Abstract
Publisher Summary This chapter focuses on the highway addressable remote transducer (HART) protocol. The HART protocol is one of a number of smart instrumentation protocols designed for collecting data from instruments, sensors, and actuators by digital communication techniques. The HART protocol can operate in a hybrid 4-20 mA digital fashion. This protocol was originally developed by Rosemount and is regarded as an open standard that is available to all manufacturers. Its main advantage is that it enables an instrumentation engineer to keep the existing 4–20 mA instrumentation cabling and to use simultaneously the same wires to carry digital information superimposed on the analog signal. HART is a hybrid analog and digital protocol, as opposed to most Fieldbus systems that are purely digital. The HART protocol uses the frequency shift keying (FSK) technique based on the Bell 202 communications standard. Two individual frequencies of 1200 and 2200 Hz, representing digits 1 and 0 respectively, are used. The average value of the sine wave (at the 1200 and 2200 Hz frequencies) that is superimposed on the 4–20 mA signal, is zero. Hence, the 4–20 mA analog information is not affected. This chapter describes the origin and benefits of the HART protocol. It also discusses the three OSI layers of the HART protocol.
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