By Thomas Kugelstadt and Kim Devlin-Allen, Texas Instruments
Modern full-duplex RS-485 transceivers offer several advantages over other communication options and suit a range of applications.
Often RS-485 networks are associated with long cable runs linking remote bus nodes whose transceivers operate in half-duplex mode. That is, they either transmit or receive data – but never do both at the same time. The length of such data links can span from 1500 to 6000 ft without repeaters, and allow for data rates of around 200 kbps.
At the other end of the frequency spectrum, high-speed transceivers operating between 10 and 50 Mbps usually manage bus lengths of less than 300 ft. However, modern time critical applications often require increased bandwidth and the ability to transmit and receive data at the same time. This has put new emphasis on full-duplex transceivers whose bus ports provide separate channels for transmit and receive function.
Bandwidth and applications
Full-duplex (FD) transceivers are available for low, medium, and high data rates. The low to medium range is mainly served by half-duplex transceivers because of the lower cabling cost of a basic two-wire bus when spanning long distances. The majority of applications for full-duplex transceivers lie in the upper frequency range with data rates from 10 to 50 Mbps covering network lengths up to 300 feet, even extending up to 1000 ft with newer transceivers.
Typical applications of full-duplex networks include:
• Telecommunication systems exchanging global positioning data (GPS) with the telephone infrastructure
• Seismic systems exchanging Earth wave data between sensor modules and system controllers
• Position encoders transferring digitized angular data to the host of a motor or motion control system
• Traffic monitoring networks reporting information on traffic density from cameras to a central control office
• Enterprise or data center networks with chassis-to-chassis or rack-to-rack fast data links.
Chosen bus configurations can vary between point-to-point connections and multi-drop/multi-point buses. Depending on system requirements, power supply lines might be run adjacent to data lines, or local, individual power supplies may power the nodes. Some configurations, though, give rise to ground potential differences (GPD) and their associated common-mode voltages.
With high-speed transmission systems pushing cable lengths to 1000 feet, transceivers with high common-mode capability are necessary. Here RS-485 offers advantages over other high-speed transmission standards, such as low-voltage differential signaling (LVDS) and Ethernet, because RS-485 demands reliable data transmission over a common-mode voltage range from –7 V to +12 V. So called super RS-485 transceivers even manage an extended common-mode range of –20 V to +25 V.
In contrast, LVDS transceivers have an input common-mode range of +0.2 V to +2.2 V. Their multi-point counterparts (MLVDS) offer slightly wider higher levels from –1 V to +3.4 V.
Ethernet overcomes ground-shifts and associated common-mode issues through the use of isolation transformers. Their implementation, however, raises system design cost. Additionally, Ethernet requires protocol software (such as HDLC) to assure reliable communication. In contrast, RS-485 is an electrical-only standard and is adaptable to any protocols using synchronous or asynchronous transmission. This is with or without signal integrity to improve coding schemes, such as Manchester coding.
Modern designs of full-duplex RS-485 transceivers offer robust internal transient protection against system-level electrostatic discharge (ESD) strikes. It also extends common-mode capability and high standoff voltage to survive even in the harshest of industrial environments.
Electrostatic discharge (ESD)
Two test methods, and their corresponding test pulse generators, determine component and equipment immunity to electrostatic discharges caused by human contact:
The human body model (HBM) generator whose electrical parameters are specified by JEDEC standard JS001-2010, delivers low-energy ESD strikes to a single integrated circuit (IC). This test method is applied in the IC manufacturing environment where ESD control measures, such as wearing ESD protective gear, are implemented to lower the electrical overstress upon an IC.
The human metal model (HMM) defined by IEC 61000-4-2, applies ESD strikes almost four-times higher in energy content to an enclosed system or equipment to simulate uncontrolled discharges in the end-user environment.
To assure equipment survival in the field, modern transceivers must provide high system-level ESD immunity following IEC 61000-4-2. For example, the SN65HVD147x family of full-duplex transceivers are fully tested for ESD ratings of 16 kV IEC 61000-4-2 contact discharge and 30 kV HBM discharge.
When determining the ESD protection of a transceiver, beware of ESD ratings without standard specification. These are commonly tested to HBM or other, less energetic ESD strikes. HBM levels higher than 30 kV are usually derived mathematically without stating the applicable test setup.
Immunity tests for high-energy surges caused by inductive switching or lightning strikes are specified in IEC 61000-4-5. The standard specifies two types of test current pulses for determining the immunity rating of symmetrical data transmission systems: a short, 8/20 μs; and a long 5/320 μs. The numbers reflect the front time, followed by the decay time to half peak value of the test pulse. The long pulse, having nine-times higher energy content, requires significantly stronger transient voltage suppressor (TVS) devices to protect subsequent circuitry.
Because IEC 61000-4-5 equivalent surges have more than a thousand times higher energy contents than ESD transients, external protection through transient voltage suppressors (TVS) are necessary. Proper surge protection design techniques require the implementation of current-limiting resistors (Rs) in series with the next stage of transient protection devices. In this case, with the internal ESD structures of the transceiver. These resistors must be pulse-proof thick-film resistors with values of 10 Ohm for the 8/20 and 20 Ohm for the 5/320 μs surge.
Devices like the SN65HVD147x family of full-duplex RS-485 transceivers are designed to survive even the harshest industrial environments.
Filed Under: Design World articles, Networks • connectivity • fieldbuses, Surge protection
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