As a standard for industrial communications, the RS-485 differential interface is useful in communicating long distances where immunity to common-mode noise is an important requirement. RS-485 ports are used effectively in the process control factory setting and are typically developed with point-to-point, multipoint, and multi-drop systems where the serial drop distances can range from a few meters to over a thousand meters. In industrial environments, transients are typically generated by conducted or induced energy from switching inductive loads. Process control systems must be designed with robust protection during installation (handling) and commissioning (miswiring), with the added ability to survive significant transients (system transients/lightning). Any of these issues can degrade the serial port’s data transfer performance, and may cause damage to devices connected to the serial bus.

There are several effective circuit protection solutions, including both single-stage and a new three-stage design, which can be implemented to meet a variety of transient standards for RS-485 serial device ports. RS-485-based systems must satisfy data transfer requirements with a high level of transient immunity, making it more important than ever for designers to choose a robust circuit protection solution.

RS-485 Port Requirements

RS-485 systems are inherently more robust than many other networks, because the signal is delivered differentially between two wires, relative to a third reference voltage. The reference voltage is often, but not always, the local earth potential. This provides substantial immunity to common mode noise, such as ground noise and induced noise from nearby motors, solenoids and transformers.

Serial port designs follow the nominal recommended voltage range for the bus pins. RS-485 defines how much protection the device has beyond the −7 V to +12 V. If the transient environment requires the data bus to survive up to ±24 V, then external protection must be added or adapted to meet these compliance standards. The selection of a device(s) for adequate circuit protection depends on the requirements of the application and maximum operating specifications of sensitive devices, such as the serial bus driver.

Table 1 displays the interface specifications for RS-485 ports. Key electrical parameters of -7 V to +12 V operating voltage with 32 Mbps data transfer at distances of 1200 meters are included. Protection requirements referenced to the IEC 61000-4-2 (ESD), IEC 61000-4-4 (EFT), and IEC 61000-4-5 (Surge) standards are often part of the system design requirements.

Single-Stage Circuit Protection

Single-stage protection normally includes a single Transient Voltage Suppression (TVS) array device on each node. This bidirectional device provides overvoltage protection for the RS-485 transceiver when the input voltage goes beyond the -7 V to +12 V common mode voltage range of the transceiver. For RS-485 systems with limited exposure, this TVS diode array may be all that is needed for reliable operation.

Three-Stage Circuit Protection

An advanced three-stage protection solution for RS-485 ports has been developed, that includes a TVS diode array for secondary protection and TBU® High-Speed Protectors which limit the current and voltage stress on the equipment and the TVS diodes. Primary protection may be provided by using a Metal Oxide Varistor (MOV) or a Gas Discharge Tube (GDT) as shown in Fig. 1. This protection scheme provides an optimal approach when protecting the RS-485 transceiver in applications located in environments where the transceiver will be exposed to fast rising, long duration, high voltage surges. The response characteristics of these components are coordinated to provide a high level of protection for the RS-485 interface that far exceeds the handling capability of a single-stage component solution.

Fig. 1 illustrates the topology for three-stage coordinated protection. Components must be carefully chosen to provide coordinated protection. The TVS diodes at the RS-485 transceiver clamp the voltage on the RS-485 signal lines to a safe level. When the transient current through the TBU® HSP circuit protection device exceeds the trigger current level, the device transitions to a blocking state, reducing the current through the device to its very low quiescent level. The fast response of the TBU® HSP device limits the let-through energy seen by the sensitive transceiver. As the transient voltage level continues to rise, a MOV or GDT primary device is triggered, which clamps or crowbars the input signal lines to a safe level protecting the TBU® HSP device from exposure to an excessive voltage, which could damage the TBU® HSP device.

High-Speed Protector Devices

TBU® HSP devices are low capacitance, high-speed series circuit protection components constructed with MOSFET semiconductor technology. They are designed to protect against faults caused by short circuits, AC power cross, induction and lightning surges up to rated limits, and then reset once the fault clears. The TBU® HSP offers a significant performance increase in terms of response time compared to protection solutions previously available.

In the three-stage topology, the TBU® HSP provides coordination between the primary and secondary protectors. During normal circuit operation, the TBU® HSP device is a low value series resistance on the communication line. As the transient current through the TBU® HSP increases to the trigger current level, the device switches to a very high resistance or protected state, effectively disconnecting the transient from the sensitive RS-485 transceiver. The fast response of the TBU® HSP limits the transient energy seen by the transceiver to a very low level.

Handling Large Surges

Power cross and high surge currents present significant challenges for circuit designers, especially when working in a finite PCB area. A circuit protection scheme that employs a tranisent voltage suppression (TVS) diode plus a resistor is effective in handling small to medium transients. However, it is not robust enough to protect against power cross and large surge currents.

By contrast, a TBU® HSP protects against these threats and offers a small footprint. Devices are available that protect against 230 Vrms as well as, 120 Vrms power cross conditions.

The purpose of the primary protector is to keep the voltage level the TBU® HSP is exposed to, below its maximum Vimpulse rating. Thus, the clamping or crowbar voltage of the primary protector must be less than Vimpulse at the maximum surge current required for the protection circuit design. The maximum surge level requirement for the design is usually specified with reference to an industry standard document, such as IEC 61000-4-5.

The secondary protector is chosen to clamp or crowbar the voltage level the transceiver is exposed to, to a safe level. Since the TBU® HSP circuit protection device has a very fast response, a minimal amount of current is let-through to the secondary device, and in the worst-case leaves a small amount of energy to be handled by the transceiver internal steering diodes. Fig. 2 shows an evaluation board with a gas discharge tube (GDT) as the primary protector. Fig. 3 is a board with a Metal Oxide Varistor (MOV) as the primary protector.

Capacitance is another important factor to consider as it could limit the bandwidth of the nodes. The total capacitance on the signal lines lumps effects from the cabling, connector, and circuit protection components. Estimating the capacitance from the cabling and connector will indicate the maximum capacitance available.

For more information about Bourns® circuit protection products and technologies, including Bourns® TBU® HSP devices and evaluation boards, visit www.Bourns.com.

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