Surge Protection for RS485 Applications

RS485

RS-485, also known as TIA-485(-A), is a standard defining the electrical characteristics of a balanced differential serial communication protocol. It's commonly used in industrial and commercial applications for its robustness, noise immunity, and ability to communicate over long distances (up to 1200 meters) at high speeds (up to 10 Mbps). RS-485 supports multi-point communication, allowing multiple devices to communicate on the same bus.

RS485 Applications

RS-485 signals are utilized in a wide range of computer and automation systems and is typically used as the physical layer underlying many standard and proprietary automation protocols used to implement industrial control systems, including the most common versions of Modbus and Profibus.

Risks, Vulnerabilities and Threats

Given the extensive use of RS485 in exposed industrial applications, RS485 circuits are at risk from overvoltage transient surge events caused by lightning strikes, electromagnetic coupling, and Ground Potential Rise (GPR). It should be noted that RS485 employs differential signaling and does resist some electromagnetic interference caused by motors and welders, for example. However, this immunity does not mitigate inherent vulnerabilities of equipment and subsequent damage caused by more extreme transient events.

The Cost of Degradation, Disruption and Destruction

Exposure to transient events can cause Degradation, Disruption and Destruction to equipment and operations resulting in expensive downtime and costs of equipment repairs. ABB Motion Services conducted a recent survey of 3,125 plant maintenance managers across the globe and concluded the typical cost of an outage that lasts for an eight-hour working shift is $1m. That’s $125,000 per hour over an eight-hour working shift, which equates easily to the time usually taken to get an operation back up and running when something goes wrong.

Investing in Reliability: Implementation of Surge Protection Strategy

The cost of implementing a comprehensive surge protection strategy is extremely low compared to the potential cost of equipment repair and downtime as revealed in the ABB study. Citel offers a broad range of data line Surge Protection Devices compatible with RS485.

Recommended Surge Protectors for RS485

Surge Protection should be installed in an RS485 network where 22 or 24 AWG twisted pair wire exit the Master/Host transceiver (near end) into the exposed environment and where the remote devices are located. (far end)

Interface	Protocol	Wiring	Max.Operating Voltage	Max. Data Rate	Part #	Picture
RS485	Modbus RTU&ASCII	2-wire	6Vdc	10 mbit/s	DLA-06D3
RS485	Interbus	2-wire	6Vdc	500kb/s	DLA-06D3
RS485	Modbus RTU&ASCII	4-wire	6Vdc	10 mbit/s	DLA2-06D3

References