18a - HART overview

Abstract

This chapter provides an insight into the highway addressable remote transducer (HART) protocol. The HART protocol is a typical smart instrumentation fieldbus that can operate in a hybrid 4-20 mA digital fashion. The main advantage of HART 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. This enables most companies to capitalize on their existing investment in 4–20 mA instrumentation cabling and associated systems and to add further capability of HART without incurring major costs. HART is a hybrid analog and digital protocol, as opposed to most fieldbus systems, which are purely digital. The HART protocol uses the frequency shift keying (FSK) technique based on the Bell 202 communications standard. The HART protocol can be used in three ways, namely, in conjunction with the 4–20 mA current signal in point-to-point mode, in conjunction with other field devices in multi-drop mode, and in point-to-point mode with only one field device broadcasting in burst mode.