Technical Article MS-2443 . RS-485 STANDARD Safeguard Your RS-485 I&I applications require data transmission between multiple Communication Networks systems, often over very long distances. The RS-485 electrical standard is one of the most widely used physical layer from Harmful EMC Events specifications in I&I applications, such as industrial by James Scanlon, Senior Evaluation Engineer, Analog automation, process control, motor control, and motion Devices, Inc., and Koenraad Rutgers, Senior Field control remote terminals building automation, such as Applications Engineer, Bourns, Inc. heating, ventilation, and air conditioning (HVAC) security systems renewable energy. IDEA IN BRIEF Some of the key features of the RS-485 that make it ideal for In real industrial and instrumentation (I&I) applications, use in I&I communications applications are: RS-485 interface links must work in harsh electromagnetic Long distance linksup to 4000 feet environments. Large transient voltages caused by lightning strikes, electrostatic discharge, and other electromagnetic Bidirectional communications possible over a single phenomenon can damage communications ports. To ensure pair of twisted cables that these data ports can survive in their final installation environments, they must meet certain electromagnetic Differential transmission increases common-mode compatibility (EMC) regulations. noise immunity and decreases noise emissions These requirements include three main transient immunity Multiple drivers and receivers can be connected on the standards: electrostatic discharge, electrical fast transients, same bus and surge. Wide common-mode range (7 V to +12 V) allows for Many EMC problems are not simple or obvious, so they must be considered at the start of the product design. Leaving differences in ground potential between the driver and these considerations to the end of the design cycle can lead receiver to overruns in engineering budget and schedule. TIA/EIA-485-A allows for data rates of up to 10s This article describes each of these main transient types and of Mbps presents and demonstrates three different EMC compliant solutions for three different cost/protection levels on RS-485 TIA/EIA-485-A describes the physical layer of the RS-485 communication ports. interface and is normally used with a higher-level protocol, such as Profibus, Interbus, Modbus, or BACnet. This allows Analog Devices, Inc., and Bourns, Inc., have partnered for robust data transmission over relatively long distances. to extend their offering of system oriented solutions by In real applications, however, lightning strikes, power co-developing the industrys first EMC compliant RS-485 induction and direct contact, power source fluctuations, interface design tool that provides up to Level 4 protection inductive switching, and electrostatic discharge can damage levels for IEC61000-4-2 ESD, IEC61000-4-4 EFT, and RS-485 transceivers by generating large transient voltages. IEC61000-4-5 surge. It gives designers the design options Designers must ensure that equipment does not only work depending on the level of protection required and available under ideal conditions but that it will also work in the real budgets. These design tools allow designers to reduce risk of world. To ensure that these designs can survive in electrically project slippage due to EMC problems by considering them harsh environments, various government agencies and at the start of the design cycle. regulatory bodies have imposed EMC regulations. Compliance with these regulations gives the end user assurance that designs will operate as desired in these harsh environments. www.analog.com Page 1 of 10 2013 Analog Devices, Inc. All rights reserved. MS-2443 Technical Article ELECTROMAGNETIC COMPATIBILITY repeatable. Therefore, contact discharge is the preferred test method. An electromagnetic environment is composed of both radiated and conducted energy, so EMC has two aspects: During testing, the data port is subjected to at least 10 positive emission and susceptibility. Thus, EMC has the ability of an and 10 negative single discharges with a one second interval electronic system to function satisfactorily in its intended between each pulse. Selection of the test voltage is dependent electromagnetic environment without introducing intolerable on the system end environment. The highest specified test is electromagnetic disturbances to that environment. This Level 4, which defines a contact discharge voltage of 8 kV article deals with increasing the protection level for EMC and an air discharge voltage of 15 kV. susceptibility of RS-485 ports against the three main EMC Figure 1 shows the 8 kV contact discharge current waveform transients. as described in the specification. Some of the key waveform The International Electrotechnical Commission (IEC) is parameters are rise times of less than 1 ns and pulse widths the worlds leading organization that prepares and publishes of approximately 60 ns. This equates to a pulse with a total international standards for all electrical, electronic, and energy in the range of 10 s of mJ. related technologies. Since 1996, all electronic equipment sold to or within the European Community must meet EMC levels as defined in specifications IEC61000-4-x. The IEC61000 specifications define the set of EMC immunity requirements that apply to electrical and electronic equipment intended for use in residential, commercial, and light industrial environments. This set of specifications includes three types of high voltage transients that electronic designers need to be concerned about for the data communication lines: IEC 61000-4-2 electrostatic discharge (ESD) IEC 61000-4-4 electrical fast transients (EFT) Figure 1. IEC61000-4-2 ESD Waveform (8 kV) IEC 61000-4-5 surge immunity ELECTRICAL FAST TRANSIENTS Each of these specifications defines a test method to assess Electrical fast transient testing involves coupling a number the immunity of electronic and electrical equipment against of extremely fast transient impulses onto the signal lines to the defined phenomenon. The following sections summarize represent transient disturbances associated with external each of these tests. switching circuits that are capacitively coupled onto the ELECTROSTATIC DISCHARGE communication ports, which may include relay and switch ESD is the sudden transfer of electrostatic charge between contact bounce or transients originating from the switching bodies at different potentials caused by near contact or of inductive or capacitive loadsall of which are very induced by an electric field. It has the characteristics of high common in industrial environments. The EFT test defined current in a short time period. The primary purpose of the in IEC61000-4-4 attempts to simulate the interference IEC61000-4-2 test is to determine the immunity of systems resulting from these types of events. to external ESD events outside the system during operation. Figure 2 shows the EFT 50 load waveform. The EFT IEC61000-4-2 describes testing using two coupling methods. waveform is described in terms of a voltage across 50 These are known as contact discharge and air-gap discharge. impedance from a generator with 50 output impe dance. Contact discharge implies a direct contact between the The output waveform consists of a 15 ms burst of 2.5 kHz discharge gun and the unit under test. During air discharge to 5 kHz high voltage transients repeated at 300 ms intervals. testing, the charged electrode of the discharge gun is moved Each individual pulse has a rise time of 5 ns and pulse duration toward the unit under test until a discharge occurs as an arc of 50 ns, measured between the 50% point on the rising and across the air gap. The discharge gun does not make direct falling edges of the waveform. The total energy in a single contact with the unit under test. A number of factors affect EFT pulse is similar to that in an ESD pulse. The total energy the results and repeatability of the air discharge test, including in a single pulse is typically 4 mJ. Voltages applied to the data humidity, temperature, barometric pressure, distance, and ports can be as high as 2 kV. rate of approach to the unit under test. This method is a better representation of an actual ESD event but is not as Page 2 of 10