23 April 2012

Integrated device addresses HART communications

The humble 4 to 20mA current loop has been a faithful servant of process control systems for decades, largely down to its reliability and simplicity.

The underlying principles of the current loop are readily understood. The output voltage from a sensor, for example, is converted into a current, with the sensor's zero level output represented by a 4mA signal and its full scale output represented by a 20mA signal. Using a current to represent the output lends itself more readily to installations where the sensor may be a long distance from the control system.

Transmitting the sensor's output voltage over a long distance raises a number of problems, including voltage drop and noise. However, the current over the link – also known as a loop – will remain unaffected.

But 4 to 20mA communications loops provide only basic information to the control system: the output is proportional to the current flowing. What if you want to add a degree of sophistication? That requirement began to be addressed in the 1980s, with the emergence of a range of so called fieldbus protocols. Even so, the 4 to 20mA loop retained its popularity. The need to add sophistication to 4 to 20mA communications was first addressed by Rosemount, now part of Emerson, when it developed an approach based on the Bell 202 communications standard. The Bell 202 modem used audio frequency shift keying (FSK) to support data rates of up to 1200bit/s in half duplex mode. With differential FSK, full duplex communications were offered at rates of up to 1800bit/s. The overlap between telephony and industrial control were not accidental; there was a significant legacy of current loop expertise in the telephony sector.

Fairly quickly, this approach turned into an open specification, under the name of the Highway Addressable Remote Transducer protocol, or HART.

HART instruments can operate in two modes: point to point, in which digital information is modulated on top of the current loop signal; or multidrop, in which the loop current is fixed at 4mA. In the latter case, digital signals are still modulated on top of the current, but an address needs to be included.

According to the HART Communications Foundation (HCF), because most automation networks in operation are based on traditional 4 to 20mA analogue wiring, HART technology serves a critical role because the digital information is simultaneously communicated with the 4 to 20mA signal. Without it, the foundation says, there would be no digital communication.

Despite HART's apparent lack of sophistication, relative to similar protocols, the approach continues to find application. Uwe Brockelmann is director of field applications with Analog Devices. He said: "There remains a large installed base of 4 to 20mA communications systems and companies are still building new equipment based on the approach." This, Brockelmann continued, is no surprise as it provides stable business opportunities for suppliers.

According to Brockelmann, there are three good reasons for using HART. "Firstly, it's a global communications standard. Secondly, it enhances measurement capability and, finally, it enables diagnostics."

Digital information is added to the underlying analogue current using FSK techniques. "A frequency of 2.2kHz is used to represent a digital 1," Brockelmann continued, "while a frequency of 1.1kHz represents a digital 0."

The demand for HART related technology remains large enough to convince Analog Devices to continue to develop products for the market. One of the latest introductions is the AD5700, which offers higher levels of integration while, the company says, simplifying applications.

The single chip solution has been developed for use as a half duplex modem. According to Analog Devices, the part includes all the necessary filtering, signal detection, modulation/demodulation and signal generation features (see fig 1). Also integrated is a buffered HART output, which the company says provides high output drive capability and removes the need for external buffering.

The modulator block contains a direct digital synthesis engine which produces either a 1.1 or a 2.2kHz sine wave in digital form, after which a d/a conversion is performed. This approach generates continuous phase signals and avoids output discontinuities. When the request to send pin is at a logic high, the demodulator is enabled, putting the device into receive mode. Analog Devices says the combination of an a/d converter, digital filtering and digital demodulation provides an accurate output.

Alongside a HART FSK modem, the chip integrates a receive band pass filter. "The AD5700 is smaller than other solutions," Brockelmann observed, "and uses less power. It also reduces the number of external components required from 13 to five."

"The AD5700 HART modem ic has been tested by the HCF to verify that products designed with the new device will adhere to HART protocol requirements," said Ron Helson, the HCF's executive director. "Compliance with HART protocol requirements is important so users worldwide can gain quick, easy visibility to devices in the field when using HART enabled handheld test, calibration devices, computers and automation systems."

Two versions of the AD5700 have been announced, both of which are supplied in a 24pin 4 x 4mm package. The AD5700-1 differs from the AD5700 in that it features an integrated 0.5% precision oscillator. Both parts are specified for use in temperatures ranging from -40 to 125°C and operate from a supply in the range from 2 to 5.5V. While the AD5700 consumes 178µA, the AD5700-1 consumes 464µW. Additionally, both parts can be used in instrument and master configurations.

When the devices are in full duplex configuration, testing is possible not only of the AD5700, but also of the complete signal path, verifying the communications loop is functional. In this mode, it is possible to improve the system's safety integrity level (SIL) rating.

The AD5700 has been designed to interface with other components from Analog Devices' industrial data conversion portfolio. The company has unveiled what it claims is a fully functional smart transmitter demo system (see fig 2). Alongside the AD5700, the circuit uses the AD5421, a 16bit loop powered 4 to 20mA d/a converter. The demo has also been approved by the HART Communication Foundation. There is also an evaluation board available for the AD5700.

Meanwhile, Analog Devices has added the ADM3054 to its CAN product portfolio. The ADM3054 is a 5kV rms signal isolated CAN physical layer transceiver which complies with ISO11898. The device, which employs Analog Devices' iCoupler technology, combines a three channel isolator and a CAN transceiver into one package. Logic is powered from a single 3.3 or 5V supply, while the bus side uses a 5V.

The ADM3054 creates an isolated interface between the CAN protocol controller and the physical layer bus. It is capable of running at data rates of up to 1Mbit/s.

Current limiting and thermal shutdown features are provided to protect against output short circuits and situations where the bus might be shorted to ground or power terminals. The part is specified for use in the industrial temperature range and is available in a 16 lead soic.

By reducing pcb component count by as much as 70% compared to traditional discrete optocoupler based component circuits, the ADM3054 simplifies design and reduces board space.

The part complements the ADM3052 and ADM3053, which provide power isolation in addition to signal isolation. The ADM3054 is targeted at applications where an isolated power rail exists or where an additional overwind from an isolation transformer can be used.

Graham Pitcher

Supporting Information



Analog Devices

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