caused by electrostatic discharge, inductive switching and lightning strikes. Therefore, the International Electrotechnical Commission (IEC) has defined three transient suppression test standards to ensure the correct operation of the circuit during and after the test.

This article briefly describes these tests and suggests that using industrial RS-485 cables and low capacitance transient voltage suppressors provides the most effective method for protecting your network.

Transient Suppression Test Series

We will discuss three IEC standards:

Electrostatic Discharge (ESD) Immunity (IEC61000-4-2)

Electrical fast transient (EFT) immunity, or burst immunity (IEC61000-4-4)

Surge immunity (IEC61000-4-5)

ESD testing simulates the impact of electrostatic discharge from the human body on electronic equipment. The test pulses generated by the ESD generator are short-duration (less than 100ns) pulses with a fast rise time of about 1ns. Although this test pulse is low energy, it can form many destructive high currents, which are enough to destroy the internal protection circuit of a transceiver. A minimum ESD test sequence consists of 20 discharges, 10 positive pulses, 10 negative pulses, and a one-second pause interval between each pulse.

Burst testing simulates everyday switching transients caused by inductive switching, relay contact bounce, etc. In contrast to ESD testing with a single test pulse, the burst generator generates a complete sequence of test pulses (called a burst). Each burst consists of about 15,000 transients. A complete test sequence of six 10s bursts (with a 10s pause interval between each other) can generate millions of pulses in a minute. A single pulse is short in duration (see Figure 1b) and therefore has low energy, but this completely endless transient bombardment of the transceiver will pose a huge challenge to its internal protection units.

The surge test is the most stringent of all tests because it simulates the switching transients caused by lightning. The transients produced by the surge generator are about 1000 times longer than ESD or burst transients. In addition, the low source impedance of the generator allows high surge currents at high voltages, representing high energy pulses. Due to its high energy component, the test sequence generally consists of five positive surge pulses and five negative surge pulses with a time interval of one minute or less between pulses.

Figure 1 Examples of ESD (left), burst (middle), and surge pulse (right)

Note: Current and voltage are normalized. For absolute values, refer to the actual standard.

Protecting bus nodes

Choosing low-cost CAT5 industrial RS-485 cables or flat-band cables can eliminate the main transient energy introduced into the bus lines. Cables such as Belden 3107A (see Figure 2) have a braided shield that can greatly reduce the noise coupled into the signal wires. Less signal line noise means less transient impact on the following protection circuits.

Figure 2 Industrial RS-485 cable with shield and drain wire

The industrial RS-485 cable shown in Figure 2 is ideal for single-duplex and half-duplex data links. It allows one signal pair to be used as the bus signal line and the other signal pair to be used as the ground line, thereby reducing the inductance of the transient current return path. Some other benefits include the 120 ohm rated characteristic impedance of the cable. This ensures impedance matching RS-485 transceiver switching characteristics and the addition of a drain wire, allowing simple grounding of the cable shield. Note that grounding should be applied only at one end of the cable, preferably at the end closest to the single ground reference bus ground reference.

Figure 3 shows a simplified schematic of a typical RS-485 node circuit with transient voltage suppressor diode (TVS) protection. Recent process technology advances have enabled the manufacture of fast, low-capacitance TVS diodes. Previous generations of TVS designs exhibited nanosecond response times that were too slow for the fast rise times of ESD and burst transients. In addition, their capacitive loading exceeded 1000 nF per TVS device, which did not provide effective multi-node network protection without reducing data rates to very low levels.

Many modern high-precision suppressor designs have response times in the low picosecond range, while having capacitances of around 10 pF to 100 pF (depending on the device topology and power rating), enabling single bus node protection.

In differential data signals, such as RS-485, three transient suppressors are generally required to simulate effective real-world transient protection: two TVS devices are used to achieve common-mode transient protection (appearing between the A line and ground and the B line and ground), and the third TVS is used to suppress differential transients between the A line and the B line.

The screw terminals that serve as RS-485 connectors connect the transmission cable to the transceiver (XCVR). Three transient voltage suppressor diodes (TVS) are used to eliminate common-mode transients between the A line and ground and the B line and ground, as well as differential transients between A and B.

Figure 3 RS-485 bus node with transient protection

Figure 4 shows the symmetrical voltage-current (VI) characteristics of a bidirectional transient voltage suppressor. At some low voltages down to the cutoff voltage VWM, the transient suppressor presents a high impedance to the signal line with only a few microamperes of device leakage current. In this state, the data link must be able to operate normally. Therefore, when selecting a TVS for an RS-485 link, its cutoff voltage must be higher than the maximum bus voltage, including the ± 7V ground potential difference (GPD) specified by the RS-485 standard, which necessarily requires VWM ≥ 12V.

In a transient event where the bus potential exceeds the TVS breakdown voltage, VBR, the device becomes a low impedance that conducts high current to ground. However, its dynamic impedance causes a voltage drop across the device that increases proportionally with the rising current. This voltage is often expressed as a clamping voltage, Vc, which can be as high as 35V, significantly exceeding the maximum rating of the transceiver bus potential.

Figure 4 VI characteristics of TVS diode

Although high ESD-rated transceivers can handle this short-term overstress, some weaker components benefit from surge-rated resistors switched in series with the transceiver bus terminations (see Figure 3). Resistors with typical values ​​of 10Ω to 20Ω reduce the current flowing to the transceiver during clamping action, minimizing the impact on its ESD cells.

In addition to these dangerous voltage and current levels, real-world transients also introduce a lot of broadband noise. For example, the noise of an ESD pulse has a spectrum of about 3 MHz to 3 GHz. Therefore, in addition to transient suppression, we also recommend the use of noise filtering and high-frequency layout methods to ensure a robust board design in the face of electromagnetic interference (EMI).

These suggestions will help you with this design. Transient protection options to consider at the beginning of your circuit design include:

Use a four-layer printed circuit board with the following stacking order: bus signal layer, ground layer, power layer, and control signal layer.

Place the ground plane immediately adjacent to the bus signal layer to establish impedance tracking control and provide a low inductance path for the return current.

Place the TVS diode as close to the bus connector as possible to prevent transients from penetrating into the board circuitry.

Place bypass capacitors (10nF – 100nF) as close as possible to all ICs on the board.

Connect the TVS and bypass capacitors to the ground plane using multiple vias (at least 2 per terminal).

The EMI filter is applied to the single-ended side of the transceiver via a simple RC low-pass filter.

Note that because high-frequency currents follow the path of least inductance, most of the above recommendations are aimed at diverting high-frequency noise through low-inductance paths.

in conclusion

Some sophisticated transient suppressor diodes have the capability to effectively protect all bus nodes in an RS-485 network. Although effective transient protection adds cost to the initial design, it can prevent future expensive field failures, network downtime, and possible product recalls.